Re: [PATCH v1] NFS: Fix possible NULL pointer dereference in nfs_inode_remove_request()

[Trond Myklebust <trondmy@kernel.org>]

On Sun, 2025-10-12 at 16:39 +0800, Baolin Liu wrote:
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> 
> From: Baolin Liu <liubaolin@kylinos.cn>
> 
> nfs_page_to_folio(req->wb_head) may return NULL in certain
> conditions,
> but the function dereferences folio->mapping and calls
> folio_end_dropbehind(folio) unconditionally. This may cause a NULL
> pointer dereference crash.
> 
> Fix this by checking folio before using it or calling
> folio_end_dropbehind().
> 
> Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
> ---
>  fs/nfs/write.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index 0fb6905736d5..e148308c1923 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -739,17 +739,18 @@ static void nfs_inode_remove_request(struct
> nfs_page *req)
>         nfs_page_group_lock(req);
>         if (nfs_page_group_sync_on_bit_locked(req, PG_REMOVE)) {
>                 struct folio *folio = nfs_page_to_folio(req-
> >wb_head);
> -               struct address_space *mapping = folio->mapping;
> 
> -               spin_lock(&mapping->i_private_lock);
>                 if (likely(folio)) {
> +                       struct address_space *mapping = folio-
> >mapping;
> +
> +                       spin_lock(&mapping->i_private_lock);
>                         folio->private = NULL;
>                         folio_clear_private(folio);
>                         clear_bit(PG_MAPPED, &req->wb_head-
> >wb_flags);
> -               }
> -               spin_unlock(&mapping->i_private_lock);
> +                       spin_unlock(&mapping->i_private_lock);
> 
> -               folio_end_dropbehind(folio);
> +                       folio_end_dropbehind(folio);
> +               }
>         }
>         nfs_page_group_unlock(req);
> 
> --
> 2.39.2
> 

What reason is there to believe that we can ever call
nfs_inode_remove_request() with a NULL value for req->wb_head-
>wb_folio, or even with a NULL value for req->wb_head->wb_folio-
>mapping?


-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com