fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

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>
Link: https://lore.kernel.org/20250620070328.803704-3-shivankg@amd.com
Acked-by: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index e51e7d88980a229fd74a6eb70c128ee5dc0ed8e2..1d847a939f29a41356af3f12e5f61372ec2fb550 100644 (file)
--- 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 4ec77da65f144f0a5c8fa775b2c7e982787380b9..3a8e643c42792e14556684af2429107a76fb3a1c 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3606,6 +3606,8 @@ 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 589b26c2d553c761648c32bcfda27590505f7334..9a11a38a677084f08398caf8de3279ca1f617dec 100644 (file)
--- 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))