KVM guest_memfd is implementing its own inodes to store metadata for backing memory using a custom filesystem. This requires the ability to allocate an anonymous inode with security context using anon_inode_make_secure_inode().
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. Signed-off-by: Shivank Garg <shiva...@amd.com> --- The handling of the S_PRIVATE flag for these inodes was discussed extensively ([1], [2]) My understanding [3], is that because KVM guest_memfd and secretmem results in user-visible file descriptors, its inodes should not bypass LSM security checks. Therefore, anon_inode_make_secure_inode() (as implemented in this patch) correctly clears the S_PRIVATE flag set by alloc_anon_inode() to ensure proper security policy enforcement. [1] https://lore.kernel.org/all/b9e5fa41-62fd-4b3d-bb2d-24ae9d3c3...@redhat.com [2] https://lore.kernel.org/all/cover.1748890962.git.ackerley...@google.com [3] https://lore.kernel.org/all/647ab7a4-790f-4858-acf2-0f6bae5b7...@amd.com fs/anon_inodes.c | 20 +++++++++++++++++--- include/linux/fs.h | 2 ++ 2 files changed, 19 insertions(+), 3 deletions(-) diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c index e51e7d88980a..441fff40b55a 100644 --- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -98,14 +98,26 @@ static struct file_system_type anon_inode_fs_type = { .kill_sb = kill_anon_super, }; -static struct inode *anon_inode_make_secure_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 +130,7 @@ static struct inode *anon_inode_make_secure_inode( } return inode; } +EXPORT_SYMBOL_GPL(anon_inode_make_secure_inode); static struct file *__anon_inode_getfile(const char *name, const struct file_operations *fops, @@ -132,7 +145,8 @@ 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 96c7925a6551..7ba45be0d7a0 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -3604,6 +3604,8 @@ 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 *); +extern 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; -- 2.43.0
