[PATCH] signals: check_kill_permission: don't check creds ifsame_thread_group()

From: Oleg Nesterov
Date: Mon May 17 2010 - 15:57:15 EST

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

This happens because check_kill_permission() doesn't allow to send
a signal to the task with the different cred->xids. But there is no
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

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.

Reported-by: Andrew Tridgell <tridge@xxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

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

--- 34-rc1/kernel/signal.c~CKP_CK_STG 2010-05-09 21:09:31.000000000 +0200
+++ 34-rc1/kernel/signal.c 2010-05-17 17:02:09.000000000 +0200
@@ -642,7 +642,7 @@ 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;

@@ -656,8 +656,10 @@ 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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