linux-kernel - [050/117] x86-64: Clear a 64-bit FS/GS base on fork if selector is nonzero

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Date:	Mon, 10 May 2010 15:32:21 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [050/117] x86-64: Clear a 64-bit FS/GS base on fork if selector is nonzero

2.6.33-stable review patch.  If anyone has any objections, please let us know.

------------------

From: H. Peter Anvin <hpa@...or.com>

commit 7ce5a2b9bb2e92902230e3121d8c3047fab9cb47 upstream.

When we do a thread switch, we clear the outgoing FS/GS base if the
corresponding selector is nonzero.  This is taken by __switch_to() as
an entry invariant; it does not verify that it is true on entry.
However, copy_thread() doesn't enforce this constraint, which can
result in inconsistent results after fork().

Make copy_thread() match the behavior of __switch_to().

Reported-and-tested-by: Samuel Thibault <samuel.thibault@...ia.fr>
Signed-off-by: H. Peter Anvin <hpa@...or.com>
LKML-Reference: <4BD1E061.8030605@...or.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 arch/x86/kernel/process_64.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -282,12 +282,12 @@ int copy_thread(unsigned long clone_flag

 	set_tsk_thread_flag(p, TIF_FORK);

-	p->thread.fs = me->thread.fs;
-	p->thread.gs = me->thread.gs;
 	p->thread.io_bitmap_ptr = NULL;

 	savesegment(gs, p->thread.gsindex);
+	p->thread.gs = p->thread.gsindex ? 0 : me->thread.gs;
 	savesegment(fs, p->thread.fsindex);
+	p->thread.fs = p->thread.fsindex ? 0 : me->thread.fs;
 	savesegment(es, p->thread.es);
 	savesegment(ds, p->thread.ds);

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