x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code

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>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 6e5feb342ee412e839748ed63aaee7fc869b79f4..ddee7b2abe25b02c9e45173f1905bfc81ad3c5dc 100644 (file)
--- 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)