Re: [PATCH v2] af_unix: Call scm_recv() only after scm_set_cred().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Date: Mon, 26 Jun 2023 19:45:37 +0200
> On Mon, Jun 26, 2023 at 7:39 PM Konrad Dybcio <konradybcio@kernel.org> wrote:
> >
> >
> >
> > On 22.06.2023 20:43, Kuniyuki Iwashima wrote:
> > > syzkaller hit a WARN_ON_ONCE(!scm->pid) in scm_pidfd_recv().
> > >
> > > In unix_stream_read_generic(), if there is no skb in the queue, we could
> > > bail out the do-while loop without calling scm_set_cred():
> > >
> > >   1. No skb in the queue
> > >   2. sk is non-blocking
> > >        or
> > >      shutdown(sk, RCV_SHUTDOWN) is called concurrently
> > >        or
> > >      peer calls close()
> > >
> > > If the socket is configured with SO_PASSCRED or SO_PASSPIDFD, scm_recv()
> > > would populate cmsg with garbage.
> > >
> > > Let's not call scm_recv() unless there is skb to receive.
> > >
> > > WARNING: CPU: 1 PID: 3245 at include/net/scm.h:138 scm_pidfd_recv include/net/scm.h:138 [inline]
> > > WARNING: CPU: 1 PID: 3245 at include/net/scm.h:138 scm_recv.constprop.0+0x754/0x850 include/net/scm.h:177
> > > Modules linked in:
> > > CPU: 1 PID: 3245 Comm: syz-executor.1 Not tainted 6.4.0-rc5-01219-gfa0e21fa4443 #2
> > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> > > RIP: 0010:scm_pidfd_recv include/net/scm.h:138 [inline]
> > > RIP: 0010:scm_recv.constprop.0+0x754/0x850 include/net/scm.h:177
> > > Code: 67 fd e9 55 fd ff ff e8 4a 70 67 fd e9 7f fd ff ff e8 40 70 67 fd e9 3e fb ff ff e8 36 70 67 fd e9 02 fd ff ff e8 8c 3a 20 fd <0f> 0b e9 fe fb ff ff e8 50 70 67 fd e9 2e f9 ff ff e8 46 70 67 fd
> > > RSP: 0018:ffffc90009af7660 EFLAGS: 00010216
> > > RAX: 00000000000000a1 RBX: ffff888041e58a80 RCX: ffffc90003852000
> > > RDX: 0000000000040000 RSI: ffffffff842675b4 RDI: 0000000000000007
> > > RBP: ffffc90009af7810 R08: 0000000000000007 R09: 0000000000000013
> > > R10: 00000000000000f8 R11: 0000000000000001 R12: ffffc90009af7db0
> > > R13: 0000000000000000 R14: ffff888041e58a88 R15: 1ffff9200135eecc
> > > FS:  00007f6b7113f640(0000) GS:ffff88806cf00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 00007f6b7111de38 CR3: 0000000012a6e002 CR4: 0000000000770ee0
> > > PKRU: 55555554
> > > Call Trace:
> > >  <TASK>
> > >  unix_stream_read_generic+0x5fe/0x1f50 net/unix/af_unix.c:2830
> > >  unix_stream_recvmsg+0x194/0x1c0 net/unix/af_unix.c:2880
> > >  sock_recvmsg_nosec net/socket.c:1019 [inline]
> > >  sock_recvmsg+0x188/0x1d0 net/socket.c:1040
> > >  ____sys_recvmsg+0x210/0x610 net/socket.c:2712
> > >  ___sys_recvmsg+0xff/0x190 net/socket.c:2754
> > >  do_recvmmsg+0x25d/0x6c0 net/socket.c:2848
> > >  __sys_recvmmsg net/socket.c:2927 [inline]
> > >  __do_sys_recvmmsg net/socket.c:2950 [inline]
> > >  __se_sys_recvmmsg net/socket.c:2943 [inline]
> > >  __x64_sys_recvmmsg+0x224/0x290 net/socket.c:2943
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x3f/0x90 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> > > RIP: 0033:0x7f6b71da2e5d
> > > Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 9f 1b 00 f7 d8 64 89 01 48
> > > RSP: 002b:00007f6b7113ecc8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> > > RAX: ffffffffffffffda RBX: 00000000004bc050 RCX: 00007f6b71da2e5d
> > > RDX: 0000000000000007 RSI: 0000000020006600 RDI: 000000000000000b
> > > RBP: 00000000004bc050 R08: 0000000000000000 R09: 0000000000000000
> > > R10: 0000000000000120 R11: 0000000000000246 R12: 0000000000000000
> > > R13: 000000000000006e R14: 00007f6b71e03530 R15: 0000000000000000
> > >  </TASK>
> > >
> > > Fixes: 5e2ff6704a27 ("scm: add SO_PASSPIDFD and SCM_PIDFD")
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Reported-by: syzkaller <syzkaller@googlegroups.com>
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > > Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
> > > ---
> > Hi, for $some_reason, this breaks reaching DE on the following setup:
> >
> > - postmarketOS (Alpine Linux w/ musl 1.2.4)
> > - busybox 1.36.1
> > - GNOME 44.1
> > - networkmanager 1.42.6
> > - openrc 0.47
> >
> > on at least 2 different Qualcomm boards.
> >
> > #regzbot introduced: 3f5f118bb657f94641ea383c7c1b8c09a5d46ea2
> >
> > Konrad
> 
> I can confirm. Right now I'm working on another patch for net-next and
> this patch makes systemd-logind.service fail to start
> in my test VM.
> 
> Kuniyuki, please let me know if you need any help with debugging this.
> Probably we need to step on more conservative
> approach that was originally proposed here
> https://lore.kernel.org/netdev/CAJqdLrrmZGNE3NQ99_+rUkUMAWmWRo04VRhjdzGKBO3zmRmpjg@mail.gmail.com/

I'll send a revert to net.git soon.

And after net-next.git is merged to net.git, I'll send another
patch to suppress the SO_PASSPIDFD warning.

Thanks!


> Kind regards,
> Alex
> 
> > > v2:
> > >   * Target to net-next (v1 was marked as ChangesRequested maybe
> > >     due to one of the blamed commits is not in net.git)
> > >
> > >   * Update changelog to clarify skb is not in the queue
> > >
> > > v1: https://lore.kernel.org/netdev/20230620000009.9675-1-kuniyu@amazon.com/
> > > ---
> > >  net/unix/af_unix.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> > > index 73c61a010b01..f9d196439b49 100644
> > > --- a/net/unix/af_unix.c
> > > +++ b/net/unix/af_unix.c
> > > @@ -2826,7 +2826,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
> > >       } while (size);
> > >
> > >       mutex_unlock(&u->iolock);
> > > -     if (state->msg)
> > > +     if (state->msg && check_creds)
> > >               scm_recv(sock, state->msg, &scm, flags);
> > >       else
> > >               scm_destroy(&scm);