Re: [PATCH] nfsd: fix NULL pointer dereference in check_export()

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2026-01-16 at 13:22 -0500, Jeff Layton wrote:
> Given the right export table, it's possible to trigger a NULL pointer
> dereference when mountd sends a path that has no export operations.
> Check that the export_ops are set and just return -EINVAL if not.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> Triggering this required a rather pathological export table (I just
> exported "/"). Given that, I'm on the fence as to whether we want to
> send this to stable.
> ---
>  fs/nfsd/export.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index 2a1499f2ad196a6033787260881e451146283bdc..4187c109d84985d33a69e19291edbf2b27b257d8 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -405,6 +405,7 @@ static struct svc_export *svc_export_lookup(struct svc_export *);
>  static int check_export(const struct path *path, int *flags, unsigned char *uuid)
>  {
>  	struct inode *inode = d_inode(path->dentry);
> +	struct export_operations *export_op = inode->i_sb->s_export_op;
>  
>  	/*
>  	 * We currently export only dirs, regular files, and (for v4
> @@ -422,14 +423,20 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
>  	if (*flags & NFSEXP_V4ROOT)
>  		*flags |= NFSEXP_READONLY;
>  
> -	/* There are two requirements on a filesystem to be exportable.
> -	 * 1:  We must be able to identify the filesystem from a number.
> +	/* There are four requirements on a filesystem to be exportable:
> +	 * 1: It must define sb->s_export_op
> +	 * 2: We must be able to identify the filesystem from a number.
>  	 *       either a device number (so FS_REQUIRES_DEV needed)
>  	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
> -	 * 2:  We must be able to find an inode from a filehandle.
> +	 * 3: We must be able to find an inode from a filehandle.
>  	 *       This means that s_export_op must be set.
> -	 * 3: We must not currently be on an idmapped mount.
> +	 * 4: We must not currently be on an idmapped mount.
>  	 */
> +	if (!export_op) {
> +		dprintk("%s: fs doesn't define export_operations!\n", __func__);
> +		return -EINVAL;
> +	}
> +
>  	if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
>  	    !(*flags & NFSEXP_FSID) &&
>  	    uuid == NULL) {
> @@ -437,7 +444,7 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
>  		return -EINVAL;
>  	}
>  
> -	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
> +	if (!exportfs_can_decode_fh(export_op)) {

Hrm, actually exportfs_can_decode_fh() already checks this. You can
disregard this patch. 
-- 
Jeff Layton <jlayton@kernel.org>