From: Nelson Elhage <nelhage@ksplice.com>
BugLink: http://bugs.launchpad.net/bugs/723945
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>
Signed-off-by: Brad Figg <brad.figg@canonical.com>
---
kernel/exit.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c
index 3d78abb..ffcc469 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -809,6 +809,15 @@ fastcall NORET_TYPE void do_exit(long code)
}
/*
+ * 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);
+
+ /*
* We're taking recursive faults here in do_exit. Safest is to just
* leave this task alone and wait for reboot.
*/
--
1.7.0.4
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do_exit(): make sure that we run with get_fs() == USER_DS
