1 files changed, 31 insertions, 8 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index bd97161f90cb..3bb2c4302df1 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1135,7 +1135,7 @@ END(paranoid_exit)
 
 /*
  * Save all registers in pt_regs, and switch gs if needed.
- * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
+ * Return: EBX=0: came from user mode; EBX=1: otherwise
  */
 ENTRY(error_entry)
         cld
@@ -1144,9 +1144,11 @@ ENTRY(error_entry)
         xorl    %ebx, %ebx
         testb   $3, CS+8(%rsp)
         jz      error_kernelspace
-error_swapgs:
+
+        /* We entered from user mode */
         SWAPGS
-error_sti:
+
+error_entry_done:
         TRACE_IRQS_OFF
         ret
 
@@ -1165,8 +1167,15 @@ 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 */
@@ -1174,16 +1183,30 @@ 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 and clear EBX so that
+         * error_exit knows that we will be returning to user mode.
+         */
         mov     %rsp, %rdi
         call    fixup_bad_iret
         mov     %rax, %rsp
-        decl    %ebx                            /* Return to usergs */
-        jmp     error_sti
+        decl    %ebx
+        jmp     error_entry_done
 END(error_entry)
 
 
-/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
+/*
+ * On entry, EBS is a "return to kernel mode" flag:
+ *   1: already in kernel mode, don't need SWAPGS
+ *   0: user gsbase is loaded, we need SWAPGS and standard preparation for return to usermode
+ */
 ENTRY(error_exit)
         movl    %ebx, %eax
         RESTORE_EXTRA_REGS