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

[Andy Lutomirski <luto@xxxxxxxxxx>]

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.]

Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for exceptions/interrupts, 
to reduce speculation attack surface")
Reported-and-tested-by: "M. Vefa Bicakci" <m.v.b@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

I could also submit the conservative fix tagged for -stable and respin
this on top of it.  Ingo, Greg, what do you prefer?

 arch/x86/entry/entry_64.S | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 73a522d53b53..8ae7ffda8f98 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -981,7 +981,7 @@ ENTRY(\sym)
 
        call    \do_sym
 
-       jmp     error_exit                      /* %ebx: no swapgs flag */
+       jmp     error_exit
        .endif
 END(\sym)
 .endm
@@ -1222,7 +1222,6 @@ END(paranoid_exit)
 
 /*
  * Save all registers in pt_regs, and switch GS if needed.
- * Return: EBX=0: came from user mode; EBX=1: otherwise
  */
 ENTRY(error_entry)
        UNWIND_HINT_FUNC
@@ -1269,7 +1268,6 @@ ENTRY(error_entry)
         * for these here too.
         */
 .Lerror_kernelspace:
-       incl    %ebx
        leaq    native_irq_return_iret(%rip), %rcx
        cmpq    %rcx, RIP+8(%rsp)
        je      .Lerror_bad_iret
@@ -1303,28 +1301,20 @@ ENTRY(error_entry)
 
        /*
         * 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.
+        * as if we faulted immediately after IRET.
         */
        mov     %rsp, %rdi
        call    fixup_bad_iret
        mov     %rax, %rsp
-       decl    %ebx
        jmp     .Lerror_entry_from_usermode_after_swapgs
 END(error_entry)
 
-
-/*
- * On entry, EBX 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)
        UNWIND_HINT_REGS
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_OFF
-       testl   %ebx, %ebx
-       jnz     retint_kernel
+       testb   $3, CS(%rsp)
+       jz      retint_kernel
        jmp     retint_user
 END(error_exit)
 
-- 
2.17.1


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