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

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Message-ID: <CANn89iK0F6W2CGCAz5HWWSzpLzV_iMvJYz0=qp3ZyrpDhjws2Q@mail.gmail.com>
Date: Fri, 6 Sep 2024 14:37:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Shoaib Rao <rao.shoaib@...cle.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, davem@...emloft.net, 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 Thu, Sep 5, 2024 at 10:48 PM Shoaib Rao <rao.shoaib@...cle.com> wrote:
>
>
> 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.

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).