Re: [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap_handler

[Eric W. Biederman]

Christian Borntraeger <borntraeger@xxxxxxxxxx> writes:

> Am 20.10.21 um 19:43 schrieb Eric W. Biederman:
>> Reading the history it is unclear why default_trap_handler calls
>> do_exit.  It is not even menthioned in the commit where the change
>> happened.  My best guess is that because it is unknown why the
>> exception happened it was desired to guarantee the process never
>> returned to userspace.
>>
>> Using do_exit(SIGSEGV) has the problem that it will only terminate one
>> thread of a process, leaving the process in an undefined state.
>>
>> Use force_sigsegv(SIGSEGV) instead which effectively has the same
>> behavior except that is uses the ordinary signal mechanism and
>> terminates all threads of a process and is generally well defined.
>
> Do I get that right, that programs can not block SIGSEGV from force_sigsegv
> with a signal handler? Thats how I read the code. If this is true
> then
>
> Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>

99% true, and it is what force_sigsegv(SIGSEGV) intends to do.

Andy Lutormorski pointed at a race where a thread can call sigaction
and change the signal handler after force_sigsegv has run but before
the process dequeues the SIGSEGV.

In principle it isn't too hard to close that race, and I was hoping to
be able to tell you that I had sorted by the time I replied.
Unfortunately it looks like it will take another week or two so will
probably not be ready by the merge window.

I am definitely going to close that race.

Eric


>> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
>> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Cc: linux-s390@xxxxxxxxxxxxxxx
>> Fixes: ca2ab03237ec ("[PATCH] s390: core changes")
>> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>   arch/s390/kernel/traps.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
>> index bcefc2173de4..51729ea2cf8e 100644
>> --- a/arch/s390/kernel/traps.c
>> +++ b/arch/s390/kernel/traps.c
>> @@ -84,7 +84,7 @@ static void default_trap_handler(struct pt_regs *regs)
>>   {
>>   	if (user_mode(regs)) {
>>   		report_user_fault(regs, SIGSEGV, 0);
>> -		do_exit(SIGSEGV);
>> +		force_sigsegv(SIGSEGV);
>>   	} else
>>   		die(regs, "Unknown program exception");
>>   }
>>