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

Helge Deller
2023-02-28T18:27:03Z

On 2/28/23 18:34, Al Viro wrote:
> On Mon, Feb 06, 2023 at 05:58:02PM +0100, Helge Deller wrote:
>> 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?
>
> Still trying to resurrect the parisc box to test on it...
> FWIW, right now I've locally confirmed that mainline has the bug
> in question and that patch fixes it for alpha, sparc32 and sparc64;
> hexagon, m68k and riscv got acks from other folks; microblaze,
> nios2 and openrisc I can't test at all (no hardware, no qemu setup);
> same for parisc64. Itanic and parisc32 I might be able to test,
> if I manage to resurrect the hardware.

I can test both parisc32 and parisc64.

> Just to confirm: your "can be killed with ^C" had been on the
> mainline parisc kernel (with userfaultfd enable, of course, or it wouldn't
> hang up at all), right?

It was a recent mainline kernel with your patch.

> Was it 32bit or 64bit kernel?

I don't remember. I think I tried both.

Helge