linux-kernel - Re: [PATCH] libfs: Fix NULL pointer access in simple_recursive

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Message-ID: <20251118170450.GG2441659@ZenIV>
Date: Tue, 18 Nov 2025 17:04:50 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "rom.wang" <r4o5m6e8o@....com>, Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yufeng Wang <wangyufeng@...inos.cn>,
	stable@...r.kernel.org
Subject: Re: [PATCH] libfs: Fix NULL pointer access in
 simple_recursive_removal

On Thu, Nov 13, 2025 at 08:25:32AM -0500, Greg KH wrote:
> On Thu, Nov 13, 2025 at 01:23:57PM +0800, rom.wang wrote:
> > From: Yufeng Wang <wangyufeng@...inos.cn>
> > 
> > There is an issue in the kernel:
> > if inode is NULL pointer. the function "inode_lock_nested"
> > (or function "inode_lock" before)
> > a crash will happen at code "&inode->i_rwsem".
> 
> How is inode NULL?  What is causing that?
> 
> > [292618.520532] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
> > [...]
> > [292618.560398] RIP: 0010:down_write+0x12/0x30
> > [292618.565580] Code: 83 f8 01 74 08 48 c7 47 20 01 00 00 00 f3 c3 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 ba 01 00 00 00 ff ff ff ff 48 89 f8 <f0> 48 0f c1 10 85 d2 74 05 e8 00 43 ff ff 65 48 8b 04 25 80 5c 01
> > [292618.587219] RSP: 0018:ffffb898dc86fc20 EFLAGS: 00010246
> > [292618.593666] RAX: 00000000000000a0 RBX: ffff94c84f363950 RCX: ffffff8000000000
> > [292618.602255] RDX: ffffffff00000001 RSI: 0000000000000063 RDI: 00000000000000a0
> > [292618.610844] RBP: ffffb898dc86fc78 R08: 0000000000000000 R09: 0000000000000000
> > [292618.619434] R10: ffffb898dc86fca8 R11: 0000000000000000 R12: 0000000000000000
> > [292618.628022] R13: ffff94c84f362a20 R14: ffff954d3f2fb4a0 R15: ffff954c3afa5010
> > [292618.636612] FS:  0000555555989cc0(0000) GS:ffff956dbf900000(0000) knlGS:0000000000000000
> > [292618.646271] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [292618.653300] CR2: 00000000000000a0 CR3: 000000fc7f25a000 CR4: 00000000003406e0
> > [292618.661888] Call Trace:
> > [292618.665225]  simple_recursive_removal+0x4f/0x230
> > [292618.670994]  ? debug_fill_super+0xe0/0xe0
> > [292618.676079]  debugfs_remove+0x40/0x60
> > [292618.680799]  kvm_vcpu_release+0x19/0x30 [kvm]
> 
> Is the kvm code doing something wrong here?  debugfs shouldn't be trying
> to remove an inode that is already removed, so please fix the root
> cause, do not paper over it.

That's... interesting.  It's not just inode that is already removed,
it's very likely a dentry that has already beed freed (dentry with
refcount greater than 1 can not become negative).