linux-kernel - Re: [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap

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Date:   Fri, 29 Oct 2021 14:32:36 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap_handler

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Christian Borntraeger <borntraeger@...ibm.com> 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@...ibm.com>
>
> 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.

I now have a simple patch that closes the sigaction vs force_sig race,
that I am adding to this set of changes.  So now I can say programs can
not block force_sigsegv(SIGSEGV) with a signal handler or any other
method.

Eric

>>> Cc: Heiko Carstens <hca@...ux.ibm.com>
>>> Cc: Vasily Gorbik <gor@...ux.ibm.com>
>>> Cc: Christian Borntraeger <borntraeger@...ibm.com>
>>> Cc: linux-s390@...r.kernel.org
>>> 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@...ssion.com>
>>> ---
>>>   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");
>>>   }
>>>