[Xen-devel] [PATCH 3.16 39/63] x86/entry/64: Remove %ebx handling from error_entry/exit

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.16.58-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream.

error_entry and error_exit communicate the user vs. kernel status of
the frame using %ebx.  This is unnecessary -- the information is in
regs->cs.  Just use regs->cs.

This makes error_entry simpler and makes error_exit more robust.

It also fixes a nasty bug.  Before all the Spectre nonsense, the
xen_failsafe_callback entry point returned like this:

        ALLOC_PT_GPREGS_ON_STACK
        SAVE_C_REGS
        SAVE_EXTRA_REGS
        ENCODE_FRAME_POINTER
        jmp     error_exit

And it did not go through error_entry.  This was bogus: RBX
contained garbage, and error_exit expected a flag in RBX.

Fortunately, it generally contained *nonzero* garbage, so the
correct code path was used.  As part of the Spectre fixes, code was
added to clear RBX to mitigate certain speculation attacks.  Now,
depending on kernel configuration, RBX got zeroed and, when running
some Wine workloads, the kernel crashes.  This was introduced by:

    commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for 
exceptions/interrupts, to reduce speculation attack surface")

With this patch applied, RBX is no longer needed as a flag, and the
problem goes away.

I suspect that malicious userspace could use this bug to crash the
kernel even without the offending patch applied, though.

[ Historical note: I wrote this patch as a cleanup before I was aware
  of the bug it fixed. ]

[ Note to stable maintainers: this should probably get applied to all
  kernels.  If you're nervous about that, a more conservative fix to
  add xorl %ebx,%ebx; incl %ebx before the jump to error_exit should
  also fix the problem. ]

Reported-and-tested-by: M. Vefa Bicakci <m.v.b@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for exceptions/interrupts, 
to reduce speculation attack surface")
Link: 
http://lkml.kernel.org/r/b5010a090d3586b2d6e06c7ad3ec5542d1241c45.1532282627.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16:
 - error_exit moved EBX to EAX before testing it, so delete both instructions
 - error_exit does RESTORE_REST earlier, so adjust the offset to saved CS
   accordingly
 - Drop inapplicable comment changes
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1135,7 +1135,7 @@ ENTRY(\sym)
        .if \paranoid
        jmp paranoid_exit               /* %ebx: no swapgs flag */
        .else
-       jmp error_exit                  /* %ebx: no swapgs flag */
+       jmp error_exit
        .endif
 
        CFI_ENDPROC
@@ -1411,7 +1411,6 @@ END(paranoid_exit)
 
 /*
  * Exception entry point. This expects an error code/orig_rax on the stack.
- * returns in "no swapgs flag" in %ebx.
  */
 ENTRY(error_entry)
        XCPT_FRAME
@@ -1440,7 +1439,6 @@ ENTRY(error_entry)
         * the kernel CR3 here.
         */
        SWITCH_KERNEL_CR3
-       xorl %ebx,%ebx
        testl $3,CS+8(%rsp)
        je error_kernelspace
 error_swapgs:
@@ -1456,7 +1454,6 @@ error_sti:
  * for these here too.
  */
 error_kernelspace:
-       incl %ebx
        leaq native_irq_return_iret(%rip),%rcx
        cmpq %rcx,RIP+8(%rsp)
        je error_bad_iret
@@ -1477,22 +1474,18 @@ error_bad_iret:
        mov %rsp,%rdi
        call fixup_bad_iret
        mov %rax,%rsp
-       decl %ebx       /* Return to usergs */
        jmp error_sti
        CFI_ENDPROC
 END(error_entry)
 
-
-/* ebx:        no swapgs flag (1: don't need swapgs, 0: need it) */
 ENTRY(error_exit)
        DEFAULT_FRAME
-       movl %ebx,%eax
        RESTORE_REST
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        GET_THREAD_INFO(%rcx)
-       testl %eax,%eax
-       jne retint_kernel
+       testb   $3, CS-ARGOFFSET(%rsp)
+       jz      retint_kernel
        LOCKDEP_SYS_EXIT_IRQ
        movl TI_flags(%rcx),%edx
        movl $_TIF_WORK_MASK,%edi


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