The primary motivation for the need for this flag is container runtimes
which have to interact with malicious root filesystems in the host
namespaces. One of the first requirements for a container runtime to be
secure against a malicious rootfs is that they correctly scope symlinks
(that is, they should be scoped as though they are chroot(2)ed into the
container's rootfs) and ".."-style paths. The already-existing AT_XDEV
and AT_NO_PROCLINKS help defend against other potential attacks in a
malicious rootfs scenario.
Currently most container runtimes try to do this resolution in
userspace[1], causing many potential race conditions. In addition, the
"obvious" alternative (actually performing a {ch,pivot_}root(2))
requires a fork+exec which is *very* costly if necessary for every
filesystem operation involving a container.
The most significant change in semantics with AT_THIS_ROOT is that
*at(2) syscalls now no longer have the property that an absolute
pathname causes the dirfd to be ignored completely (if LOOKUP_CHROOT is
specified). The reasoning behind this is that AT_THIS_ROOT necessarily
has to chroot-scope symlinks with absolute paths to dirfd, and so doing
it for the base path seems to be the most consistent behaviour (and also
avoids foot-gunning users who want to chroot-scope paths that might be
absolute).
Currently this is only enabled for the stat(2) and openat(2) family (the
latter has its own flag O_THISROOT with the same semantics). Ideally
this flag would be supported by all *at(2) syscalls, but this will
require adding flags arguments to many of them (and will be done in a
separate patchset).
[1]: https://github.com/cyphar/filepath-securejoin
Cc: Eric Biederman <ebied...@xmission.com>
Cc: Christian Brauner <christ...@brauner.io>
Signed-off-by: Aleksa Sarai <cyp...@cyphar.com>
---
fs/fcntl.c | 2 +-
fs/namei.c | 121 +++++++++++++++++--------------
fs/open.c | 2 +
fs/stat.c | 4 +-
include/linux/fcntl.h | 2 +-
include/linux/namei.h | 1 +
include/uapi/asm-generic/fcntl.h | 3 +
include/uapi/linux/fcntl.h | 2 +
8 files changed, 81 insertions(+), 56 deletions(-)
diff --git a/fs/fcntl.c b/fs/fcntl.c
index e343618736f7..4c36c5b9fdb9 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1031,7 +1031,7 @@ static int __init fcntl_init(void)
* Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
* is defined as O_NONBLOCK on some platforms and not on others.
*/
- BUILD_BUG_ON(25 - 1 /* for O_RDONLY being 0 */ !=
+ BUILD_BUG_ON(26 - 1 /* for O_RDONLY being 0 */ !=
HWEIGHT32(
(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
__FMODE_EXEC | __FMODE_NONOTIFY));
diff --git a/fs/namei.c b/fs/namei.c
index 757dd783771c..1b984f0dbbb4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2193,9 +2193,64 @@ static int link_path_walk(const char *name, struct
nameidata *nd)
}
}
+/*
+ * Configure nd->path based on the nd->dfd. This is only used as part of
+ * path_init().
+ */
+static inline int dirfd_path_init(struct nameidata *nd)
+{
+ if (nd->dfd == AT_FDCWD) {
+ if (nd->flags & LOOKUP_RCU) {
+ struct fs_struct *fs = current->fs;
+ unsigned seq;
+
+ do {
+ seq = read_seqcount_begin(&fs->seq);
+ nd->path = fs->pwd;
+ nd->inode = nd->path.dentry->d_inode;
+ nd->seq =
__read_seqcount_begin(&nd->path.dentry->d_seq);
+ } while (read_seqcount_retry(&fs->seq, seq));
+ } else {
+ get_fs_pwd(current->fs, &nd->path);
+ nd->inode = nd->path.dentry->d_inode;
+ }
+ } else {
+ /* Caller must check execute permissions on the starting path
component */
+ struct fd f = fdget_raw(nd->dfd);
+ struct dentry *dentry;
+
+ if (!f.file)
+ return -EBADF;
+
+ dentry = f.file->f_path.dentry;
+
+ if (*nd->name->name && unlikely(!d_can_lookup(dentry))) {
+ fdput(f);
+ return -ENOTDIR;
+ }
+
+ nd->path = f.file->f_path;
+ if (nd->flags & LOOKUP_RCU) {
+ nd->inode = nd->path.dentry->d_inode;
+ nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
+ } else {
+ path_get(&nd->path);
+ nd->inode = nd->path.dentry->d_inode;
+ }
+ fdput(f);
+ }
+ if (unlikely(nd->flags & (LOOKUP_CHROOT | LOOKUP_BENEATH))) {
+ nd->root = nd->path;
+ if (!(nd->flags & LOOKUP_RCU))
+ path_get(&nd->root);
+ }
+ return 0;
+}
+
/* must be paired with terminate_walk() */
static const char *path_init(struct nameidata *nd, unsigned flags)
{
+ int error;
const char *s = nd->name->name;
if (!*s)
@@ -2230,65 +2285,25 @@ static const char *path_init(struct nameidata *nd,
unsigned flags)
nd->path.dentry = NULL;
nd->m_seq = read_seqbegin(&mount_lock);
