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ptrace: use fsuid, fsgid, effective creds for fs access checks

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By checking the effective credentials instead of the real UID / permitted
capabilities, ensure that the calling process actually intended to use its
credentials.

To ensure that all ptrace checks use the correct caller credentials (e.g.
in case out-of-tree code or newly added code omits the PTRACE_MODE_*CREDS
flag), use two new flags and require one of them to be set.

The problem was that when a privileged task had temporarily dropped its
privileges, e.g.  by calling setreuid(0, user_uid), with the intent to
perform following syscalls with the credentials of a user, it still passed
ptrace access checks that the user would not be able to pass.

While an attacker should not be able to convince the privileged task to
perform a ptrace() syscall, this is a problem because the ptrace access
check is reused for things in procfs.

In particular, the following somewhat interesting procfs entries only rely
on ptrace access checks:

 /proc/$pid/stat - uses the check for determining whether pointers
     should be visible, useful for bypassing ASLR
 /proc/$pid/maps - also useful for bypassing ASLR
 /proc/$pid/cwd - useful for gaining access to restricted
     directories that contain files with lax permissions, e.g. in
     this scenario:
     lrwxrwxrwx root root /proc/13020/cwd -> /root/foobar
     drwx------ root root /root
     drwxr-xr-x root root /root/foobar
     -rw-r--r-- root root /root/foobar/secret

Therefore, on a system where a root-owned mode 6755 binary changes its
effective credentials as described and then dumps a user-specified file,
this could be used by an attacker to reveal the memory layout of root's
processes or reveal the contents of files he is not allowed to access
(through /proc/$pid/cwd).

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jann Horn 2016-01-20 15:00:04 -08:00 committed by Linus Torvalds
parent 3dfb7d8cdb
commit caaee6234d
11 changed files with 80 additions and 29 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
fs/proc/array.c
View file

@ -395,7 +395,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
state = *get_task_state(task);
vsize = eip = esp = 0;
permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS | PTRACE_MODE_NOAUDIT);
mm = get_task_mm(task);
if (mm) {
vsize = task_vsize(mm);

21
fs/proc/base.c
View file

@ -403,7 +403,7 @@ static const struct file_operations proc_pid_cmdline_ops = {
static int proc_pid_auxv(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *task)
{
struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ);
struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
if (mm && !IS_ERR(mm)) {
unsigned int nwords = 0;
do {
@ -430,7 +430,8 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
wchan = get_wchan(task);
if (wchan && ptrace_may_access(task, PTRACE_MODE_READ) && !lookup_symbol_name(wchan, symname))
if (wchan && ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
&& !lookup_symbol_name(wchan, symname))
seq_printf(m, "%s", symname);
else
seq_putc(m, '0');
@ -444,7 +445,7 @@ static int lock_trace(struct task_struct *task)
int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
if (err)
return err;
if (!ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_FSCREDS)) {
mutex_unlock(&task->signal->cred_guard_mutex);
return -EPERM;
}
@ -697,7 +698,7 @@ static int proc_fd_access_allowed(struct inode *inode)
*/
task = get_proc_task(inode);
if (task) {
allowed = ptrace_may_access(task, PTRACE_MODE_READ);
allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
put_task_struct(task);
}
return allowed;
@ -732,7 +733,7 @@ static bool has_pid_permissions(struct pid_namespace *pid,
return true;
if (in_group_p(pid->pid_gid))
return true;
return ptrace_may_access(task, PTRACE_MODE_READ);
return ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
}
@ -809,7 +810,7 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
struct mm_struct *mm = ERR_PTR(-ESRCH);
if (task) {
mm = mm_access(task, mode);
mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
put_task_struct(task);
if (!IS_ERR_OR_NULL(mm)) {
@ -1860,7 +1861,7 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
if (!task)
goto out_notask;
mm = mm_access(task, PTRACE_MODE_READ);
mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
if (IS_ERR_OR_NULL(mm))
goto out;
@ -2013,7 +2014,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
goto out;
result = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out_put_task;
result = -ENOENT;
@ -2066,7 +2067,7 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
goto out;
ret = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out_put_task;
ret = 0;
@ -2533,7 +2534,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
if (result)
return result;
if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
result = -EACCES;
goto out_unlock;
}

4
fs/proc/namespaces.c
View file

@ -46,7 +46,7 @@ static const char *proc_ns_get_link(struct dentry *dentry,
if (!task)
return error;
if (ptrace_may_access(task, PTRACE_MODE_READ)) {
if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
error = ns_get_path(&ns_path, task, ns_ops);
if (!error)
nd_jump_link(&ns_path);
@ -67,7 +67,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
if (!task)
return res;
if (ptrace_may_access(task, PTRACE_MODE_READ)) {
if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
res = ns_get_name(name, sizeof(name), task, ns_ops);
if (res >= 0)
res = readlink_copy(buffer, buflen, name);

