ACK: [PATCH 1/1] x86/entry/64: Remove %ebx handling from error_entry/exit

[Tyler Hicks]

On 2019-02-18 16:08:46, Aaron Ma wrote:
> From: Andy Lutomirski <luto at kernel.org>
> 
> 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 at runbox.com>
> Signed-off-by: Andy Lutomirski <luto at kernel.org>
> Cc: Boris Ostrovsky <boris.ostrovsky at oracle.com>
> Cc: Borislav Petkov <bp at alien8.de>
> Cc: Brian Gerst <brgerst at gmail.com>
> Cc: Dave Hansen <dave.hansen at linux.intel.com>
> Cc: Denys Vlasenko <dvlasenk at redhat.com>
> Cc: Dominik Brodowski <linux at dominikbrodowski.net>
> Cc: Greg KH <gregkh at linuxfoundation.org>
> Cc: H. Peter Anvin <hpa at zytor.com>
> Cc: Josh Poimboeuf <jpoimboe at redhat.com>
> Cc: Juergen Gross <jgross at suse.com>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: stable at vger.kernel.org
> Cc: xen-devel at lists.xenproject.org
> 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@kernel.org
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> 
> CVE-2018-14678
> 
> (cherry picked from commit b3681dd548d06deb2e1573890829dff4b15abf46)
> Signed-off-by: Aaron Ma <aaron.ma at canonical.com>

Clean cherry pick, we've had this patch applied in Xenial's kernel, and
it looks safe to me.

Acked-by: Tyler Hicks <tyhicks at canonical.com>

Thanks!

Tyler

> ---
>  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 3a7d58384479..c0636b7ccc86 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -937,7 +937,7 @@ ENTRY(\sym)
>  
>  	call	\do_sym
>  
> -	jmp	error_exit			/* %ebx: no swapgs flag */
> +	jmp	error_exit
>  	.endif
>  END(\sym)
>  .endm
> @@ -1172,7 +1172,6 @@ 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
> @@ -1219,7 +1218,6 @@ 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
> @@ -1253,28 +1251,20 @@ 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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