The patch titled
do_exit(): make sure that we run with get_fs() == USER_DS
has been removed from the -mm tree. Its filename was
do_exit-make-sure-we-run-with-get_fs-==-user_ds.patch
This patch was dropped because it was merged into mainline or a subsystem tree
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: do_exit(): make sure that we run with get_fs() == USER_DS
From: Nelson Elhage <nelhage@xxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxx: update code comment]
Signed-off-by: Nelson Elhage <nelhage@xxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/exit.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff -puN kernel/exit.c~do_exit-make-sure-we-run-with-get_fs-==-user_ds kernel/exit.c
--- a/kernel/exit.c~do_exit-make-sure-we-run-with-get_fs-==-user_ds
+++ a/kernel/exit.c
@@ -914,6 +914,15 @@ 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);
_
Patches currently in -mm which might be from nelhage@xxxxxxxxxxx are
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