[PATCH 4/8] x86/segments/64: When load_gs_index fails, clear the base

[Andy Lutomirski]

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

Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/entry/entry_64.S | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1693c17dbf81..6344629ae1ce 100644
--- 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
-- 
2.5.5