Re: [PATCH v3] fs/kernfs: null-ptr deref in simple_xattrs_free()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Dec 16, 2025 at 09:43:35AM -0700, Will Rosenberg wrote:
> There exists a null pointer dereference in simple_xattrs_free() as
> part of the __kernfs_new_node() routine. Within __kernfs_new_node(),
> err_out4 calls simple_xattr_free(), but kn->iattr may be NULL if
> __kernfs_setattr() was never called. As a result, the first argument to
> simple_xattrs_free() may be NULL + 0x38, and no NULL check is done
> internally, causing an incorrect pointer dereference.
> 
> Add a check to ensure kn->iattr is not NULL, meaning __kernfs_setattr()
> has been called and kn->iattr is allocated. Note that struct kernfs_node
> kn is allocated with kmem_cache_zalloc, so we can assume kn->iattr will
> be NULL if not allocated.
> 
> An alternative fix could be to not call simple_xattr_free() at all. As
> was previously discussed during the initial patch, simple_xattr_free()
> is not strictly needed and is included to be consistent with
> kernfs_free_rcu(), which also helps the function maintain correctness if
> changes are made in __kernfs_new_node().
> 
> Reported-by: syzbot+6aaf7f48ae034ab0ea97@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=6aaf7f48ae034ab0ea97
> Fixes: 382b1e8f30f7 ("kernfs: fix memory leak of kernfs_iattrs in __kernfs_new_node")
> Cc: stable@vger.kernel.org
> Signed-off-by: Will Rosenberg <whrosenb@asu.edu>
> ---
>  fs/kernfs/dir.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 5c0efd6b239f..29baeeb97871 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -681,8 +681,10 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
>  	return kn;
>  
>   err_out4:
> -	simple_xattrs_free(&kn->iattr->xattrs, NULL);
> -	kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
> +	if (kn->iattr) {
> +		simple_xattrs_free(&kn->iattr->xattrs, NULL);
> +		kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
> +	}
>   err_out3:
>  	spin_lock(&root->kernfs_idr_lock);
>  	idr_remove(&root->ino_idr, (u32)kernfs_ino(kn));
> 
> base-commit: d358e5254674b70f34c847715ca509e46eb81e6f
> -- 
> 2.34.1
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
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- This looks like a new version of a previously submitted patch, but you
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  Please read the section entitled "The canonical patch format" in the
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  needs to be done here to properly describe this.

If you wish to discuss this problem further, or you have questions about
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thanks,

greg k-h's patch email bot