Re: [PATCH 08/10] parisc: fix livelock in uaccess

[Helge Deller <deller@gmx.de>]

Hi Al,

On 1/31/23 21:06, Al Viro wrote:
> parisc equivalent of 26178ec11ef3 "x86: mm: consolidate VM_FAULT_RETRY handling"
> If e.g. get_user() triggers a page fault and a fatal signal is caught, we might
> end up with handle_mm_fault() returning VM_FAULT_RETRY and not doing anything
> to page tables.  In such case we must *not* return to the faulting insn -
> that would repeat the entire thing without making any progress; what we need
> instead is to treat that as failed (user) memory access.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>   arch/parisc/mm/fault.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
> index 869204e97ec9..bb30ff6a3e19 100644
> --- a/arch/parisc/mm/fault.c
> +++ b/arch/parisc/mm/fault.c
> @@ -308,8 +308,11 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
>
>   	fault = handle_mm_fault(vma, address, flags, regs);
>
> -	if (fault_signal_pending(fault, regs))
> +	if (fault_signal_pending(fault, regs)) {
> +		if (!user_mode(regs))
> +			goto no_context;
>   		return;
> +	}

The testcase in
   https://lore.kernel.org/lkml/20170822102527.GA14671@leverpostej/
   https://lore.kernel.org/linux-arch/20210121123140.GD48431@C02TD0UTHF1T.local/
does hang with and without above patch on parisc.
It does not consume CPU in that state and can be killed with ^C.

Any idea?

Helge