tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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do_exit(): make sure that we run with get_fs() == USER_DS

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>

authored by

Nelson Elhage and committed by
Linus Torvalds 33dd94ae a0b0f58c

+9
+9
kernel/exit.c
··· 914 914 if (unlikely(!tsk->pid)) 915 915 panic("Attempted to kill the idle task!"); 916 916 917 + /* 918 + * If do_exit is called because this processes oopsed, it's possible 919 + * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before 920 + * continuing. Amongst other possible reasons, this is to prevent 921 + * mm_release()->clear_child_tid() from writing to a user-controlled 922 + * kernel address. 923 + */ 924 + set_fs(USER_DS); 925 + 917 926 tracehook_report_exit(&code); 918 927 919 928 validate_creds_for_do_exit(tsk);