+ if (unlikely(flags & LOOKUP_CHROOT)) {
+ error = dirfd_path_init(nd);
+ if (unlikely(error))
+ return ERR_PTR(error);
+ }
if (*s == '/') {
- int error;
- set_root(nd);
+ if (likely(!nd->root.mnt))
+ set_root(nd);
error = nd_jump_root(nd);
if (unlikely(error))
s = ERR_PTR(error);
return s;
- } else if (nd->dfd == AT_FDCWD) {
- if (flags & LOOKUP_RCU) {
- struct fs_struct *fs = current->fs;
- unsigned seq;
-
- do {
- seq = read_seqcount_begin(&fs->seq);
- nd->path = fs->pwd;
- nd->inode = nd->path.dentry->d_inode;
- nd->seq =
__read_seqcount_begin(&nd->path.dentry->d_seq);
- } while (read_seqcount_retry(&fs->seq, seq));
- } else {
- get_fs_pwd(current->fs, &nd->path);
- nd->inode = nd->path.dentry->d_inode;
- }
- if (unlikely(flags & LOOKUP_BENEATH)) {
- nd->root = nd->path;
- if (!(flags & LOOKUP_RCU))
- path_get(&nd->root);
- }
- return s;
- } else {
- /* Caller must check execute permissions on the starting path
component */
- struct fd f = fdget_raw(nd->dfd);
- struct dentry *dentry;
-
- if (!f.file)
- return ERR_PTR(-EBADF);
-
- dentry = f.file->f_path.dentry;
-
- if (*s && unlikely(!d_can_lookup(dentry))) {
- fdput(f);
- return ERR_PTR(-ENOTDIR);
- }
-
- nd->path = f.file->f_path;
- if (flags & LOOKUP_RCU) {
- nd->inode = nd->path.dentry->d_inode;
- nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
- } else {
- path_get(&nd->path);
- nd->inode = nd->path.dentry->d_inode;
- }
- if (unlikely(flags & LOOKUP_BENEATH)) {
- nd->root = nd->path;
- if (!(flags & LOOKUP_RCU))
- path_get(&nd->root);
- }
- fdput(f);
- return s;
}
+ if (likely(!nd->path.mnt)) {
+ error = dirfd_path_init(nd);
+ if (unlikely(error))
+ return ERR_PTR(error);
+ }
+ return s;
}
static const char *trailing_symlink(struct nameidata *nd)
diff --git a/fs/open.c b/fs/open.c
index 80f5f566a5ff..81d148f626cd 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -996,6 +996,8 @@ static inline int build_open_flags(int flags, umode_t mode,
struct open_flags *o
lookup_flags |= LOOKUP_NO_PROCLINKS;
if (flags & O_NOSYMLINKS)
lookup_flags |= LOOKUP_NO_SYMLINKS;
+ if (flags & O_THISROOT)
+ lookup_flags |= LOOKUP_CHROOT;
op->lookup_flags = lookup_flags;
return 0;
}
diff --git a/fs/stat.c b/fs/stat.c
index 791e61b916ae..e8366e4812c3 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -172,7 +172,7 @@ int vfs_statx(int dfd, const char __user *filename, int
flags,
if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT | AT_EMPTY_PATH |
KSTAT_QUERY_FLAGS | AT_BENEATH | AT_XDEV |
- AT_NO_PROCLINKS | AT_NO_SYMLINKS))
+ AT_NO_PROCLINKS | AT_NO_SYMLINKS | AT_THIS_ROOT))
return -EINVAL;
if (flags & AT_SYMLINK_NOFOLLOW)
@@ -189,6 +189,8 @@ int vfs_statx(int dfd, const char __user *filename, int
flags,
lookup_flags |= LOOKUP_NO_PROCLINKS;
if (flags & AT_NO_SYMLINKS)
lookup_flags |= LOOKUP_NO_SYMLINKS;
+ if (flags & AT_THIS_ROOT)
+ lookup_flags |= LOOKUP_CHROOT;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index ad5bba4b5b12..95480cd4c09d 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -10,7 +10,7 @@
O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_BENEATH | O_XDEV | \
- O_NOPROCLINKS | O_NOSYMLINKS)
+ O_NOPROCLINKS | O_NOSYMLINKS | O_THISROOT)
#ifndef force_o_largefile
#define force_o_largefile() (BITS_PER_LONG != 32)
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 5ff7f3362d1b..7ec9e2d84649 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -53,6 +53,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
#define LOOKUP_NO_PROCLINKS 0x040000 /* No /proc/$pid/fd/ "symlink"
crossing. */
#define LOOKUP_NO_SYMLINKS 0x080000 /* No symlink crossing *at all*.
Implies LOOKUP_NO_PROCLINKS. */
+#define LOOKUP_CHROOT 0x100000 /* Treat dirfd as %current->fs->root.
*/
extern int path_pts(struct path *path);
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index c2bf5983e46a..11206b0e927c 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -113,6 +113,9 @@
#ifndef O_NOSYMLINKS
#define O_NOSYMLINKS 01000000000
#endif
+#ifndef O_THISROOT
+#define O_THISROOT 02000000000
+#endif
#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 551a9e2166a8..ea978457b68f 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -99,6 +99,8 @@
#define AT_NO_PROCLINKS 0x40000 /* No /proc/$pid/fd/...
"symlinks". */
#define AT_NO_SYMLINKS 0x80000 /* No symlinks *at all*.
Implies AT_NO_PROCLINKS. */
+#define AT_THIS_ROOT 0x100000 /* Path resolution acts as though
+ it is chroot-ed into dirfd. */
#endif /* _UAPI_LINUX_FCNTL_H */
--
2.19.0
