linux-kernel - [122/200] signals: check_kill_permission(): dont check creds if same_thread

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Date:	Thu, 01 Jul 2010 10:43:32 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...isplace.org>,
	Jakub Jelinek <jakub@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Stephen Smalley <sds@...ho.nsa.gov>
Subject: [122/200] signals: check_kill_permission(): dont check creds if same_thread_group()

2.6.34-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@...hat.com>

commit 065add3941bdca54fe04ed3471a96bce9af88793 upstream.

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 <tridge@...ba.org>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Acked-by: Roland McGrath <roland@...hat.com>
Acked-by: David Howells <dhowells@...hat.com>
Cc: Eric Paris <eparis@...isplace.org>
Cc: Jakub Jelinek <jakub@...hat.com>
Cc: James Morris <jmorris@...ei.org>
Cc: Roland McGrath <roland@...hat.com>
Cc: Stephen Smalley <sds@...ho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

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

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -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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