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Subject[PATCH 3.16 122/157] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code
3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@kernel.org>

commit 539f5113650068ba221197f190267ab727296ef5 upstream.

The error_entry/error_exit code to handle gsbase and whether we
return to user mdoe was a mess:

- error_sti was misnamed. In particular, it did not enable interrupts.

- Error handling for gs_change was hopelessly tangled the normal
usermode path. Separate it out. This saves a branch in normal
entries from kernel mode.

- The comments were bad.

Fix it up. As a nice side effect, there's now a code path that
happens on error entries from user mode. We'll use it soon.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.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/f1be898ab93360169fb845ab85185948832209ee.1433878454.git.luto@kernel.org
[ Prettified it, clarified comments some more. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[bwh: Backported to 3.16 as dependency of commit 18ec54fdd6d1
"x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations":
- We do not use %ebx as a flag since we already have a backport of commit
b3681dd548d0 "x86/entry/64: Remove %ebx handling from error_entry/exit",
so don't add the comments about that
- Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1446,9 +1446,11 @@ ENTRY(error_entry)
SWITCH_KERNEL_CR3
testl $3,CS+8(%rsp)
je error_kernelspace
-error_swapgs:
+
+ /* We entered from user mode */
SWAPGS
-error_sti:
+
+error_entry_done:
TRACE_IRQS_OFF
ret

@@ -1466,8 +1468,15 @@ error_kernelspace:
cmpq %rax,RIP+8(%rsp)
je bstep_iret
cmpq $gs_change,RIP+8(%rsp)
- je error_swapgs
- jmp error_sti
+ jne error_entry_done
+
+ /*
+ * hack: gs_change can fail with user gsbase. If this happens, fix up
+ * 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

bstep_iret:
/* Fix truncated RIP */
@@ -1475,11 +1484,20 @@ bstep_iret:
/* fall through */

error_bad_iret:
+ /*
+ * We came from an IRET to user mode, so we have user gsbase.
+ * Switch to kernel gsbase:
+ */
SWAPGS
+
+ /*
+ * Pretend that the exception came from user mode: set up pt_regs
+ * as if we faulted immediately after IRET.
+ */
mov %rsp,%rdi
call fixup_bad_iret
mov %rax,%rsp
- jmp error_sti
+ jmp error_entry_done
CFI_ENDPROC
END(error_entry)

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