[44/98] x86-64: Clear a 64-bit FS/GS base on fork if selector is nonzero

[Greg KH]

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

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

From: H. Peter Anvin <hpa@xxxxxxxxx>

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@xxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
LKML-Reference: <4BD1E061.8030605@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 arch/x86/kernel/process_64.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -295,11 +295,10 @@ 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;
-
 	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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