[PATCH v4 10/17] x86/entry/64: Really create an error-entry-from-usermode code path

Andy Lutomirski
2015-06-29T19:35:01Z

In 539f51136500 ("x86/asm/entry/64: Disentangle error_entry/exit
gsbase/ebx/usermode code"), I arranged the code slightly wrong --
IRET faults would skip the code path that was intended to execute on
all error entries from user mode. Fix it up.

This does not fix a bug, but we'll need it, and it slightly shrinks
the code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/entry_64.S | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 141a5d49dddc..cd9cbc62159c 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1145,9 +1145,14 @@ ENTRY(error_entry)
testb $3, CS+8(%rsp)
jz error_kernelspace

- /* We entered from user mode */
+error_entry_from_usermode_swapgs:
+ /*
+ * We entered from user mode or we're pretending to have entered
+ * from user mode due to an IRET fault.
+ */
SWAPGS

+error_entry_from_usermode_after_swapgs:
error_entry_done:
TRACE_IRQS_OFF
ret
@@ -1174,8 +1179,7 @@ error_kernelspace:
* gsbase and proceed. We'll fix up the exception and land in
* gs_change's error handler with kernel gsbase.
*/
- SWAPGS
- jmp error_entry_done
+ jmp error_entry_from_usermode_swapgs

bstep_iret:
/* Fix truncated RIP */
@@ -1198,7 +1202,7 @@ error_bad_iret:
call fixup_bad_iret
mov %rax, %rsp
decl %ebx
- jmp error_entry_done
+ jmp error_entry_from_usermode_after_swapgs
END(error_entry)

--
2.4.3