[PATCH bpf-next v2] af_unix: fix NULL pointer bug in unix_shutdown

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

From:   Jiang Wang <jiang.wang@bytedance.com>
Date:   Sat, 21 Aug 2021 18:07:36 +0000
> Commit 94531cfcbe79 ("af_unix: Add unix_stream_proto for sockmap") 
> introduced a bug for af_unix SEQPACKET type. In unix_shutdown, the
> unhash function will call prot->unhash(), which is NULL for SEQPACKET.
> And kernel will panic. On ARM32, it will show following messages: (it 
> likely affects x86 too).
> 
> Fix the bug by checking the prot->unhash is NULL or not first.
> 
> Kernel log:
> <--- cut here ---
>  Unable to handle kernel NULL pointer dereference at virtual address
> 00000000
>  pgd = 2fba1ffb
>  *pgd=00000000
>  Internal error: Oops: 80000005 [#1] PREEMPT SMP THUMB2
>  Modules linked in:
>  CPU: 1 PID: 1999 Comm: falkon Tainted: G        W
> 5.14.0-rc5-01175-g94531cfcbe79-dirty #9240
>  Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
>  PC is at 0x0
>  LR is at unix_shutdown+0x81/0x1a8
>  pc : [<00000000>]    lr : [<c08f3311>]    psr: 600f0013
>  sp : e45aff70  ip : e463a3c0  fp : beb54f04
>  r10: 00000125  r9 : e45ae000  r8 : c4a56664
>  r7 : 00000001  r6 : c4a56464  r5 : 00000001  r4 : c4a56400
>  r3 : 00000000  r2 : c5a6b180  r1 : 00000000  r0 : c4a56400
>  Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
>  Control: 50c5387d  Table: 05aa804a  DAC: 00000051
>  Register r0 information: slab PING start c4a56400 pointer offset 0
>  Register r1 information: NULL pointer
>  Register r2 information: slab task_struct start c5a6b180 pointer offset 0
>  Register r3 information: NULL pointer
>  Register r4 information: slab PING start c4a56400 pointer offset 0
>  Register r5 information: non-paged memory
>  Register r6 information: slab PING start c4a56400 pointer offset 100
>  Register r7 information: non-paged memory
>  Register r8 information: slab PING start c4a56400 pointer offset 612
>  Register r9 information: non-slab/vmalloc memory
>  Register r10 information: non-paged memory
>  Register r11 information: non-paged memory
>  Register r12 information: slab filp start e463a3c0 pointer offset 0
>  Process falkon (pid: 1999, stack limit = 0x9ec48895)
>  Stack: (0xe45aff70 to 0xe45b0000)
>  ff60:                                     e45ae000 c5f26a00 00000000 00000125
>  ff80: c0100264 c07f7fa3 beb54f04 fffffff7 00000001 e6f3fc0e b5e5e9ec beb54ec4
>  ffa0: b5da0ccc c010024b b5e5e9ec beb54ec4 0000000f 00000000 00000000 beb54ebc
>  ffc0: b5e5e9ec beb54ec4 b5da0ccc 00000125 beb54f58 00785238 beb5529c beb54f04
>  ffe0: b5da1e24 beb54eac b301385c b62b6ee8 600f0030 0000000f 00000000 00000000
>  [<c08f3311>] (unix_shutdown) from [<c07f7fa3>] (__sys_shutdown+0x2f/0x50)
>  [<c07f7fa3>] (__sys_shutdown) from [<c010024b>]
> (__sys_trace_return+0x1/0x16)
>  Exception stack(0xe45affa8 to 0xe45afff0)
> 
> Signed-off-by: Jiang Wang <jiang.wang@bytedance.com>
> Reported-by: Dmitry Osipenko <digetx@gmail.com>
> Tested-by: Dmitry Osipenko <digetx@gmail.com>

Fixes: 94531cfcbe79 ("af_unix: Add unix_stream_proto for sockmap")
Acked-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>

LGTM, thank you.


> ---
> v1 -> v2: check prot->unhash directly.
> 
>  net/unix/af_unix.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 443c49081636..15c1e4e4012d 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2847,7 +2847,8 @@ static int unix_shutdown(struct socket *sock, int mode)
>  		int peer_mode = 0;
>  		const struct proto *prot = READ_ONCE(other->sk_prot);
>  
> -		prot->unhash(other);
> +		if (prot->unhash)
> +			prot->unhash(other);
>  		if (mode&RCV_SHUTDOWN)
>  			peer_mode |= SEND_SHUTDOWN;
>  		if (mode&SEND_SHUTDOWN)
> -- 
> 2.20.1