[PATCH v2] landlock: Explain file descriptor access rights

[Mickaël Salaün]

On 12/12/2022 08:39, Günther Noack wrote:
> On Fri, Dec 09, 2022 at 08:38:13PM +0100, Mickaël Salaün wrote:
>> 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 at gmail.com>
>> Signed-off-by: Mickaël Salaün <mic at digikod.net>
>> Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@digikod.net
>> ---
>>
>> Changes since v1:
>> https://lore.kernel.org/r/20221205112621.3530557-1-mic@digikod.net
>> * 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
> 
> Optional nit: Maybe add "at the time of resource acquisition" to stress that part?

I included this suggestion in -next: 
https://git.kernel.org/mic/c/4dd6da345ac2

Thanks!

> 
>> +  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
>>
> 
> Reviewed-by: Günther Noack <gnoack3000 at gmail.com>
> 
> Thank you!
>