seccomp: Make syscall skipping and nr changes more consistent
This fixes two issues that could cause incompatibility between kernel versions: - If a tracer uses SECCOMP_RET_TRACE to select a syscall number higher than the largest known syscall, emulate the unknown vsyscall by returning -ENOSYS. (This is unlikely to make a noticeable difference on x86-64 due to the way the system call entry works.) - On x86-64 with vsyscall=emulate, skipped vsyscalls were buggy. This updates the documentation accordingly. Signed-off-by: Andy Lutomirski <luto@amacapital.net> Acked-by: Will Drewry <wad@chromium.org> Signed-off-by: James Morris <james.l.morris@oracle.com>
This commit is contained in:
Andy Lutomirski
James Morris
parent
bf53083445
commit
87b526d349
3 changed files with 138 additions and 61 deletions
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@ -95,12 +95,15 @@ SECCOMP_RET_KILL:
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SECCOMP_RET_TRAP:
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Results in the kernel sending a SIGSYS signal to the triggering
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task without executing the system call. The kernel will
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rollback the register state to just before the system call
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entry such that a signal handler in the task will be able to
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inspect the ucontext_t->uc_mcontext registers and emulate
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system call success or failure upon return from the signal
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handler.
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task without executing the system call. siginfo->si_call_addr
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will show the address of the system call instruction, and
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siginfo->si_syscall and siginfo->si_arch will indicate which
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syscall was attempted. The program counter will be as though
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the syscall happened (i.e. it will not point to the syscall
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instruction). The return value register will contain an arch-
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dependent value -- if resuming execution, set it to something
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sensible. (The architecture dependency is because replacing
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it with -ENOSYS could overwrite some useful information.)
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The SECCOMP_RET_DATA portion of the return value will be passed
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as si_errno.
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@ -123,6 +126,18 @@ SECCOMP_RET_TRACE:
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the BPF program return value will be available to the tracer
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via PTRACE_GETEVENTMSG.
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The tracer can skip the system call by changing the syscall number
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to -1. Alternatively, the tracer can change the system call
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requested by changing the system call to a valid syscall number. If
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the tracer asks to skip the system call, then the system call will
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appear to return the value that the tracer puts in the return value
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register.
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The seccomp check will not be run again after the tracer is
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notified. (This means that seccomp-based sandboxes MUST NOT
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allow use of ptrace, even of other sandboxed processes, without
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extreme care; ptracers can use this mechanism to escape.)
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SECCOMP_RET_ALLOW:
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Results in the system call being executed.
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@ -161,3 +176,50 @@ architecture supports both ptrace_event and seccomp, it will be able to
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support seccomp filter with minor fixup: SIGSYS support and seccomp return
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value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
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to its arch-specific Kconfig.
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Caveats
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-------
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The vDSO can cause some system calls to run entirely in userspace,
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leading to surprises when you run programs on different machines that
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fall back to real syscalls. To minimize these surprises on x86, make
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sure you test with
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/sys/devices/system/clocksource/clocksource0/current_clocksource set to
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something like acpi_pm.
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On x86-64, vsyscall emulation is enabled by default. (vsyscalls are
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legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccomp, with a few oddities:
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- A return value of SECCOMP_RET_TRAP will set a si_call_addr pointing to
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the vsyscall entry for the given call and not the address after the
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'syscall' instruction. Any code which wants to restart the call
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should be aware that (a) a ret instruction has been emulated and (b)
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trying to resume the syscall will again trigger the standard vsyscall
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emulation security checks, making resuming the syscall mostly
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pointless.
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- A return value of SECCOMP_RET_TRACE will signal the tracer as usual,
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but the syscall may not be changed to another system call using the
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orig_rax register. It may only be changed to -1 order to skip the
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currently emulated call. Any other change MAY terminate the process.
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The rip value seen by the tracer will be the syscall entry address;
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this is different from normal behavior. The tracer MUST NOT modify
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rip or rsp. (Do not rely on other changes terminating the process.
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They might work. For example, on some kernels, choosing a syscall
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that only exists in future kernels will be correctly emulated (by
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returning -ENOSYS).
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To detect this quirky behavior, check for addr & ~0x0C00 ==
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0xFFFFFFFFFF600000. (For SECCOMP_RET_TRACE, use rip. For
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SECCOMP_RET_TRAP, use siginfo->si_call_addr.) Do not check any other
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condition: future kernels may improve vsyscall emulation and current
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kernels in vsyscall=native mode will behave differently, but the
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instructions at 0xF...F600{0,4,8,C}00 will not be system calls in these
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cases.
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Note that modern systems are unlikely to use vsyscalls at all -- they
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are a legacy feature and they are considerably slower than standard
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syscalls. New code will use the vDSO, and vDSO-issued system calls
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are indistinguishable from normal system calls.
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@ -136,19 +136,6 @@ static int addr_to_vsyscall_nr(unsigned long addr)
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return nr;
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}
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#ifdef CONFIG_SECCOMP
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static int vsyscall_seccomp(struct task_struct *tsk, int syscall_nr)
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{
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if (!seccomp_mode(&tsk->seccomp))
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return 0;
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task_pt_regs(tsk)->orig_ax = syscall_nr;
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task_pt_regs(tsk)->ax = syscall_nr;
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return __secure_computing(syscall_nr);
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}
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#else
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#define vsyscall_seccomp(_tsk, _nr) 0
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#endif
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static bool write_ok_or_segv(unsigned long ptr, size_t size)
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{
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/*
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@ -181,10 +168,9 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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{
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struct task_struct *tsk;
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unsigned long caller;
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int vsyscall_nr;
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int vsyscall_nr, syscall_nr, tmp;
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int prev_sig_on_uaccess_error;
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long ret;
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int skip;
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/*
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* No point in checking CS -- the only way to get here is a user mode
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@ -216,6 +202,64 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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}
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tsk = current;
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/*
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* Check for access_ok violations and find the syscall nr.
