Re: KASAN: null-ptr-deref Read in x25_connect

[Dmitry Vyukov]

On Tue, Jun 11, 2019 at 2:39 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Tue, Jun 11, 2019 at 9:18 AM syzbot
> <syzbot+777a2aab6ffd397407b5@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    f4cfcfbd net: dsa: sja1105: Fix link speed not working at ..
> > git tree:       net
> > console output: https://syzkaller.appspot.com/x/log.txt?x=16815cd2a00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=4f721a391cd46ea
> > dashboard link: https://syzkaller.appspot.com/bug?extid=777a2aab6ffd397407b5
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+777a2aab6ffd397407b5@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> Not sure why I was on Cc on this (I know nothing about x25),

syzbot uses get_maintainer.pl to find relevant people. This was
attributed to net/x25/af_x25.c file, which looks correct. And then
get_maintainer.pl points to you for this file:

$ scripts/get_maintainer.pl -f net/x25/af_x25.c
Andrew Hendry <andrew.hendry@xxxxxxxxx> (odd fixer:X.25 NETWORK LAYER)
"David S. Miller" <davem@xxxxxxxxxxxxx> (maintainer:NETWORKING
[GENERAL],commit_signer:7/9=78%)
Eric Dumazet <edumazet@xxxxxxxxxx>
(commit_signer:4/9=44%,authored:4/9=44%,added_lines:16/30=53%,removed_lines:10/50=20%)
Martin Schiller <ms@xxxxxxxxxx>
(commit_signer:2/9=22%,authored:2/9=22%,added_lines:11/30=37%,removed_lines:7/50=14%)
Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> (commit_signer:1/9=11%)
Willem de Bruijn <willemb@xxxxxxxxxx> (commit_signer:1/9=11%)
Arnd Bergmann <arnd@xxxxxxxx> (authored:1/9=11%,removed_lines:26/50=52%)
Thomas Gleixner <tglx@xxxxxxxxxxxxx> (authored:1/9=11%,removed_lines:6/50=12%)
linux-x25@xxxxxxxxxxxxxxx (open list:X.25 NETWORK LAYER)
netdev@xxxxxxxxxxxxxxx (open list:NETWORKING [GENERAL])
linux-kernel@xxxxxxxxxxxxxxx (open list)




> but I had
> a brief look and found that this is in the error path of x25_connect,
> after "goto out_put_neigh", with x25->neighbour==NULL.
>
> This would indicate that either 'x25' is being freed between the
> "if (!x25->neighbour)" check in that function and the
> x25_neigh_put(x25->neighbour), or that there are two concurrent
> calls to x25_connect, with both failing, so one sets
> x25->neighbour=NULL before the other one checks it.
>
>     Arnd
>
> > ==================================================================
> > BUG: KASAN: null-ptr-deref in atomic_read
> > include/asm-generic/atomic-instrumented.h:26 [inline]
> > BUG: KASAN: null-ptr-deref in refcount_sub_and_test_checked+0x87/0x200
> > lib/refcount.c:182
> > Read of size 4 at addr 00000000000000c8 by task syz-executor.2/16959
> >
> > CPU: 0 PID: 16959 Comm: syz-executor.2 Not tainted 5.2.0-rc2+ #40
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Call Trace:
> >   __dump_stack lib/dump_stack.c:77 [inline]
> >   dump_stack+0x172/0x1f0 lib/dump_stack.c:113
> >   __kasan_report.cold+0x5/0x40 mm/kasan/report.c:321
> >   kasan_report+0x12/0x20 mm/kasan/common.c:614
> >   check_memory_region_inline mm/kasan/generic.c:185 [inline]
> >   check_memory_region+0x123/0x190 mm/kasan/generic.c:191
> >   kasan_check_read+0x11/0x20 mm/kasan/common.c:94
> >   atomic_read include/asm-generic/atomic-instrumented.h:26 [inline]
> >   refcount_sub_and_test_checked+0x87/0x200 lib/refcount.c:182
> >   refcount_dec_and_test_checked+0x1b/0x20 lib/refcount.c:220
> >   x25_neigh_put include/net/x25.h:252 [inline]
> >   x25_connect+0x8d8/0xea0 net/x25/af_x25.c:820
> >   __sys_connect+0x264/0x330 net/socket.c:1840
> >   __do_sys_connect net/socket.c:1851 [inline]
> >   __se_sys_connect net/socket.c:1848 [inline]
> >   __x64_sys_connect+0x73/0xb0 net/socket.c:1848
> >   do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
> >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x459279
> > Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f09776b4c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
> > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459279
> > RDX: 0000000000000012 RSI: 0000000020000280 RDI: 0000000000000004
> > RBP: 000000000075bfc0 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00007f09776b56d4
> > R13: 00000000004bf854 R14: 00000000004d0e08 R15: 00000000ffffffff
> > ==================================================================
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
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