From 065add3941bdca54fe04ed3471a96bce9af88793 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 26 May 2010 14:42:54 -0700 Subject: signals: check_kill_permission(): don't check creds if same_thread_group() Andrew Tridgell reports that aio_read(SIGEV_SIGNAL) can fail if the notification from the helper thread races with setresuid(), see http://samba.org/~tridge/junkcode/aio_uid.c 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() twice. 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 Signed-off-by: Oleg Nesterov Acked-by: Roland McGrath Acked-by: David Howells Cc: Eric Paris Cc: Jakub Jelinek Cc: James Morris Cc: Roland McGrath Cc: Stephen Smalley Cc: [all kernel versions] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/signal.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'kernel/signal.c') diff --git a/kernel/signal.c b/kernel/signal.c index 825a3f24ad76..2ee9573ed11f 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -642,7 +642,7 @@ static inline bool si_fromuser(const struct siginfo *info) 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, struct siginfo *info, 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) && -- cgit v1.2.1