linux-kernel - Re: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory.

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Message-ID: <e8d3ce30-8c61-048e-2606-f8a4e8f08d87@i-love.sakura.ne.jp>
Date:   Thu, 22 Aug 2019 23:00:59 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        syzbot <syzbot+8ab2d0f39fb79fe6ca40@...kaller.appspotmail.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory.

On 2019/08/22 22:35, Greg Kroah-Hartman wrote:
> On Thu, Aug 22, 2019 at 06:59:25PM +0900, Tetsuo Handa wrote:
>> Tetsuo Handa wrote:
>>> Greg Kroah-Hartman wrote:
>>>> Oh, nice!  This shouldn't break anything that is assuming that the read
>>>> will complete before a signal is delivered, right?
>>>>
>>>> I know userspace handling of "short" reads is almost always not there...
>>>
>>> Since this check will give up upon SIGKILL, userspace won't be able to see
>>> the return value from read(). Thus, returning 0 upon SIGKILL will be safe. ;-)
>>> Maybe we also want to add cond_resched()...
>>>
>>> By the way, do we want similar check on write_mem() side?
>>> If aborting "write to /dev/mem" upon SIGKILL (results in partial write) is
>>> unexpected, we might want to ignore SIGKILL for write_mem() case.
>>> But copying data from killed threads (especially when killed by OOM killer
>>> and userspace memory is reclaimed by OOM reaper before write_mem() returns)
>>> would be after all unexpected. Then, it might be preferable to check SIGKILL
>>> on write_mem() side...
>>>
>>
>> Ha, ha. syzbot reported the same problem using write_mem().
>> https://syzkaller.appspot.com/text?tag=CrashLog&x=1018055a600000
>> We want fatal_signal_pending() check on both sides.
> 
> Ok, want to send a patch for that?

Yes. But before sending a patch, I'm trying to dump values using debug printk().

> 
> And does anything use /dev/mem anymore?  I think X stopped using it a
> long time ago.
> 
>> By the way, write_mem() worries me whether there is possibility of replacing
>> kernel code/data with user-defined memory data supplied from userspace.
>> If write_mem() were by chance replaced with code that does
>>
>>    while (1);
>>
>> we won't be able to return from write_mem() even if we added fatal_signal_pending() check.
>> Ditto for replacing local variables with unexpected values...
> 
> I'm sorry, I don't really understand what you mean here, but I haven't
> had my morning coffee...  Any hints as to an example?

Probably similar idea: "lockdown: Restrict /dev/{mem,kmem,port} when the kernel is locked down"

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/char/mem.c?h=next-20190822&id=9b9d8dda1ed72e9bd560ab0ca93d322a9440510e

Then, syzbot might want to blacklist writing to /dev/mem .