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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Shoaib Rao <rao.shoaib@oracle.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@oracle.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.