This is a note to let you know that I've just added the patch titled
do_exit(): make sure that we run with get_fs() == USER_DS
to the 2.6.36-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
do_exit-make-sure-that-we-run-with-get_fs-user_ds.patch
and it can be found in the queue-2.6.36 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177 Mon Sep 17 00:00:00 2001
From: Nelson Elhage <[email protected]>
Date: Thu, 2 Dec 2010 14:31:21 -0800
Subject: do_exit(): make sure that we run with get_fs() == USER_DS
From: Nelson Elhage <[email protected]>
commit 33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177 upstream.
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.
[[email protected]: update code comment]
Signed-off-by: Nelson Elhage <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
kernel/exit.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -903,6 +903,15 @@ 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);
Patches currently in stable-queue which might be from [email protected] are
queue-2.6.36/do_exit-make-sure-that-we-run-with-get_fs-user_ds.patch
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