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*
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* NULL is a valid user pointer (in the access_ok sense) on 32-bit and
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* 64-bit, so we don't need to special-case it here. For all the
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* vsyscalls, NULL means "don't write anything" not "write it at
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* address 0".
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*/
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switch (vsyscall_nr) {
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case 0:
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if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
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!write_ok_or_segv(regs->si, sizeof(struct timezone))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_gettimeofday;
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break;
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case 1:
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if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_time;
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break;
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case 2:
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if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
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!write_ok_or_segv(regs->si, sizeof(unsigned))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_getcpu;
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break;
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}
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/*
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* Handle seccomp. regs->ip must be the original value.
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* See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
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*
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* We could optimize the seccomp disabled case, but performance
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* here doesn't matter.
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*/
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regs->orig_ax = syscall_nr;
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regs->ax = -ENOSYS;
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tmp = secure_computing(syscall_nr);
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if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
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warn_bad_vsyscall(KERN_DEBUG, regs,
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"seccomp tried to change syscall nr or ip");
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do_exit(SIGSYS);
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}
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if (tmp)
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goto do_ret; /* skip requested */
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/*
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* With a real vsyscall, page faults cause SIGSEGV. We want to
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* preserve that behavior to make writing exploits harder.
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@ -223,49 +267,19 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
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current_thread_info()->sig_on_uaccess_error = 1;
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/*
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* NULL is a valid user pointer (in the access_ok sense) on 32-bit and
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* 64-bit, so we don't need to special-case it here. For all the
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* vsyscalls, NULL means "don't write anything" not "write it at
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* address 0".
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*/
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ret = -EFAULT;
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skip = 0;
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switch (vsyscall_nr) {
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case 0:
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skip = vsyscall_seccomp(tsk, __NR_gettimeofday);
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if (skip)
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break;
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if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
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!write_ok_or_segv(regs->si, sizeof(struct timezone)))
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break;
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ret = sys_gettimeofday(
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(struct timeval __user *)regs->di,
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(struct timezone __user *)regs->si);
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break;
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case 1:
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skip = vsyscall_seccomp(tsk, __NR_time);
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if (skip)
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break;
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if (!write_ok_or_segv(regs->di, sizeof(time_t)))
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break;
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ret = sys_time((time_t __user *)regs->di);
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break;
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case 2:
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skip = vsyscall_seccomp(tsk, __NR_getcpu);
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if (skip)
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break;
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if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
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!write_ok_or_segv(regs->si, sizeof(unsigned)))
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break;
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ret = sys_getcpu((unsigned __user *)regs->di,
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(unsigned __user *)regs->si,
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NULL);
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@ -274,12 +288,7 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;
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if (skip) {
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if ((long)regs->ax <= 0L) /* seccomp errno emulation */
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goto do_ret;
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goto done; /* seccomp trace/trap */
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}
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check_fault:
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if (ret == -EFAULT) {
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/* Bad news -- userspace fed a bad pointer to a vsyscall. */
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warn_bad_vsyscall(KERN_INFO, regs,
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@ -302,7 +311,6 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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/* Emulate a ret instruction. */
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regs->ip = caller;
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regs->sp += 8;
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done:
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return true;
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sigsegv:
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@ -396,25 +396,29 @@ int __secure_computing(int this_syscall)
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#ifdef CONFIG_SECCOMP_FILTER
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case SECCOMP_MODE_FILTER: {
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int data;
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struct pt_regs *regs = task_pt_regs(current);
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ret = seccomp_run_filters(this_syscall);
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data = ret & SECCOMP_RET_DATA;
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ret &= SECCOMP_RET_ACTION;
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switch (ret) {
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case SECCOMP_RET_ERRNO:
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/* Set the low-order 16-bits as a errno. */
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syscall_set_return_value(current, task_pt_regs(current),
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syscall_set_return_value(current, regs,
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-data, 0);
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goto skip;
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case SECCOMP_RET_TRAP:
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/* Show the handler the original registers. */
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syscall_rollback(current, task_pt_regs(current));
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syscall_rollback(current, regs);
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/* Let the filter pass back 16 bits of data. */
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seccomp_send_sigsys(this_syscall, data);
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goto skip;
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case SECCOMP_RET_TRACE:
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/* Skip these calls if there is no tracer. */
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if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP))
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if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
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syscall_set_return_value(current, regs,
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-ENOSYS, 0);
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goto skip;
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}
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/* Allow the BPF to provide the event message */
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ptrace_event(PTRACE_EVENT_SECCOMP, data);
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/*
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@ -425,6 +429,9 @@ int __secure_computing(int this_syscall)
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*/
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if (fatal_signal_pending(current))
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break;
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if (syscall_get_nr(current, regs) < 0)
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goto skip; /* Explicit request to skip. */
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return 0;
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case SECCOMP_RET_ALLOW:
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return 0;
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