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

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@inria.fr>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <4BD1E061.8030605@zytor.com>
Cc: <stable@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index dc9690b4c4cc..17cb3295cbf7 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -276,12 +276,12 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
         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);