Re: KASAN: use-after-free Read in __do_page_fault

[Andrea Arcangeli]

On Tue, Oct 31, 2017 at 03:28:26PM +0100, Vlastimil Babka wrote:
> Hmm that could indeed work, Dmitry can you try the patch below?
> But it still seems rather fragile so I'd hope Andrea can do it more
> robust, or at least make sure that we don't reintroduce this kind of
> problem in the future (explicitly set vma to NULL with a comment?).

Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

> 
> ----8<----
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index e2baeaa053a5..9bd16fc621db 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1441,6 +1441,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  	 * the fault.  Since we never set FAULT_FLAG_RETRY_NOWAIT, if
>  	 * we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
>  	 */
> +	pkey = vma_pkey(vma);
>  	fault = handle_mm_fault(vma, address, flags);
>  	major |= fault & VM_FAULT_MAJOR;
>  
> @@ -1467,7 +1468,6 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		return;
>  	}
>  
> -	pkey = vma_pkey(vma);
>  	up_read(&mm->mmap_sem);
>  	if (unlikely(fault & VM_FAULT_ERROR)) {
>  		mm_fault_error(regs, error_code, address, &pkey, fault);
>