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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>, linux-s390@vger.kernel.org
In-Reply-To: <7c99f791-4a87-ae52-bee7-cb794b0741d2@de.ibm.com> (Christian
        Borntraeger's message of "Tue, 26 Oct 2021 11:38:44 +0200")
References: <87y26nmwkb.fsf@disp2133>
        <20211020174406.17889-11-ebiederm@xmission.com>
        <7c99f791-4a87-ae52-bee7-cb794b0741d2@de.ibm.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
Date:   Thu, 28 Oct 2021 10:56:24 -0500
Message-ID: <87y26dp2hj.fsf@disp2133>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap_handler
Precedence: bulk

Christian Borntraeger <borntraeger@de.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@de.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.

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@linux.ibm.com>
>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>> Cc: linux-s390@vger.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@xmission.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");
>>   }
>>