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

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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>
This commit is contained in:
Nelson Elhage 2010-12-02 14:31:21 -08:00 committed by Linus Torvalds
parent a0b0f58cdd
commit 33dd94ae1c
1 changed files with 9 additions and 0 deletions
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9
kernel/exit.c
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@ -914,6 +914,15 @@ NORET_TYPE void do_exit(long code)
if (unlikely(!tsk->pid)) if (unlikely(!tsk->pid))
panic("Attempted to kill the idle task!"); 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); tracehook_report_exit(&code);
validate_creds_for_do_exit(tsk); validate_creds_for_do_exit(tsk);