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

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Message-ID: <d7073535-b435-418e-a58f-46728fc79c1e@oracle.com>
Date: Thu, 5 Sep 2024 13:41:26 -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 1:37 PM, Shoaib Rao wrote:
>
> On 9/5/2024 1:15 PM, Shoaib Rao wrote:
>>
>> 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.
>>
>> Shoaib
>>
> Can you provide the full test case that you used to reproduce the issue.
>
> Thanks,
>
> Shoaib
>
>
 From the recvmsg man page


    Return Value

These calls return the number of bytes received, or -1 if an error 
occurred. The return value will be 0 when the peer has performed an 
orderly shutdown.

*EFAULT*

The receive buffer*pointer*(s) point outside the process's address space.

I still do not understand why if I had all the check on and the issue 
occured, why the kernel did not panic. Maybe, running the exact test 
case will help me understand.

Shoaib