On Fri, Jun 20, 2025 at 07:03:30AM +0000, Shivank Garg wrote:
> Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create
> anonymous inodes with proper security context. This replaces the current
> pattern of calling alloc_anon_inode() followed by
> inode_init_security_anon() for creating security context manually.
>
> This change also fixes a security regression in secretmem where the
> S_PRIVATE flag was not cleared after alloc_anon_inode(), causing
> LSM/SELinux checks to be bypassed for secretmem file descriptors.
>
> As guest_memfd currently resides in the KVM module, we need to export this
> symbol for use outside the core kernel. In the future, guest_memfd might be
> moved to core-mm, at which point the symbols no longer would have to be
> exported. When/if that happens is still unclear.
>
> Fixes: 2bfe15c52612 ("mm: create security context for memfd_secret inodes")
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Mike Rapoport <rppt@xxxxxxxxxx>
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> ---
> The handling of the S_PRIVATE flag for these inodes was discussed
> extensively ([1], [2], [3]).
>
> As per discussion [3] with Mike and Paul, KVM guest_memfd and secretmem
> result in user-visible file descriptors, so they should be subject to
> LSM/SELinux security policies rather than bypassing them with S_PRIVATE.
>
> [1] https://lore.kernel.org/all/b9e5fa41-62fd-4b3d-bb2d-24ae9d3c33da@xxxxxxxxxx
> [2] https://lore.kernel.org/all/cover.1748890962.git.ackerleytng@xxxxxxxxxx
> [3] https://lore.kernel.org/all/aFOh8N_rRdSi_Fbc@xxxxxxxxxx
>
> V1->V2: Use EXPORT_SYMBOL_GPL_FOR_MODULES() since KVM is the only user.
>
> fs/anon_inodes.c | 23 ++++++++++++++++++-----
> include/linux/fs.h | 2 ++
> mm/secretmem.c | 9 +--------
> 3 files changed, 21 insertions(+), 13 deletions(-)
>
> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index e51e7d88980a..1d847a939f29 100644
> --- a/fs/anon_inodes.c
> +++ b/fs/anon_inodes.c
> @@ -98,14 +98,25 @@ static struct file_system_type anon_inode_fs_type = {
> .kill_sb = kill_anon_super,
> };
>
> -static struct inode *anon_inode_make_secure_inode(
> - const char *name,
> - const struct inode *context_inode)
> +/**
> + * anon_inode_make_secure_inode - allocate an anonymous inode with security context
> + * @sb: [in] Superblock to allocate from
> + * @name: [in] Name of the class of the newfile (e.g., "secretmem")
> + * @context_inode:
> + * [in] Optional parent inode for security inheritance
> + *
> + * The function ensures proper security initialization through the LSM hook
> + * security_inode_init_security_anon().
> + *
> + * Return: Pointer to new inode on success, ERR_PTR on failure.
> + */
> +struct inode *anon_inode_make_secure_inode(struct super_block *sb, const char *name,
> + const struct inode *context_inode)
> {
> struct inode *inode;
> int error;
>
> - inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> + inode = alloc_anon_inode(sb);
> if (IS_ERR(inode))
> return inode;
> inode->i_flags &= ~S_PRIVATE;
> @@ -118,6 +129,7 @@ static struct inode *anon_inode_make_secure_inode(
> }
> return inode;
> }
> +EXPORT_SYMBOL_GPL_FOR_MODULES(anon_inode_make_secure_inode, "kvm");
>
> static struct file *__anon_inode_getfile(const char *name,
> const struct file_operations *fops,
> @@ -132,7 +144,8 @@ static struct file *__anon_inode_getfile(const char *name,
> return ERR_PTR(-ENOENT);
>
> if (make_inode) {
> - inode = anon_inode_make_secure_inode(name, context_inode);
> + inode = anon_inode_make_secure_inode(anon_inode_mnt->mnt_sb,
> + name, context_inode);
> if (IS_ERR(inode)) {
> file = ERR_CAST(inode);
> goto err;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b085f161ed22..040c0036320f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3608,6 +3608,8 @@ extern int simple_write_begin(struct file *file, struct address_space *mapping,
> extern const struct address_space_operations ram_aops;
> extern int always_delete_dentry(const struct dentry *);
> extern struct inode *alloc_anon_inode(struct super_block *);
> +struct inode *anon_inode_make_secure_inode(struct super_block *sb, const char *name,
> + const struct inode *context_inode);
> extern int simple_nosetlease(struct file *, int, struct file_lease **, void **);
> extern const struct dentry_operations simple_dentry_operations;
>
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 589b26c2d553..9a11a38a6770 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -195,18 +195,11 @@ static struct file *secretmem_file_create(unsigned long flags)
> struct file *file;
> struct inode *inode;
> const char *anon_name = "[secretmem]";
> - int err;
>
> - inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
> + inode = anon_inode_make_secure_inode(secretmem_mnt->mnt_sb, anon_name, NULL);
> if (IS_ERR(inode))
> return ERR_CAST(inode);
>
> - err = security_inode_init_security_anon(inode, &QSTR(anon_name), NULL);
> - if (err) {
> - file = ERR_PTR(err);
> - goto err_free_inode;
> - }
> -
> file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
> O_RDWR, &secretmem_fops);
> if (IS_ERR(file))
> --
> 2.43.0
>
--
Sincerely yours,
Mike.
