linux-kernel - [PATCH v2] do_exit(): Make sure we run with get_fs() == USER

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Date:	Tue, 30 Nov 2010 21:27:36 -0500
From:	Nelson Elhage <nelhage@...lice.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Nelson Elhage <nelhage@...lice.com>
Subject: [PATCH v2] do_exit(): Make sure 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.

A more logical place to put this 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.

Signed-off-by: Nelson Elhage <nelhage@...lice.com>
---
 kernel/exit.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 21aa7b3..68899b3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -914,6 +914,14 @@ 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. This is relevant at least for clearing clear_child_tid in
+	 * mm_release.
+	 */
+	set_fs(USER_DS);
+
 	tracehook_report_exit(&code);

 	validate_creds_for_do_exit(tsk);
-- 
1.7.1.31.g6297e

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