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Subject: Re: [syzbot] WARNING in xfrm_alloc_compat (2)
To:     Eric Dumazet <eric.dumazet@gmail.com>,
        Dmitry Safonov <dima@arista.com>,
        syzbot <syzbot+834ffd1afc7212eb8147@syzkaller.appspotmail.com>,
        davem@davemloft.net, herbert@gondor.apana.org.au, kuba@kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        steffen.klassert@secunet.com, syzkaller-bugs@googlegroups.com
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From:   Dmitry Safonov <0x7f454c46@gmail.com>
Message-ID: <e675bba0-b14d-1448-172e-e3ed66c2c9ef@gmail.com>
Date:   Mon, 29 Mar 2021 21:42:07 +0100
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On 3/29/21 9:31 PM, Eric Dumazet wrote:
> 
> 
> On 3/29/21 9:57 PM, Dmitry Safonov wrote:
[..]
>>> ------------[ cut here ]------------
>>> unsupported nla_type 356
>>
>> This doesn't seem to be an issue.
>> Userspace sent message with nla_type 356, which is > __XFRM_MSG_MAX, so
>> this warning is expected. I've added it so when a new XFRM_MSG_* will be
>> added, to make sure that there will be translations to such messages in
>> xfrm_compat.ko (if the translation needed).
>> This is WARN_ON_ONCE(), so it can't be used as DOS.
>>
>> Ping me if you'd expect something else than once-a-boot warning for
>> this. Not sure how-to close syzkaller bug, docs have only `invalid' tag,
>> which isn't `not-a-bug'/`expected' tag:
>> https://github.com/google/syzkaller/blob/master/docs/syzbot.md
>>
> 
> You should not use WARN_ON_ONCE() for this case (if user space can trigger it)
> 
> https://lwn.net/Articles/769365/
> 
> <quote>
> Greg Kroah-Hartman raised the problem of core kernel API code that will use WARN_ON_ONCE() to complain about bad usage; that will not generate the desired result if WARN_ON_ONCE() is configured to crash the machine. He was told that the code should just call pr_warn() instead, and that the called function should return an error in such situations. It was generally agreed that any WARN_ON() or WARN_ON_ONCE() calls that can be triggered from user space need to be fixed. 
> </quote>

Yeah, fair enough, I've already thought after sending the reply that
WARN_ON*() prints registers and that may be unwanted.
I'll cook a patch to convert this and other WARNs in the module.

I wish there was something like pr_warn_backtrace_once(), but I guess
it's fine without dumpstack(), after all.

Thanks,
         Dmitry