The problem is that a pathname can contain absolute symlinks and now
they are resolved relative to the current root.
It may be a problem if you want to open a file in another namespace.
For example, you open /proc/PID/root for a process from the target
namespace and then you use openat() to open a file from this namespace.
If a path to the file contains an absolute symlink, you will open a file
from the current namespace, because a symlink will be resolved relative
to the current root.
A proposed solution adds a new flag which means that dirfd should be
set as a root for a current system call (openat(), statat(), etc).
Here are examples how we can open a file in a contex of another process.
How we can do this without these changes:
old_root = open("/", O_PATH);
old_cwd = open(".", O_PATH);
chroot("/proc/PID/root");
fd = open(pathname, O_RDONLY);
fchdir(old_root); /* emulate fchroot() */
chroot(".");
fchdir(old_cwd);
close(old_cwd);
close(old_root);
How this code is simplified with new flags:
dirfd = open("/proc/PID/root", O_PATH);
fd = open(dirfd, pathname, O_RDONLY | O_ATROOT);
close(dirfd);
One more thing is that chroot isn't available for unprivileged users.
We met this problem, when we tryed to dump an ubuntu container and
failed to resolve /proc/PID/root/var/run/mysqld/mysqld.sock, because
/var/run was a symlink to /run.
Changes since the first version:
- change a value of O_ATROOT to not intersect with other constants.
Changes since the second version:
- initialize nd->root_seq (thanks to Omar Sandoval for reporting and
fixing this issue)
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: "J. Bruce Fields" <bfields@xxxxxxxxxx>
Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Cc: NeilBrown <neilb@xxxxxxx>
Cc: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
Cc: Omar Sandoval <osandov@xxxxxxxxxxx>
Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
Andrey Vagin (3):
namei: add LOOKUP_DFD_ROOT to use dfd as root
fs: allow to use dirfd as root for openat and other *at syscalls
selftests: check O_ATROOT and AT_FDROOT flags
fs/exec.c | 4 +-
fs/namei.c | 42 +++++++++++----
fs/open.c | 6 ++-
fs/stat.c | 4 +-
fs/utimes.c | 4 +-
include/linux/namei.h | 2 +
include/uapi/asm-generic/fcntl.h | 4 ++
include/uapi/linux/fcntl.h | 1 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/lookup/.gitignore | 1 +
tools/testing/selftests/lookup/Makefile | 8 +++
tools/testing/selftests/lookup/lookup_at_root.c | 71 +++++++++++++++++++++++++
tools/testing/selftests/lookup/run.sh | 14 +++++
13 files changed, 148 insertions(+), 14 deletions(-)
create mode 100644 tools/testing/selftests/lookup/.gitignore
create mode 100644 tools/testing/selftests/lookup/Makefile
create mode 100644 tools/testing/selftests/lookup/lookup_at_root.c
create mode 100755 tools/testing/selftests/lookup/run.sh
--
2.5.5
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