Starting with LANDLOCK_ACCESS_FS_TRUNCATE, it is worth explaining why we choose to restrict access checks at open time. This new "File descriptor access rights" section is complementary to the existing "Inode access rights" section. Add a new guiding principle related to this section. Cc: Günther Noack <gnoack3000@xxxxxxxxx> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@xxxxxxxxxxx --- Changes since v1: https://lore.kernel.org/r/20221205112621.3530557-1-mic@xxxxxxxxxxx * Reworded the new section based on Günther suggestions. * Added a new guiding principle. * Update date. --- Documentation/security/landlock.rst | 33 ++++++++++++++++++++++++++--- 1 file changed, 30 insertions(+), 3 deletions(-) diff --git a/Documentation/security/landlock.rst b/Documentation/security/landlock.rst index c0029d5d02eb..95a0e4726dc5 100644 --- a/Documentation/security/landlock.rst +++ b/Documentation/security/landlock.rst @@ -7,7 +7,7 @@ Landlock LSM: kernel documentation ================================== :Author: Mickaël Salaün -:Date: September 2022 +:Date: December 2022 Landlock's goal is to create scoped access-control (i.e. sandboxing). To harden a whole system, this feature should be available to any process, @@ -41,12 +41,15 @@ Guiding principles for safe access controls processes. * Computation related to Landlock operations (e.g. enforcing a ruleset) shall only impact the processes requesting them. +* Resources (e.g. file descriptors) directly obtained from the kernel by a + sandboxed process shall retain their scoped accesses whatever process use + them. Cf. `File descriptor access rights`_. Design choices ============== -Filesystem access rights ------------------------- +Inode access rights +------------------- All access rights are tied to an inode and what can be accessed through it. Reading the content of a directory does not imply to be allowed to read the @@ -57,6 +60,30 @@ directory, not the unlinked inode. This is the reason why ``LANDLOCK_ACCESS_FS_REMOVE_FILE`` or ``LANDLOCK_ACCESS_FS_REFER`` are not allowed to be tied to files but only to directories. +File descriptor access rights +----------------------------- + +Access rights are checked and tied to file descriptors at open time. The +underlying principle is that equivalent sequences of operations should lead to +the same results, when they are executed under the same Landlock domain. + +Taking the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right as an example, it may be +allowed to open a file for writing without being allowed to +:manpage:`ftruncate` the resulting file descriptor if the related file +hierarchy doesn't grant such access right. The following sequences of +operations have the same semantic and should then have the same result: + +* ``truncate(path);`` +* ``int fd = open(path, O_WRONLY); ftruncate(fd); close(fd);`` + +Similarly to file access modes (e.g. ``O_RDWR``), Landlock access rights +attached to file descriptors are retained even if they are passed between +processes (e.g. through a Unix domain socket). Such access rights will then be +enforced even if the receiving process is not sandboxed by Landlock. Indeed, +this is required to keep a consistent access control over the whole system, and +avoid unattended bypasses through file descriptor passing (i.e. confused deputy +attack). + Tests ===== base-commit: 0b4ab8cd635e8b21e42c14b9e4810ca701babd11 -- 2.38.1
