Re: [PATCH] [PATCH] x86: Don't clobber top of pt_regs in nested NMI

[Steven Rostedt]

On Mon, 2012-10-01 at 17:29 -0700, Salman Qazi wrote:
> The nested NMI modifies the place (instruction, flags and stack)
> that the first NMI will iret to.  However, the copy of registers
> modified is exactly the one that is the part of pt_regs in
> the first NMI.  This can change the behaviour of the first NMI.
> 
> In particular, Google's arch_trigger_all_cpu_backtrace handler
> also prints regions of memory surrounding addresses appearing in
> registers.  This results in handled exceptions, after which nested NMIs
> start coming in.  These nested NMIs change the value of registers
> in pt_regs.  This can cause the original NMI handler to produce
> incorrect output.
> 
> We solve this problem by interchanging the position of the preserved
> copy of the iret registers ("saved") and the copy subject to being
> trampled by nested NMI ("copied").
> 

I was all ready to push this forward, but on my last final review I
found some nits that prevent me from doing so.

> Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
> ---
>  arch/x86/kernel/entry_64.S |   41 +++++++++++++++++++++++++++--------------
>  1 files changed, 27 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 44531ac..b5d6e43 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1739,9 +1739,10 @@ nested_nmi:
>  
>  1:
>  	/* Set up the interrupted NMIs stack to jump to repeat_nmi */
> -	leaq -6*8(%rsp), %rdx
> +	leaq -1*8(%rsp), %rdx
>  	movq %rdx, %rsp
> -	CFI_ADJUST_CFA_OFFSET 6*8
> +	CFI_ADJUST_CFA_OFFSET 1*8
> +	leaq -10*8(%rsp), %rdx
>  	pushq_cfi $__KERNEL_DS
>  	pushq_cfi %rdx
>  	pushfq_cfi
> @@ -1749,8 +1750,8 @@ nested_nmi:
>  	pushq_cfi $repeat_nmi
>  
>  	/* Put stack back */
> -	addq $(11*8), %rsp
> -	CFI_ADJUST_CFA_OFFSET -11*8
> +	addq $(6*8), %rsp
> +	CFI_ADJUST_CFA_OFFSET -6*8
>  
>  nested_nmi_out:
>  	popq_cfi %rdx
> @@ -1776,18 +1777,18 @@ first_nmi:
>  	 * +-------------------------+
>  	 * | NMI executing variable  |
>  	 * +-------------------------+
> -	 * | Saved SS                |
> -	 * | Saved Return RSP        |
> -	 * | Saved RFLAGS            |
> -	 * | Saved CS                |
> -	 * | Saved RIP               |
> -	 * +-------------------------+
>  	 * | copied SS               |
>  	 * | copied Return RSP       |
>  	 * | copied RFLAGS           |
>  	 * | copied CS               |
>  	 * | copied RIP              |
>  	 * +-------------------------+
> +	 * | Saved SS                |
> +	 * | Saved Return RSP        |
> +	 * | Saved RFLAGS            |
> +	 * | Saved CS                |
> +	 * | Saved RIP               |
> +	 * +-------------------------+
>  	 * | pt_regs                 |
>  	 * +-------------------------+
>  	 *
> @@ -1803,9 +1804,14 @@ first_nmi:
>  	/* Set the NMI executing variable on the stack. */
>  	pushq_cfi $1
>  
> +	/*
> +	 * Leave room for the "copied" frame
> +	 */
> +	subq $(5*8), %rsp
> +
>  	/* Copy the stack frame to the Saved frame */
>  	.rept 5
> -	pushq_cfi 6*8(%rsp)
> +	pushq_cfi 11*8(%rsp)
>  	.endr
>  	CFI_DEF_CFA_OFFSET SS+8-RIP
>  
> @@ -1826,12 +1832,15 @@ repeat_nmi:
>  	 * is benign for the non-repeat case, where 1 was pushed just above
>  	 * to this very stack slot).
>  	 */
> -	movq $1, 5*8(%rsp)
> +	movq $1, 10*8(%rsp)
>  
>  	/* Make another copy, this one may be modified by nested NMIs */
> +	addq $(10*8), %rsp

This breaks the CFI magic.

>  	.rept 5
> -	pushq_cfi 4*8(%rsp)
> +	pushq_cfi -6*8(%rsp)
>  	.endr
> +	subq $(5*8), %rsp

So does this.

This needs to be annotated correctly before I can push it out. But the
good news is, I stressed tested this change, and it all works out.

Jan, can you help out here?

-- Steve

> +
>  	CFI_DEF_CFA_OFFSET SS+8-RIP
>  end_repeat_nmi:
>  
> @@ -1882,8 +1891,12 @@ nmi_swapgs:
>  	SWAPGS_UNSAFE_STACK
>  nmi_restore:
>  	RESTORE_ALL 8
> +
> +	/* Pop the extra iret frame */
> +	addq $(5*8), %rsp
> +
>  	/* Clear the NMI executing stack variable */
> -	movq $0, 10*8(%rsp)
> +	movq $0, 5*8(%rsp)
>  	jmp irq_return
>  	CFI_ENDPROC
>  END(nmi)


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