Re: [PATCH] x86/entry/64: Prevent clobbering of saved CR2 value

[Peter Zijlstra]

On Sat, Jul 20, 2019 at 10:56:41AM +0200, Thomas Gleixner wrote:
> The recent fix for CR2 corruption introduced a new way to reliably corrupt
> the saved CR2 value.
> 
> CR2 is saved early in the entry code in RDX, which is the third argument to
> the fault handling functions. But it missed that between saving and
> invoking the fault handler enter_from_user_mode() can be called. RDX is a
> caller saved register so the invoked function can freely clobber it with
> the obvious consequences.
> 
> The TRACE_IRQS_OFF call is safe as it calls through the thunk which
> preserves RDX.
> 
> Store CR2 in R12 instead which is a callee saved register and move R12 to
> RDX just before calling the fault handler.
> 
> Fixes: a0d14b8909de ("x86/mm, tracing: Fix CR2 corruption")
> Reported-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

D'0h :-( Sorry about that.

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  arch/x86/entry/entry_64.S |   11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -875,7 +875,12 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work
>  	UNWIND_HINT_REGS
>  
>  	.if \read_cr2
> -	GET_CR2_INTO(%rdx);			/* can clobber %rax */
> +	/*
> +	 * Store CR2 early so subsequent faults cannot clobber it. Use R12 as
> +	 * intermediate storage as RDX can be clobbered in enter_from_user_mode().
> +	 * GET_CR2_INTO can clobber RAX.
> +	 */
> +	GET_CR2_INTO(%r12);
>  	.endif
>  
>  	.if \shift_ist != -1
> @@ -904,6 +909,10 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work
>  	subq	$\ist_offset, CPU_TSS_IST(\shift_ist)
>  	.endif
>  
> +	.if \read_cr2
> +	movq	%r12, %rdx			/* Move CR2 into 3rd argument */
> +	.endif
> +
>  	call	\do_sym
>  
>  	.if \shift_ist != -1