netdev - Re: [bpf PATCH] bpf: sockmap, restore sk_write

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Date:   Thu, 23 May 2019 16:15:00 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Sitnicki <jakub@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [bpf PATCH] bpf: sockmap, restore sk_write_space when psock gets
 dropped

On 05/22/2019 12:01 PM, Jakub Sitnicki wrote:
> Once psock gets unlinked from its sock (sk_psock_drop), user-space can
> still trigger a call to sk->sk_write_space by setting TCP_NOTSENT_LOWAT
> socket option. This causes a null-ptr-deref because we try to read
> psock->saved_write_space from sk_psock_write_space:
> 
> ==================================================================
> BUG: KASAN: null-ptr-deref in sk_psock_write_space+0x69/0x80
> Read of size 8 at addr 00000000000001a0 by task sockmap-echo/131
> 
> CPU: 0 PID: 131 Comm: sockmap-echo Not tainted 5.2.0-rc1-00094-gf49aa1de9836 #81
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29 04/01/2014
> Call Trace:
>  ? sk_psock_write_space+0x69/0x80
>  __kasan_report.cold.2+0x5/0x3f
>  ? sk_psock_write_space+0x69/0x80
>  kasan_report+0xe/0x20
>  sk_psock_write_space+0x69/0x80
>  tcp_setsockopt+0x69a/0xfc0
>  ? tcp_shutdown+0x70/0x70
>  ? fsnotify+0x5b0/0x5f0
>  ? remove_wait_queue+0x90/0x90
>  ? __fget_light+0xa5/0xf0
>  __sys_setsockopt+0xe6/0x180
>  ? sockfd_lookup_light+0xb0/0xb0
>  ? vfs_write+0x195/0x210
>  ? ksys_write+0xc9/0x150
>  ? __x64_sys_read+0x50/0x50
>  ? __bpf_trace_x86_fpu+0x10/0x10
>  __x64_sys_setsockopt+0x61/0x70
>  do_syscall_64+0xc5/0x520
>  ? vmacache_find+0xc0/0x110
>  ? syscall_return_slowpath+0x110/0x110
>  ? handle_mm_fault+0xb4/0x110
>  ? entry_SYSCALL_64_after_hwframe+0x3e/0xbe
>  ? trace_hardirqs_off_caller+0x4b/0x120
>  ? trace_hardirqs_off_thunk+0x1a/0x3a
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x7f2e5e7cdcce
> Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb b1 66 2e 0f 1f 84 00 00 00 00 00
> 0f 1f 44 00 00 f3 0f 1e fa 49 89 ca b8 36 00 00 00 0f 05 <48> 3d 01 f0 ff
> ff 73 01 c3 48 8b 0d 8a 11 0c 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffed011b778 EFLAGS: 00000206 ORIG_RAX: 0000000000000036
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f2e5e7cdcce
> RDX: 0000000000000019 RSI: 0000000000000006 RDI: 0000000000000007
> RBP: 00007ffed011b790 R08: 0000000000000004 R09: 00007f2e5e84ee80
> R10: 00007ffed011b788 R11: 0000000000000206 R12: 00007ffed011b78c
> R13: 00007ffed011b788 R14: 0000000000000007 R15: 0000000000000068
> ==================================================================
> 
> Restore the saved sk_write_space callback when psock is being dropped to
> fix the crash.
> 
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>

LGTM, applied thanks!