[PATCH 03/46] mm/memfd: check for real shmem inodes before accessing shmem state

[Mike Rapoport]

On Tue, Oct 27, 2026 at 12:17:23AM +0100, David Hildenbrand (Arm) wrote:
> shmem_file() only checks a file's backing mapping. That mapping can belong
> to another file, as with a Coda file backed by a tmpfs container after
> mmap(). It does not establish that file_inode(file) is a shmem inode.
> 
> Seal handling and memfd preservation use this check before accessing
> shmem inode state, so they can access an inode of the wrong type.
> 
> Add file_inode_is_shmem() to check the inode's file operations in both
> paths. This identifies regular shmem inodes, including those used by
> memfds and ordinary tmpfs files, independently of wrapper file operations.
> 
> Fixes: c49fcfcda8b5 ("mm/shmem: update file sealing comments and file checking")
> Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
> Cc: stable at vger.kernel.org
> Reported-by: Chris Mason <mason at kernel.org>
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Reviewed-by: Mike Rapoport (Microsoft) <rppt at kernel.org>

With a small nit below

> ---
>  include/linux/shmem_fs.h | 6 ++++++
>  mm/memfd.c               | 8 +++++---
>  mm/memfd_luo.c           | 2 +-
>  mm/shmem.c               | 5 +++++
>  4 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index a7c7a96a7cbf9..2ceca7993e4f5 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -117,8 +117,14 @@ extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
>  		unsigned long len, unsigned long pgoff, unsigned long flags);
>  extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);
>  #ifdef CONFIG_SHMEM
> +bool file_inode_is_shmem(const struct inode *inode);

Do we really need the file_ prefix in this name?

>  bool shmem_mapping(const struct address_space *mapping);

-- 
Sincerely yours,
Mike.