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

[Shoaib Rao]

On 9/6/2024 5:37 AM, Eric Dumazet wrote:
> On Thu, Sep 5, 2024 at 10:48 PM Shoaib Rao <rao.shoaib@xxxxxxxxxx> wrote:
> > 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.
> KASAN does not panic unless you request it.
>
> Documentation/dev-tools/kasan.rst
>
> KASAN is affected by the generic ``panic_on_warn`` command line parameter.
> When it is enabled, KASAN panics the kernel after printing a bug report.
>
> By default, KASAN prints a bug report only for the first invalid memory access.
> With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
> effectively disables ``panic_on_warn`` for KASAN reports.
>
> Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot
> parameter can be used to control panic and reporting behaviour:
>
> - ``kasan.fault=report``, ``=panic``, or ``=panic_on_write`` controls whether
>    to only print a KASAN report, panic the kernel, or panic the kernel on
>    invalid writes only (default: ``report``). The panic happens even if
>    ``kasan_multi_shot`` is enabled. Note that when using asynchronous mode of
>    Hardware Tag-Based KASAN, ``kasan.fault=panic_on_write`` always panics on
>    asynchronously checked accesses (including reads).

Hi Eric,

Thanks for the update. I forgot to mention that I I did set 
/proc/sys/kernel/panic_on_warn to 1. I ran the program over night in two 
separate windows, there are no reports and no panic. I first try to 
reproduce the issue, because if I can not, how can I be sure that I have 
fixed that bug? I may find another issue and fix it but not the one that 
I was trying to. Please be assured that I am not done, I continue to 
investigate the issue.

If someone has a way of reproducing the failure please kindly let me know.

Kind regards,

Shoaib