Commit-ID: fc0e81b2bea0ebceb71889b61d2240856141c9ee
Gitweb: http://git.kernel.org/tip/fc0e81b2bea0ebceb71889b61d2240856141c9ee
Author: Andy Lutomirski <[email protected]>
AuthorDate: Sat, 19 Nov 2016 18:42:40 -0800
Committer: Ingo Molnar <[email protected]>
CommitDate: Mon, 21 Nov 2016 08:06:54 +0100
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 <[email protected]>
Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Fixes: 4c5023a3fa2e ("x86-32: Handle exception table entries during early boot")
Link:
http://lkml.kernel.org/r/cb32c69920e58a1a58e7b5cad975038a69c0ce7d.1479609510.git.l...@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>
---
arch/x86/mm/extable.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 79ae939..fcd06f7 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;
/*
