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

Michael Schmitz
2023-02-28T19:19:10Z

Guenter,

On 1/03/23 04:22, Guenter Roeck wrote:
> On Tue, Jan 31, 2023 at 08:06:27PM +0000, 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;
> 0-day rightfully complains that this leaves 'msg' uninitialized.
>
> arch/parisc/mm/fault.c:427 do_page_fault() error: uninitialized symbol 'msg'
>
> Guenter

What happens if you initialize msg to "Page fault: no context" right at
the start of do_page_fault (and drop the assignment a few lines down as
that's now redundant)?

(Wondering if the zero page access on parisc could cause a trip right
back into do_page_fault, ad infinitum...)

Cheers,

Michael

>> return;
>> + }
>>
>> /* The fault is fully completed (including releasing mmap lock) */
>> if (fault & VM_FAULT_COMPLETED)