From: Nelson Elhage Date: Thu, 2 Dec 2010 22:31:21 +0000 (-0800) Subject: do_exit(): make sure that we run with get_fs() == USER_DS X-Git-Tag: v2.6.35.10~51 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=fa21da1870e6626274c934722801914690b2dd55;p=karo-tx-linux.git do_exit(): make sure that we run with get_fs() == USER_DS 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. [akpm@linux-foundation.org: update code comment] Signed-off-by: Nelson Elhage Cc: KOSAKI Motohiro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Andi Kleen --- diff --git a/kernel/exit.c b/kernel/exit.c index ac90425ce996..d1e6789064cf 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -900,6 +900,15 @@ 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);