linux-kernel - Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 22 Apr 2021 16:31:26 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     David Laight <David.Laight@...LAB.COM>
CC:     "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "carlos@...hat.com" <carlos@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow

On Apr 22, 2021, at 01:46, David Laight <David.Laight@...LAB.COM> wrote:
> From: Chang S. Bae
>> Sent: 22 April 2021 05:49
>> 
>> The kernel pushes context on to the userspace stack to prepare for the
>> user's signal handler. When the user has supplied an alternate signal
>> stack, via sigaltstack(2), it is easy for the kernel to verify that the
>> stack size is sufficient for the current hardware context.
>> 
>> Check if writing the hardware context to the alternate stack will exceed
>> it's size. If yes, then instead of corrupting user-data and proceeding with
>> the original signal handler, an immediate SIGSEGV signal is delivered.
> 
> What happens if SIGSEGV is caught?

Boris pointed out the relevant notes before [1]. I think "unpredictable
results" is a somewhat vague statement but process termination is unavoidable
in this situation.

In the thread [1], a new signal number was discussed for the signal delivery
failure, but my takeaway is this SIGSEGV is still recognizable.

FWIW, Len summarized other possible approaches as well [2].

>> Refactor the stack pointer check code from on_sig_stack() and use the new
>> helper.
>> 
>> While the kernel allows new source code to discover and use a sufficient
>> alternate signal stack size, this check is still necessary to protect
>> binaries with insufficient alternate signal stack size from data
>> corruption.
> ...
>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>> index 3f6a0fcaa10c..ae60f838ebb9 100644
>> --- a/include/linux/sched/signal.h
>> +++ b/include/linux/sched/signal.h
>> @@ -537,6 +537,17 @@ static inline int kill_cad_pid(int sig, int priv)
>> #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0)
>> #define SEND_SIG_PRIV	((struct kernel_siginfo *) 1)
>> 
>> +static inline int __on_sig_stack(unsigned long sp)
>> +{
>> +#ifdef CONFIG_STACK_GROWSUP
>> +	return sp >= current->sas_ss_sp &&
>> +		sp - current->sas_ss_sp < current->sas_ss_size;
>> +#else
>> +	return sp > current->sas_ss_sp &&
>> +		sp - current->sas_ss_sp <= current->sas_ss_size;
>> +#endif
>> +}
>> +
> 
> Those don't look different enough.

The difference is on the SS_AUTODISARM flag check.  This refactoring was
suggested as on_sig_stack() brought confusion [3].

Thanks,
Chang

[1] https://lore.kernel.org/lkml/20210414120608.GE10709@zn.tnic/
[2] https://lore.kernel.org/lkml/CAJvTdKnpWL8y4N_BrCiK7fU0UXERwuuM8o84LUpp7Watxd8STw@mail.gmail.com/
[3] https://lore.kernel.org/lkml/20210325212733.GC32296@zn.tnic/