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authorAndy Lutomirski <luto@kernel.org>2016-11-20 05:42:40 +0300
committerIngo Molnar <mingo@kernel.org>2016-11-21 10:06:54 +0300
commitfc0e81b2bea0ebceb71889b61d2240856141c9ee (patch)
tree3f4f58c25eb8d5ae8d8e3f6578e93164dd2ca1c2
parent91e08ab0c8515450258d7ad9033bfe69bebad25a (diff)
downloadlinux-fc0e81b2bea0ebceb71889b61d2240856141c9ee.tar.xz
x86/traps: Ignore high word of regs->cs in early_fixup_exception()
On the 80486 DX, it seems that some exceptions may leave garbage in the high bits of CS. This causes sporadic failures in which early_fixup_exception() refuses to fix up an exception. As far as I can tell, this has been buggy for a long time, but the problem seems to have been exacerbated by commits: 1e02ce4cccdc ("x86: Store a per-cpu shadow copy of CR4") e1bfc11c5a6f ("x86/init: Fix cr4_init_shadow() on CR4-less machines") This appears to have broken for as long as we've had early exception handling. [ Note to stable maintainers: This patch is needed all the way back to 3.4, but it will only apply to 4.6 and up, as it depends on commit: 0e861fbb5bda ("x86/head: Move early exception panic code into early_fixup_exception()") If you want to backport to kernels before 4.6, please don't backport the prerequisites (there was a big chain of them that rewrote a lot of the early exception machinery); instead, ask me and I can send you a one-liner that will apply. ] Reported-by: Matthew Whitehead <tedheadster@gmail.com> Signed-off-by: Andy Lutomirski <luto@kernel.org> 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: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org Fixes: 4c5023a3fa2e ("x86-32: Handle exception table entries during early boot") Link: http://lkml.kernel.org/r/cb32c69920e58a1a58e7b5cad975038a69c0ce7d.1479609510.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--arch/x86/mm/extable.c7
1 files changed, 6 insertions, 1 deletions
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 79ae939970d3..fcd06f7526de 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -135,7 +135,12 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
if (early_recursion_flag > 2)
goto halt_loop;
- if (regs->cs != __KERNEL_CS)
+ /*
+ * Old CPUs leave the high bits of CS on the stack
+ * undefined. I'm not sure which CPUs do this, but at least
+ * the 486 DX works this way.
+ */
+ if ((regs->cs & 0xFFFF) != __KERNEL_CS)
goto fail;
/*