Re: [PATCH V2] fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

[Mike Rapoport <rppt@kernel.org>]

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@redhat.com>
> Suggested-by: Mike Rapoport <rppt@kernel.org>
> Signed-off-by: Shivank Garg <shivankg@amd.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> 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@redhat.com
> [2] https://lore.kernel.org/all/cover.1748890962.git.ackerleytng@google.com
> [3] https://lore.kernel.org/all/aFOh8N_rRdSi_Fbc@kernel.org
> 
> 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.