WARNING in __kernel_read

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: a581387e Merge tag 'io_uring-5.8-2020-07-10' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13e730eb100000
kernel config: https://syzkaller.appspot.com/x/.config?x=66ad203c2bb6d8b
dashboard link: https://syzkaller.appspot.com/bug?extid=d012ca3f813739c37c25
compiler: gcc (GCC) 10.1.0-syz 20200507
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12e0222b100000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=162a004f100000

The bug was bisected to:

commit 6209dd9132e8ea5545cffc84483841e88ea8cc5b
Author: Christoph Hellwig <h...@lst.de>
Date: Fri May 8 07:00:28 2020 +0000

fs: implement kernel_read using __kernel_read

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=152d91fb100000
final crash: https://syzkaller.appspot.com/x/report.txt?x=172d91fb100000
console output: https://syzkaller.appspot.com/x/log.txt?x=132d91fb100000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d012ca...@syzkaller.appspotmail.com
Fixes: 6209dd9132e8 ("fs: implement kernel_read using __kernel_read")

------------[ cut here ]------------
WARNING: CPU: 0 PID: 5 at fs/read_write.c:427 __kernel_read+0x41d/0x4d0 fs/read_write.c:427
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 5 Comm: kworker/0:0 Not tainted 5.8.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events p9_read_work
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x18f/0x20d lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:231
__warn.cold+0x20/0x45 kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
handle_bug+0x38/0x90 arch/x86/kernel/traps.c:235
exc_invalid_op+0x13/0x40 arch/x86/kernel/traps.c:255
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:542
RIP: 0010:__kernel_read+0x41d/0x4d0 fs/read_write.c:427
Code: fd ff ff e8 75 19 b6 ff 45 31 c9 45 31 c0 b9 01 00 00 00 4c 89 f2 89 ee 4c 89 ef e8 5d 22 12 00 e9 46 ff ff ff e8 53 19 b6 ff <0f> 0b 49 c7 c4 ea ff ff ff e9 11 fe ff ff 4c 89 f7 e8 2d 76 f5 ff
RSP: 0018:ffffc90000cbfbc8 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff8880a9786ac0 RCX: ffffffff81bd9ac4
RDX: ffff8880a95a2140 RSI: ffffffff81bd9e3d RDI: 0000000000000005
RBP: ffff888096bc8060 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 1ffffffff1829bdd R12: 00000000081d801e
R13: ffffc90000cbfc98 R14: ffff8880a9786b44 R15: 0000000000000007
kernel_read+0x52/0x70 fs/read_write.c:457
p9_fd_read net/9p/trans_fd.c:263 [inline]
p9_read_work+0x2ac/0xff0 net/9p/trans_fd.c:298
process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
kthread+0x3b5/0x4a0 kernel/kthread.c:291
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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[Christoph Hellwig]

On Mon, Jul 13, 2020 at 12:03:17AM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:

This is not a crash, but a WARN_ON_ONCE, someone really needs to fix
syzbot to report this correctly.

The fix should be queued up by the 9p maintainers.

---end quoted text---

[Cengiz Can]

Hello,

I'm trying to help clean up syzkaller submissions and this caught my
attention and I wanted to get your advice.

With commit: 6209dd9132e8ea5545cffc84483841e88ea8cc5b `kernel_read` was
modified to use `__kernel_read` by Christoph Hellwig.

One of the syzkaller tests executes following system calls:

open("./file0", O_WRONLY|O_CREAT|O_EXCL|O_DIRECT|0x4, 000) = 5
open("/dev/char/4:1", O_RDWR) = 6
mount(NULL, "./file0", "9p", 0,
"trans=fd,rfdno=0x0000000000000005,wfdno=0x0000000000000006,"

This initiates a `__kernel_read` call from `p9_read_work` (and
`p9_fd_read`) and since the `file->f_mode` does not contain FMODE_READ
, a WARN_ON_ONCE is thrown.

```
if (WARN_ON_ONCE(!(file->f_mode & FMODE_READ)))
return -EINVAL;
```

Can you help me understand what's wrong and fix this issue?
Is it already being worked on?

Thank you

Cengiz Can

[Eric Biggers]

Hi Cengiz,

On Thu, Jul 23, 2020 at 05:17:25PM +0300, Cengiz Can wrote:
> Hello,
>
> I'm trying to help clean up syzkaller submissions and this caught my
> attention and I wanted to get your advice.
>
> With commit: 6209dd9132e8ea5545cffc84483841e88ea8cc5b `kernel_read` was
> modified to use `__kernel_read` by Christoph Hellwig.
>
> One of the syzkaller tests executes following system calls:
>
> open("./file0", O_WRONLY|O_CREAT|O_EXCL|O_DIRECT|0x4, 000) = 5
> open("/dev/char/4:1", O_RDWR) = 6
> mount(NULL, "./file0", "9p", 0,
> "trans=fd,rfdno=0x0000000000000005,wfdno=0x0000000000000006,"
>
> This initiates a `__kernel_read` call from `p9_read_work` (and
> `p9_fd_read`) and since the `file->f_mode` does not contain FMODE_READ
> , a WARN_ON_ONCE is thrown.
>
> ```
> if (WARN_ON_ONCE(!(file->f_mode & FMODE_READ)))
> return -EINVAL;
> ```
>
> Can you help me understand what's wrong and fix this issue?
> Is it already being worked on?
>

Looks like this was already fixed in linux-next by:

commit a39c46067c845a8a2d7144836e9468b7f072343e
Author: Christoph Hellwig <h...@lst.de>
Date: Fri Jul 10 10:57:22 2020 +0200

net/9p: validate fds in p9_fd_open

Let's tell syzbot so that it closes this bug report:

#syz fix: net/9p: validate fds in p9_fd_open