Patchwork [Karmic,CVE-2010-4258,1/1] do_exit(): make sure that we run with get_fs() == USER_DS

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Submitter Brad Figg
Date Feb. 28, 2011, 6:40 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/84859/
State Accepted
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Brad Figg - Feb. 28, 2011, 6:40 p.m.
From: Nelson Elhage <>


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.

[ update code comment]
Signed-off-by: Nelson Elhage <>
Cc: KOSAKI Motohiro <>
Cc: <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Brad Figg <>
 kernel/exit.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)


diff --git a/kernel/exit.c b/kernel/exit.c
index b64937a..69f4445 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -907,6 +907,15 @@  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);