54 lines
2.1 KiB
Diff
54 lines
2.1 KiB
Diff
From: Nelson Elhage <nelhage@ksplice.com>
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Date: Thu, 2 Dec 2010 22:31:21 +0000 (-0800)
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Subject: do_exit(): make sure that we run with get_fs() == USER_DS
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X-Git-Tag: v2.6.37-rc5~17
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X-Git-Url: http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177
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do_exit(): make sure that we run with get_fs() == USER_DS
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If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not
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otherwise reset before do_exit(). do_exit may later (via mm_release in
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fork.c) do a put_user to a user-controlled address, potentially allowing
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a user to leverage an oops into a controlled write into kernel memory.
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This is only triggerable in the presence of another bug, but this
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potentially turns a lot of DoS bugs into privilege escalations, so it's
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worth fixing. I have proof-of-concept code which uses this bug along
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with CVE-2010-3849 to write a zero to an arbitrary kernel address, so
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I've tested that this is not theoretical.
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A more logical place to put this fix might be when we know an oops has
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occurred, before we call do_exit(), but that would involve changing
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every architecture, in multiple places.
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Let's just stick it in do_exit instead.
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[akpm@linux-foundation.org: update code comment]
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Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
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Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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Cc: <stable@kernel.org>
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Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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---
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diff --git a/kernel/exit.c b/kernel/exit.c
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index 21aa7b3..676149a 100644
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--- a/kernel/exit.c
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+++ b/kernel/exit.c
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@@ -914,6 +914,15 @@ NORET_TYPE void do_exit(long code)
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if (unlikely(!tsk->pid))
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panic("Attempted to kill the idle task!");
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+ /*
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+ * If do_exit is called because this processes oopsed, it's possible
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+ * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
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+ * continuing. Amongst other possible reasons, this is to prevent
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+ * mm_release()->clear_child_tid() from writing to a user-controlled
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+ * kernel address.
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+ */
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+ set_fs(USER_DS);
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+
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tracehook_report_exit(&code);
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validate_creds_for_do_exit(tsk);
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