Re: [PATCH 2/2] arm: mm: abort uaccess retries upon fatal signal

Mark Rutland
2017-08-22T10:42:10Z

On Tue, Jul 11, 2017 at 03:19:23PM +0100, Mark Rutland wrote:
> When there's a fatal signal pending, arm's do_page_fault()
> implementation returns 0. The intent is that we'll return to the
> faulting userspace instruction, delivering the signal on the way.
>
> However, if we take a fatal signal during fixing up a uaccess, this
> results in a return to the faulting kernel instruction, which will be
> instantly retried, resulting in the same fault being taken forever. As
> the task never reaches userspace, the signal is not delivered, and the
> task is left unkillable. While the task is stuck in this state, it can
> inhibit the forward progress of the system.
>
> To avoid this, we must ensure that when a fatal signal is pending, we
> apply any necessary fixup for a faulting kernel instruction. Thus we
> will return to an error path, and it is up to that code to make forward
> progress towards delivering the fatal signal.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Reviewed-by: Steve Capper <steve.capper@arm.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: stable@vger.kernel.org

Russell, on the assumption that you're happy with this as-is, I've
dropped it into the patch system as 8692/1.

Thanks,
Mark.

> ---
> arch/arm/mm/fault.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index ff8b0aa..42f5853 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -315,8 +315,11 @@ static inline bool access_error(unsigned int fsr, struct vm_area_struct *vma)
> * signal first. We do not need to release the mmap_sem because
> * it would already be released in __lock_page_or_retry in
> * mm/filemap.c. */
> - if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> + if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
> + if (!user_mode(regs))
> + goto no_context;
> return 0;
> + }
>
> /*
> * Major/minor page fault accounting is only done on the
> --
> 1.9.1
>