linux-kernel - Re: [PATCH] signal/x86: Delay calling signals in atomic

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Date:   Mon, 28 Mar 2022 09:48:48 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] signal/x86: Delay calling signals in atomic

"Eric W. Biederman" <ebiederm@...ssion.com> writes:

>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index 9b04631acde8f..cb2b28c17c0a5 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -1327,6 +1327,34 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
>>  	struct k_sigaction *action;
>>  	int sig = info->si_signo;
        ^^^^^^^^^^^^^^^^^^^^^^^^^
See info->si_signo is unconditionally dereferenced.
>>  
>> +	/*
>> +	 * On some archs, PREEMPT_RT has to delay sending a signal from a trap
>> +	 * since it can not enable preemption, and the signal code's spin_locks
>> +	 * turn into mutexes. Instead, it must set TIF_NOTIFY_RESUME which will
>> +	 * send the signal on exit of the trap.
>> +	 */
>> +#ifdef ARCH_RT_DELAYS_SIGNAL_SEND
>> +	if (in_atomic()) {
>> +		struct task_struct *t = current;
>> +
>> +		if (WARN_ON_ONCE(t->forced_info.si_signo))
>> +			return 0;
>> +
>> +		if (is_si_special(info)) {
                    ^^^^^^^^^^^^^^^^^^^
This tests to see if info is either 0 or 1 cased to as "kernel_siginfo *"

Which is a long way of saying this code already guarantees that
is_si_special is guaranteed to be false so most of this code is
completely unnecessary.


>> +			WARN_ON_ONCE(info != SEND_SIG_PRIV);
>> +			t->forced_info.si_signo = info->si_signo;
>> +			t->forced_info.si_errno = 0;
>> +			t->forced_info.si_code = SI_KERNEL;
>> +			t->forced_info.si_pid = 0;
>> +			t->forced_info.si_uid = 0;
>> +		} else {
>> +			t->forced_info = *info;
>> +		}
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
All of that can just be:

		copy_siginfo(&t->force_info, info);

Using a structure copy here that gcc is allowed to not copy bytes in the
padding is wrong in that it allows an information leak to userspace.


>> +		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
>> +		return 0;
>> +	}
>> +#endif
>>  	spin_lock_irqsave(&t->sighand->siglock, flags);
>>  	action = &t->sighand->action[sig-1];
>>  	ignored = action->sa.sa_handler == SIG_IGN;

Eric