Re: [PATCH] sysfs: prevent writing excessively large files

[Greg KH]

On Tue, Sep 15, 2026 at 08:20:17PM +0800, Edward Adam Davis wrote:
> Since atomic_write_len is not configured for sysfs_file_kfops_rw, a large
> file write via sysfs_kf_write() may result in an out-of-bounds read when
> checking for the null terminator of a string element in the kobject_actions
> array within kobject_action_type(), potentially hitting:
> 
> BUG: KASAN: global-out-of-bounds in kobject_action_type lib/kobject_uevent.c:86 [inline]
> BUG: KASAN: global-out-of-bounds in kobject_synth_uevent+0x79d/0x7d0 lib/kobject_uevent.c:200
> Read of size 1 at addr ffffffff8d72559f by task syz.0.17/5917
> Call Trace:
>  kobject_action_type lib/kobject_uevent.c:86 [inline]
>  kobject_synth_uevent+0x79d/0x7d0 lib/kobject_uevent.c:200
>  bus_uevent_store+0x3d/0x90 drivers/base/bus.c:917
>  bus_attr_store+0x74/0xb0 drivers/base/bus.c:172
>  sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:145
>  kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:345
>  new_sync_write fs/read_write.c:595 [inline]
>  vfs_write+0x6af/0x1050 fs/read_write.c:687
> 
> Add atomic_write_len for sysfs_file_kfops_rw and sysfs_file_kfops_wo
> properly.
> 
> Fixes: f6acf8bb6a40 ("sysfs, kernfs: introduce kernfs_ops")
> Reported-by: syzbot+9a321aea9d851b299486@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=9a321aea9d851b299486
> Tested-by: syzbot+9a321aea9d851b299486@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxxxx>
> ---
>  fs/sysfs/file.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index cd5bb0f9fee6..a63130d18680 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -228,10 +228,12 @@ static const struct kernfs_ops sysfs_file_kfops_ro = {
>  };
>  
>  static const struct kernfs_ops sysfs_file_kfops_wo = {
> +	.atomic_write_len	= PAGE_SIZE,
>  	.write		= sysfs_kf_write,
>  };
>  
>  static const struct kernfs_ops sysfs_file_kfops_rw = {
> +	.atomic_write_len	= PAGE_SIZE,
>  	.seq_show	= sysfs_kf_seq_show,
>  	.write		= sysfs_kf_write,
>  };
> -- 
> 2.43.0
> 

Are you sure this will not break those sysfs files that want larger page
sizes?  Given the age of this "issue" it's really worrying to me to
change it now...

thanks,

greg k-h