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

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2018-08-08 at 10:35 -0700, Sarah Newman wrote:
> commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream.
> 
> This version applies to v4.9.

I think you can kill the 'xorl %ebx,%ebx' from error_entry too but yes,
this does want to go to 4.9 and earlier because the 'Fixes:' tag is a
bit of a lie — the problem existed before that, at least in theory.

> From Andy Lutomirski, original author:
> 
> 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.]
> 
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: xen-devel@lists.xenproject.org
> Cc: x86@kernel.org
> Cc: stable@vger.kernel.org
> Cc: Andy Lutomirski <luto@kernel.org>
> 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@runbox.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Sarah Newman <srn@prgmr.com>
> ---
>  arch/x86/entry/entry_64.S | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index d58d8dc..0dab47a 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -774,7 +774,7 @@ ENTRY(\sym)
>  
>  	call	\do_sym
>  
> -	jmp	error_exit			/* %ebx: no
> swapgs flag */
> +	jmp	error_exit
>  	.endif
>  END(\sym)
>  .endm
> @@ -1043,7 +1043,6 @@ 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)
>  	cld
> @@ -1087,7 +1086,6 @@ 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
> @@ -1119,28 +1117,19 @@ 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)
> -	movl	%ebx, %eax
>  	DISABLE_INTERRUPTS(CLBR_NONE)
>  	TRACE_IRQS_OFF
> -	testl	%eax, %eax
> -	jnz	retint_kernel
> +	testb	$3, CS(%rsp)
> +	jz	retint_kernel
>  	jmp	retint_user
>  END(error_exit)
>  

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