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x86/entry/64: Remove %ebx handling from error_entry/exit 

commit b3681dd548 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 3ac6d8c787 ("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@runbox.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Fixes: 3ac6d8c787 ("x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface")
Link: http://lkml.kernel.org/r/b5010a090d3586b2d6e06c7ad3ec5542d1241c45.1532282627.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
pull/10/head
Andy Lutomirski 2018-07-22 11:05:09 -07:00 committed by Greg Kroah-Hartman
parent 7cf6b325f5
commit c1a29c2d00
1 changed files with 4 additions and 14 deletions
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18
arch/x86/entry/entry_64.S
View File

@ -933,7 +933,7 @@ ENTRY(\sym)
call \do_sym
jmp error_exit /* %ebx: no swapgs flag */
jmp error_exit
.endif
END(\sym)
.endm
@ -1166,7 +1166,6 @@ 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
@ -1213,7 +1212,6 @@ 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
@ -1247,28 +1245,20 @@ 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)