linux-kernel - Re: [PATCH] fs/notify: call exportfs_encode_fid with s

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Message-ID: <e4o5wuh2h7viev2khbr7excdm7xv6ubw3va55e56q4apjno62s@hu3ybnftbhhz>
Date: Wed, 1 Oct 2025 14:15:11 +0200
From: Jan Kara <jack@...e.cz>
To: Jakub Acs <acsjakub@...zon.de>
Cc: linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>, 
	Amir Goldstein <amir73il@...il.com>, Miklos Szeredi <miklos@...redi.hu>, 
	Christian Brauner <brauner@...nel.org>, linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] fs/notify: call exportfs_encode_fid with s_umount

On Wed 01-10-25 10:09:55, Jakub Acs wrote:
> Calling intotify_show_fdinfo() on fd watching an overlayfs inode, while
> the overlayfs is being unmounted, can lead to dereferencing NULL ptr.
> 
> This issue was found by syzkaller.
> 
> Race Condition Diagram:
> 
> Thread 1                           Thread 2
> --------                           --------
> 
> generic_shutdown_super()
>  shrink_dcache_for_umount
>   sb->s_root = NULL
> 
>                     |
>                     |             vfs_read()
>                     |              inotify_fdinfo()
>                     |               * inode get from mark *
>                     |               show_mark_fhandle(m, inode)
>                     |                exportfs_encode_fid(inode, ..)
>                     |                 ovl_encode_fh(inode, ..)
>                     |                  ovl_check_encode_origin(inode)
>                     |                   * deref i_sb->s_root *
>                     |
>                     |
>                     v
>  fsnotify_sb_delete(sb)
> 
> Which then leads to:
> 
> [   32.133461] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
> [   32.134438] KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
> [   32.135032] CPU: 1 UID: 0 PID: 4468 Comm: systemd-coredum Not tainted 6.17.0-rc6 #22 PREEMPT(none)
> 
> <snip registers, unreliable trace>
> 
> [   32.143353] Call Trace:
> [   32.143732]  ovl_encode_fh+0xd5/0x170
> [   32.144031]  exportfs_encode_inode_fh+0x12f/0x300
> [   32.144425]  show_mark_fhandle+0xbe/0x1f0
> [   32.145805]  inotify_fdinfo+0x226/0x2d0
> [   32.146442]  inotify_show_fdinfo+0x1c5/0x350
> [   32.147168]  seq_show+0x530/0x6f0
> [   32.147449]  seq_read_iter+0x503/0x12a0
> [   32.148419]  seq_read+0x31f/0x410
> [   32.150714]  vfs_read+0x1f0/0x9e0
> [   32.152297]  ksys_read+0x125/0x240
> 
> IOW ovl_check_encode_origin derefs inode->i_sb->s_root, after it was set
> to NULL in the unmount path.
> 
> Fix it by protecting calling exportfs_encode_fid() from
> show_mark_fhandle() with s_umount lock.
> 
> This form of fix was suggested by Amir in [1].
> 
> [1]: https://lore.kernel.org/all/CAOQ4uxhbDwhb+2Brs1UdkoF0a3NSdBAOQPNfEHjahrgoKJpLEw@mail.gmail.com/
> 
> Fixes: c45beebfde34 ("ovl: support encoding fid from inode with no alias")
> Signed-off-by: Jakub Acs <acsjakub@...zon.de>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Amir Goldstein <amir73il@...il.com>
> Cc: Miklos Szeredi <miklos@...redi.hu>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: linux-unionfs@...r.kernel.org
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: stable@...r.kernel.org
> ---
> 
> This issue was already discussed in [1] with no consensus reached on the
> fix.
> 
> This form was suggested as a band-aid fix, without explicity yes/no
> reaction. Hence reviving the discussion around the band-aid.

FWIW I'm working on a proper fix. But it's a larger rework so it will take
some time to settle. For the time being, since this seems to happen in
practical workloads, I guess we can live with this workaround so I'll pick
this patch, add some comment about band-aid into the code and push it to
Linus. Thanks!

								Honza

> 
>  fs/notify/fdinfo.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
> index 1161eabf11ee..9cc7eb863643 100644
> --- a/fs/notify/fdinfo.c
> +++ b/fs/notify/fdinfo.c
> @@ -17,6 +17,7 @@
>  #include "fanotify/fanotify.h"
>  #include "fdinfo.h"
>  #include "fsnotify.h"
> +#include "../internal.h"
>  
>  #if defined(CONFIG_PROC_FS)
>  
> @@ -46,7 +47,12 @@ static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
>  
>  	size = f->handle_bytes >> 2;
>  
> +	if (!super_trylock_shared(inode->i_sb))
> +		return;
> +
>  	ret = exportfs_encode_fid(inode, (struct fid *)f->f_handle, &size);
> +	up_read(&inode->i_sb->s_umount);
> +
>  	if ((ret == FILEID_INVALID) || (ret < 0))
>  		return;
>  
> -- 
> 2.47.3
> 
> 
> 
> 
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-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR