Commit 33dd94ae authored Dec 02, 2010 by Nelson Elhage Committed by Linus Torvalds Dec 02, 2010

do_exit(): make sure that we run with get_fs() == USER_DS



If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not
otherwise reset before do_exit().  do_exit may later (via mm_release in
fork.c) do a put_user to a user-controlled address, potentially allowing
a user to leverage an oops into a controlled write into kernel memory.

This is only triggerable in the presence of another bug, but this
potentially turns a lot of DoS bugs into privilege escalations, so it's
worth fixing.  I have proof-of-concept code which uses this bug along
with CVE-2010-3849 to write a zero to an arbitrary kernel address, so
I've tested that this is not theoretical.

A more logical place to put this fix might be when we know an oops has
occurred, before we call do_exit(), but that would involve changing
every architecture, in multiple places.

Let's just stick it in do_exit instead.

[akpm@linux-foundation.org: update code comment]
Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent a0b0f58c

kernel/exit.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -914,6 +914,15 @@ NORET_TYPE void do_exit(long code)
		if (unlikely(!tsk->pid))
		panic("Attempted to kill the idle task!");

		/*
		* If do_exit is called because this processes oopsed, it's possible
		* that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
		* continuing. Amongst other possible reasons, this is to prevent
		* mm_release()->clear_child_tid() from writing to a user-controlled
		* kernel address.
		*/
		set_fs(USER_DS);

		tracehook_report_exit(&code);

		validate_creds_for_do_exit(tsk);

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