On 19/03/2021 20:19, Mickaël Salaün wrote: > > On 19/03/2021 19:57, Kees Cook wrote: >> On Tue, Mar 16, 2021 at 09:42:47PM +0100, Mickaël Salaün wrote: >>> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >>> >>> Using Landlock objects and ruleset, it is possible to tag inodes >>> according to a process's domain. To enable an unprivileged process to >>> express a file hierarchy, it first needs to open a directory (or a file) >>> and pass this file descriptor to the kernel through >>> landlock_add_rule(2). When checking if a file access request is >>> allowed, we walk from the requested dentry to the real root, following >>> the different mount layers. The access to each "tagged" inodes are >>> collected according to their rule layer level, and ANDed to create >>> access to the requested file hierarchy. This makes possible to identify >>> a lot of files without tagging every inodes nor modifying the >>> filesystem, while still following the view and understanding the user >>> has from the filesystem. >>> >>> Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not >>> keep the same struct inodes for the same inodes whereas these inodes are >>> in use. >>> >>> This commit adds a minimal set of supported filesystem access-control >>> which doesn't enable to restrict all file-related actions. This is the >>> result of multiple discussions to minimize the code of Landlock to ease >>> review. Thanks to the Landlock design, extending this access-control >>> without breaking user space will not be a problem. Moreover, seccomp >>> filters can be used to restrict the use of syscall families which may >>> not be currently handled by Landlock. >>> >>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> >>> Cc: Anton Ivanov <anton.ivanov@xxxxxxxxxxxxxxxxxx> >>> Cc: James Morris <jmorris@xxxxxxxxx> >>> Cc: Jann Horn <jannh@xxxxxxxxxx> >>> Cc: Jeff Dike <jdike@xxxxxxxxxxx> >>> Cc: Kees Cook <keescook@xxxxxxxxxxxx> >>> Cc: Richard Weinberger <richard@xxxxxx> >>> Cc: Serge E. Hallyn <serge@xxxxxxxxxx> >>> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >>> Link: https://lore.kernel.org/r/20210316204252.427806-8-mic@xxxxxxxxxxx >>> [...] >>> + spin_lock(&sb->s_inode_list_lock); >>> + list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { >>> + struct landlock_object *object; >>> + >>> + /* Only handles referenced inodes. */ >>> + if (!atomic_read(&inode->i_count)) >>> + continue; >>> + >>> + /* >>> + * Checks I_FREEING and I_WILL_FREE to protect against a race >>> + * condition when release_inode() just called iput(), which >>> + * could lead to a NULL dereference of inode->security or a >>> + * second call to iput() for the same Landlock object. Also >>> + * checks I_NEW because such inode cannot be tied to an object. >>> + */ >>> + spin_lock(&inode->i_lock); >>> + if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) { >>> + spin_unlock(&inode->i_lock); >>> + continue; >>> + } >> >> This (and elsewhere here) seems like a lot of inode internals getting >> exposed. Can any of this be repurposed into helpers? I see this test >> scattered around the kernel a fair bit: >> >> $ git grep I_FREEING | grep I_WILL_FREE | grep I_NEW | wc -l >> 9 > > Dealing with the filesystem is complex. Some helpers could probably be > added, but with a series dedicated to the filesystem. I can work on that > once this series is merged. > >> >>> +static inline u32 get_mode_access(const umode_t mode) >>> +{ >>> + switch (mode & S_IFMT) { >>> + case S_IFLNK: >>> + return LANDLOCK_ACCESS_FS_MAKE_SYM; >>> + case 0: >>> + /* A zero mode translates to S_IFREG. */ >>> + case S_IFREG: >>> + return LANDLOCK_ACCESS_FS_MAKE_REG; >>> + case S_IFDIR: >>> + return LANDLOCK_ACCESS_FS_MAKE_DIR; >>> + case S_IFCHR: >>> + return LANDLOCK_ACCESS_FS_MAKE_CHAR; >>> + case S_IFBLK: >>> + return LANDLOCK_ACCESS_FS_MAKE_BLOCK; >>> + case S_IFIFO: >>> + return LANDLOCK_ACCESS_FS_MAKE_FIFO; >>> + case S_IFSOCK: >>> + return LANDLOCK_ACCESS_FS_MAKE_SOCK; >>> + default: >>> + WARN_ON_ONCE(1); >>> + return 0; >>> + } >> >> I'm assuming this won't be reachable from userspace. > > It should not, only a bogus kernel code could. > >> >>> [...] >>> index a5d6ef334991..f8e8e980454c 100644 >>> --- a/security/landlock/setup.c >>> +++ b/security/landlock/setup.c >>> @@ -11,17 +11,24 @@ >>> >>> #include "common.h" >>> #include "cred.h" >>> +#include "fs.h" >>> #include "ptrace.h" >>> #include "setup.h" >>> >>> +bool landlock_initialized __lsm_ro_after_init = false; >>> + >>> struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = { >>> .lbs_cred = sizeof(struct landlock_cred_security), >>> + .lbs_inode = sizeof(struct landlock_inode_security), >>> + .lbs_superblock = sizeof(struct landlock_superblock_security), >>> }; >>> >>> static int __init landlock_init(void) >>> { >>> landlock_add_cred_hooks(); >>> landlock_add_ptrace_hooks(); >>> + landlock_add_fs_hooks(); >>> + landlock_initialized = true; >> >> I think this landlock_initialized is logically separate from the optional >> DEFINE_LSM "enabled" variable, but I thought I'd double check. :) > > An LSM can be marked as enabled (at boot) but not yet initialized. > >> >> It seems like it's used here to avoid releasing superblocks before >> landlock_init() is called? What is the scenario where that happens? > > It is a condition for LSM hooks, syscalls and superblock management. > >> >>> pr_info("Up and running.\n"); >>> return 0; >>> } >>> diff --git a/security/landlock/setup.h b/security/landlock/setup.h >>> index 9fdbf33fcc33..1daffab1ab4b 100644 >>> --- a/security/landlock/setup.h >>> +++ b/security/landlock/setup.h >>> @@ -11,6 +11,8 @@ >>> >>> #include <linux/lsm_hooks.h> >>> >>> +extern bool landlock_initialized; >>> + >>> extern struct lsm_blob_sizes landlock_blob_sizes; >>> >>> #endif /* _SECURITY_LANDLOCK_SETUP_H */ >>> -- >>> 2.30.2 >>> >> >> The locking and inode semantics are pretty complex, but since, again, >> it's got significant test and syzkaller coverage, it looks good to me. >> >> With the inode helper cleanup: I think the inode helper would have to be in a separate patch focused on fs/ (like all matches of your greps, except Landlock). Are you OK if I send a patch for that once Landlock is merged? >> >> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> >>