24
include/linux/ptrace.h
View file

@ -57,7 +57,29 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
#define PTRACE_MODE_READ 0x01
#define PTRACE_MODE_ATTACH 0x02
#define PTRACE_MODE_NOAUDIT 0x04
/* Returns true on success, false on denial. */
#define PTRACE_MODE_FSCREDS 0x08
#define PTRACE_MODE_REALCREDS 0x10
/* shorthands for READ/ATTACH and FSCREDS/REALCREDS combinations */
#define PTRACE_MODE_READ_FSCREDS (PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_READ_REALCREDS (PTRACE_MODE_READ | PTRACE_MODE_REALCREDS)
#define PTRACE_MODE_ATTACH_FSCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_ATTACH_REALCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS)
/**
* ptrace_may_access - check whether the caller is permitted to access
* a target task.
* @task: target task
* @mode: selects type of access and caller credentials
*
* Returns true on success, false on denial.
*
* One of the flags PTRACE_MODE_FSCREDS and PTRACE_MODE_REALCREDS must
* be set in @mode to specify whether the access was requested through
* a filesystem syscall (should use effective capabilities and fsuid
* of the caller) or through an explicit syscall such as
* process_vm_writev or ptrace (and should use the real credentials).
*/
extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
static inline int ptrace_reparented(struct task_struct *child)

2
kernel/events/core.c
View file

@ -3376,7 +3376,7 @@ find_lively_task_by_vpid(pid_t vpid)
/* Reuse ptrace permission checks for now. */
err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
goto errout;
return task;

2
kernel/futex.c
View file

@ -2884,7 +2884,7 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
}
ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ))
if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock;
head = p->robust_list;

2
kernel/futex_compat.c
View file

@ -155,7 +155,7 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
}
ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ))
if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock;
head = p->compat_robust_list;

4
kernel/kcmp.c
View file

@ -122,8 +122,8 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
&task2->signal->cred_guard_mutex);
if (ret)
goto err;
if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
!ptrace_may_access(task2, PTRACE_MODE_READ)) {
if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
!ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
ret = -EPERM;
goto err_unlock;
}

39
kernel/ptrace.c
View file

@ -219,6 +219,14 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
{
const struct cred *cred = current_cred(), *tcred;
int dumpable = 0;
kuid_t caller_uid;
kgid_t caller_gid;
if (!(mode & PTRACE_MODE_FSCREDS) == !(mode & PTRACE_MODE_REALCREDS)) {
WARN(1, "denying ptrace access check without PTRACE_MODE_*CREDS\n");
return -EPERM;
}
/* May we inspect the given task?
* This check is used both for attaching with ptrace
@ -228,18 +236,33 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
* because setting up the necessary parent/child relationship
* or halting the specified task is impossible.
*/
int dumpable = 0;
/* Don't let security modules deny introspection */
if (same_thread_group(task, current))
return 0;
rcu_read_lock();
if (mode & PTRACE_MODE_FSCREDS) {
caller_uid = cred->fsuid;
caller_gid = cred->fsgid;
} else {
/*
* Using the euid would make more sense here, but something
* in userland might rely on the old behavior, and this
* shouldn't be a security problem since
* PTRACE_MODE_REALCREDS implies that the caller explicitly
* used a syscall that requests access to another process
* (and not a filesystem syscall to procfs).
*/
caller_uid = cred->uid;
caller_gid = cred->gid;
}
tcred = __task_cred(task);
if (uid_eq(cred->uid, tcred->euid) &&
uid_eq(cred->uid, tcred->suid) &&
uid_eq(cred->uid, tcred->uid) &&
gid_eq(cred->gid, tcred->egid) &&
gid_eq(cred->gid, tcred->sgid) &&
gid_eq(cred->gid, tcred->gid))
if (uid_eq(caller_uid, tcred->euid) &&
uid_eq(caller_uid, tcred->suid) &&
uid_eq(caller_uid, tcred->uid) &&
gid_eq(caller_gid, tcred->egid) &&
gid_eq(caller_gid, tcred->sgid) &&
gid_eq(caller_gid, tcred->gid))
goto ok;
if (ptrace_has_cap(tcred->user_ns, mode))
goto ok;
@ -306,7 +329,7 @@ static int ptrace_attach(struct task_struct *task, long request,
goto out;
task_lock(task);
retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
task_unlock(task);
if (retval)
goto unlock_creds;

2
mm/process_vm_access.c
View file

@ -194,7 +194,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter,
goto free_proc_pages;
}
mm = mm_access(task, PTRACE_MODE_ATTACH);
mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS);
if (!mm || IS_ERR(mm)) {
rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
/*

7
security/commoncap.c
View file

@ -137,12 +137,17 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
{
int ret = 0;
const struct cred *cred, *child_cred;
const kernel_cap_t *caller_caps;
rcu_read_lock();
cred = current_cred();
child_cred = __task_cred(child);
if (mode & PTRACE_MODE_FSCREDS)
caller_caps = &cred->cap_effective;
else
caller_caps = &cred->cap_permitted;
if (cred->user_ns == child_cred->user_ns &&
cap_issubset(child_cred->cap_permitted, cred->cap_permitted))
cap_issubset(child_cred->cap_permitted, *caller_caps))
goto out;
if (ns_capable(child_cred->user_ns, CAP_SYS_PTRACE))
goto out;