Re: [syzbot] [net?] KASAN: slab-use-after-free Read in unix_stream_read_actor (2)

[Shoaib Rao]

On 9/5/2024 1:35 PM, Kuniyuki Iwashima wrote:
> From: Shoaib Rao <rao.shoaib@xxxxxxxxxx>
> Date: Thu, 5 Sep 2024 13:15:18 -0700
> > On 9/5/2024 12:46 PM, Kuniyuki Iwashima wrote:
> > > From: Shoaib Rao <rao.shoaib@xxxxxxxxxx>
> > > 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@xxxxxxxxxx/__;!!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