linux-kernel - Re: [syzbot] [net?] KASAN: slab-use-after-free Read in unix_stream_read

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Message-ID: <19ce4e18-f1e0-44c4-b006-83001eb6ae24@oracle.com>
Date: Thu, 5 Sep 2024 13:48:16 -0700
From: Shoaib Rao <rao.shoaib@...cle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        pabeni@...hat.com,
        syzbot+8811381d455e3e9ec788@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] KASAN: slab-use-after-free Read in
 unix_stream_read_actor (2)


On 9/5/2024 1:35 PM, Kuniyuki Iwashima wrote:
> From: Shoaib Rao <rao.shoaib@...cle.com>
> Date: Thu, 5 Sep 2024 13:15:18 -0700
>> On 9/5/2024 12:46 PM, Kuniyuki Iwashima wrote:
>>> From: Shoaib Rao <rao.shoaib@...cle.com>
>>> Date: Thu, 5 Sep 2024 00:35:35 -0700
>>>> Hi All,
>>>>
>>>> I am not able to reproduce the issue. I have run the C program at least
>>>> 100 times in a loop. In the I do get an EFAULT, not sure if that is
>>>> intentional or not but no panic. Should I be doing something
>>>> differently? The kernel version I am using is
>>>> v6.11-rc6-70-gc763c4339688. Later I can try with the exact version.
>>> The -EFAULT is the bug meaning that we were trying to read an consumed skb.
>>>
>>> But the first bug is in recvfrom() that shouldn't be able to read OOB skb
>>> without MSG_OOB, which doesn't clear unix_sk(sk)->oob_skb, and later
>>> something bad happens.
>>>
>>>     socketpair(AF_UNIX, SOCK_STREAM, 0, [3, 4]) = 0
>>>     sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\333", iov_len=1}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, MSG_OOB|MSG_DONTWAIT) = 1
>>>     recvmsg(3, {msg_name=NULL, msg_namelen=0, msg_iov=NULL, msg_iovlen=0, msg_controllen=0, msg_flags=MSG_OOB}, MSG_OOB|MSG_WAITFORONE) = 1
>>>     sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\21", iov_len=1}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, MSG_OOB|MSG_NOSIGNAL|MSG_MORE) = 1
>>>> recvfrom(3, "\21", 125, MSG_DONTROUTE|MSG_TRUNC|MSG_DONTWAIT, NULL, NULL) = 1
>>>     recvmsg(3, {msg_namelen=0}, MSG_OOB|MSG_ERRQUEUE) = -1 EFAULT (Bad address)
>>>
>>> I posted a fix officially:
>>> https://urldefense.com/v3/__https://lore.kernel.org/netdev/20240905193240.17565-5-kuniyu@amazon.com/__;!!ACWV5N9M2RV99hQ!IJeFvLdaXIRN2ABsMFVaKOEjI3oZb2kUr6ld6ZRJCPAVum4vuyyYwUP6_5ZH9mGZiJDn6vrbxBAOqYI$
>> Thanks that is great. Isn't EFAULT,  normally indicative of an issue
>> with the user provided address of the buffer, not the kernel buffer.
> Normally, it's used when copy_to_user() or copy_from_user() or
> something similar failed.
>
> But this time, if you turn KASAN off, you'll see the last recvmsg()
> returns 1-byte garbage instead of -EFAULT, so actually KASAN worked
> on your host, I guess.

No it did not work. As soon as KASAN detected read after free it should 
have paniced as it did in the report and I have been running the 
syzbot's C program in a continuous loop. I would like to reproduce the 
issue before we can accept the fix -- If that is alright with you. I 
will try your new test case later and report back. Thanks for the patch 
though.

Regards,

Shoaib