Re: [syzbot] [v9fs?] UBSAN: shift-out-of-bounds in v9fs_get_tree

Dominique Martinet
2025-08-21T03:48:31Z

Hi Eric,

syzbot wrote on Wed, Aug 20, 2025 at 07:58:31PM -0700:
> UBSAN: shift-out-of-bounds in fs/9p/vfs_super.c:57:22
> shift exponent 32 is too large for 32-bit type 'int'
> Call Trace:
> <TASK>
> dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
> ubsan_epilogue+0xa/0x40 lib/ubsan.c:233
> __ubsan_handle_shift_out_of_bounds+0x386/0x410 lib/ubsan.c:494
> v9fs_fill_super fs/9p/vfs_super.c:57 [inline]
> v9fs_get_tree+0x957/0xa90 fs/9p/vfs_super.c:125
> vfs_get_tree+0x8f/0x2b0 fs/super.c:1752
> do_new_mount+0x2a2/0xa30 fs/namespace.c:3810
> do_mount fs/namespace.c:4138 [inline]

I thinks the mount rework triggered this one (full copy below or at [1])
[1] https://lore.kernel.org/all/68a68b57.050a0220.3d78fd.0012.GAE@google.com/T/#u

From a quick look the old code bound msize to 4k-INT_MAX, but the new
code accepts higher uint32 values.
To be honest I'm not sure INT_MAX even makes sense as later allocations
are likely to work :) but for now something as simple as this is likely
to work (I'm not sure I got the test thing right, let's see...)

Shall I just roll that into your patch, unless you know of a more
appropriate limit?
There doesn't seem to be any easy to use variable about max allocation
size, a limit of a few MB is probably sensible but I don't like
artificial restrictions just to please syzbot so happy to defer to
someone else here.

#syz test

diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index 55ba26186351..cc65330ee684 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -302,6 +302,10 @@ int v9fs_parse_param(struct fs_context *fc, struct fs_parameter *param)
p9_debug(P9_DEBUG_ERROR, "msize should be at least 4k\n");
return -EINVAL;
}
+ if (result.uint_32 > INT_MAX) {
+ p9_debug(P9_DEBUG_ERROR, "msize too big\n");
+ return -EINVAL;
+ }
clnt->msize = result.uint_32;
break;
case Opt_trans:

syzbot wrote on Wed, Aug 20, 2025 at 07:58:31PM -0700:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 3ac864c2d9bb Add linux-next specific files for 20250818
> git tree: linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=13706442580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=6d1acc6b9e1fca1b
> dashboard link: https://syzkaller.appspot.com/bug?extid=30c83da54e948f6e9436
> compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=141586f0580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=124c9ba2580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/37dbe82593f0/disk-3ac864c2.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/d2fea0824445/vmlinux-3ac864c2.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/6f2a83735a01/bzImage-3ac864c2.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+30c83da54e948f6e9436@syzkaller.appspotmail.com
>
> ------------[ cut here ]------------
> UBSAN: shift-out-of-bounds in fs/9p/vfs_super.c:57:22
> shift exponent 32 is too large for 32-bit type 'int'
> CPU: 0 UID: 0 PID: 5861 Comm: syz-executor379 Not tainted 6.17.0-rc2-next-20250818-syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> Call Trace:
> <TASK>
> dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
> ubsan_epilogue+0xa/0x40 lib/ubsan.c:233
> __ubsan_handle_shift_out_of_bounds+0x386/0x410 lib/ubsan.c:494
> v9fs_fill_super fs/9p/vfs_super.c:57 [inline]
> v9fs_get_tree+0x957/0xa90 fs/9p/vfs_super.c:125
> vfs_get_tree+0x8f/0x2b0 fs/super.c:1752
> do_new_mount+0x2a2/0xa30 fs/namespace.c:3810
> do_mount fs/namespace.c:4138 [inline]
> __do_sys_mount fs/namespace.c:4349 [inline]
> __se_sys_mount+0x317/0x410 fs/namespace.c:4326
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7ff35edd46a9
> Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 00 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 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffeee8a4078 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 00002000000025c0 RCX: 00007ff35edd46a9
> RDX: 00002000000000c0 RSI: 00002000000025c0 RDI: 0000000000000000
> RBP: 0000200000000280 R08: 0000200000000280 R09: 00007ffeee8a4258
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007ff35ee1d017
> R13: 00007ffeee8a4248 R14: 0000000000000001 R15: 0000000000000001
> </TASK>
> ---[ end trace ]---
>
>
> ---
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>
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--
Dominique Martinet | Asmadeus