Re: [PATCH 1/2] arm64: mm: abort uaccess retries upon fatal signal

[Will Deacon]

On Tue, Jul 11, 2017 at 03:19:22PM +0100, Mark Rutland wrote:
> When there's a fatal signal pending, arm64'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@xxxxxxx>
> Reviewed-by: Steve Capper <steve.capper@xxxxxxx>
> Tested-by: Steve Capper <steve.capper@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/arm64/mm/fault.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 37b95df..3952d5e 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -397,8 +397,11 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>  	 * 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;
> +	}

This will need rebasing at -rc1 (take a look at current HEAD).

Also, I think it introduces a weird corner case where we take a page fault
when writing the signal frame to the user stack to deliver a SIGSEGV. If
we end up with VM_FAULT_RETRY and somebody has sent a SIGKILL to the task,
then we'll fail setup_sigframe and force an un-handleable SIGSEGV instead
of SIGKILL.

The end result (task is killed) is the same, but the fatal signal is wrong.

Will