x86/segments/64: When load_gs_index fails, clear the base

On AMD CPUs, a failed load_gs_base currently may not clear the FS
base.  Fix it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1a6c4d3a8a4e7be79ba448b42685e0321d50c14c.1461698311.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1693c17..6344629 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -794,6 +794,12 @@ END(native_load_gs_index)
        /* running with kernelgs */
 bad_gs:
        SWAPGS                                  /* switch back to user gs */
+.macro ZAP_GS
+       /* This can't be a string because the preprocessor needs to see it. */
+       movl $__USER_DS, %eax
+       movl %eax, %gs
+.endm
+       ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
        xorl    %eax, %eax
        movl    %eax, %gs
        jmp     2b