From: Greg KH on
2.6.33-stable review patch. If anyone has any objections, please let me know.


From: Oleg Nesterov <oleg(a)>

commit 065add3941bdca54fe04ed3471a96bce9af88793 upstream.

Andrew Tridgell reports that aio_read(SIGEV_SIGNAL) can fail if the
notification from the helper thread races with setresuid(), see

This happens because check_kill_permission() doesn't permit sending a
signal to the task with the different cred->xids. But there is not any
security reason to check ->cred's when the task sends a signal (private or
group-wide) to its sub-thread. Whatever we do, any thread can bypass all
security checks and send SIGKILL to all threads, or it can block a signal
SIG and do kill(gettid(), SIG) to deliver this signal to another
sub-thread. Not to mention that CLONE_THREAD implies CLONE_VM.

Change check_kill_permission() to avoid the credentials check when the
sender and the target are from the same thread group.

Also, move "cred = current_cred()" down to avoid calling get_current()

Note: David Howells pointed out we could relax this even more, the
CLONE_SIGHAND (without CLONE_THREAD) case probably does not need
these checks too.

Roland said:
: The glibc (libpthread) that does set*id across threads has
: been in use for a while (2.3.4?), probably in distro's using kernels as old
: or older than any active -stable streams. In the race in question, this
: kernel bug is breaking valid POSIX application expectations.

Reported-by: Andrew Tridgell <tridge(a)>
Signed-off-by: Oleg Nesterov <oleg(a)>
Acked-by: Roland McGrath <roland(a)>
Acked-by: David Howells <dhowells(a)>
Cc: Eric Paris <eparis(a)>
Cc: Jakub Jelinek <jakub(a)>
Cc: James Morris <jmorris(a)>
Cc: Roland McGrath <roland(a)>
Cc: Stephen Smalley <sds(a)>
Signed-off-by: Andrew Morton <akpm(a)>
Signed-off-by: Linus Torvalds <torvalds(a)>
Signed-off-by: Greg Kroah-Hartman <gregkh(a)>

kernel/signal.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -625,7 +625,7 @@ static inline bool si_fromuser(const str
static int check_kill_permission(int sig, struct siginfo *info,
struct task_struct *t)
- const struct cred *cred = current_cred(), *tcred;
+ const struct cred *cred, *tcred;
struct pid *sid;
int error;

@@ -639,8 +639,10 @@ static int check_kill_permission(int sig
if (error)
return error;

+ cred = current_cred();
tcred = __task_cred(t);
- if ((cred->euid ^ tcred->suid) &&
+ if (!same_thread_group(current, t) &&
+ (cred->euid ^ tcred->suid) &&
(cred->euid ^ tcred->uid) &&
(cred->uid ^ tcred->suid) &&
(cred->uid ^ tcred->uid) &&

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