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The layer masks data structure tracks the requested but unfulfilled
access rights during an operation's security check. It stores one bit
for each combination of access right and layer index. If the bit is
set, that access right is not granted (yet) in the given layer and we
have to traverse the path further upwards to grant it.
Previously, the layer masks were stored as arrays mapping from access
right indices to layer_mask_t. The layer_mask_t value then indicates
all layers in which the given access right is still (tentatively)
denied.
This patch introduces struct layer_access_masks instead: This struct
contains an array with the access_mask_t of each (tentatively) denied
access right in that layer.
The hypothesis of this patch is that this simplifies the code enough
so that the resulting code will run faster:
* We can use bitwise operations in multiple places where we previously
looped over bits individually with macros. (Should require less
branch speculation and lends itself to better loop unrolling.)
* Code is ~75 lines smaller.
Other noteworthy changes:
* In no_more_access(), call a new helper function may_refer(), which
only solves the asymmetric case. Previously, the code interleaved
the checks for the two symmetric cases in RENAME_EXCHANGE. It feels
that the code is clearer when renames without RENAME_EXCHANGE are
more obviously the normal case.
Tradeoffs:
This change improves performance, at a slight size increase to the
layer masks data structure.
This fixes the size of the data structure at 32 bytes for all types of
access rights. (64, once we introduce a 17th filesystem access right).
For filesystem access rights, at the moment, the data structure has
the same size as before, but once we introduce the 17th filesystem
access right, it will double in size (from 32 to 64 bytes), as
access_mask_t grows from 16 to 32 bit [1].
Link: https://lore.kernel.org/all/20260120.haeCh4li9Vae@digikod.net/ [1]
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260206151154.97915-5-gnoack3000@gmail.com
[mic: Cosmetic fixes, moved struct layer_access_masks definition]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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This helper function checks whether an access_mask_t has a subset of the
bits enabled than another one. This expresses the intent a bit smoother
in the code and does not cost us anything when it gets inlined.
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260206151154.97915-4-gnoack3000@gmail.com
[mic: Improve subject]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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Add audit support to the file_truncate and file_ioctl hooks.
Add a deny_masks_t type and related helpers to store the domain's layer
level per optional access rights (i.e. LANDLOCK_ACCESS_FS_TRUNCATE and
LANDLOCK_ACCESS_FS_IOCTL_DEV) when opening a file, which cannot be
inferred later. In practice, the landlock_file_security aligned blob size is
still 16 bytes because this new one-byte deny_masks field follows the
existing two-bytes allowed_access field and precede the packed
fown_subject.
Implementing deny_masks_t with a bitfield instead of a struct enables a
generic implementation to store and extract layer levels.
Add KUnit tests to check the identification of a layer level from a
deny_masks_t, and the computation of a deny_masks_t from an access right
with its layer level or a layer_mask_t array.
Audit event sample:
type=LANDLOCK_DENY msg=audit(1729738800.349:44): domain=195ba459b blockers=fs.ioctl_dev path="/dev/tty" dev="devtmpfs" ino=9 ioctlcmd=0x5401
Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250320190717.2287696-15-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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Upgrade domain's handled access masks when creating a domain from a
ruleset, instead of converting them at runtime. This is more consistent
and helps with audit support.
Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-7-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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Move LANDLOCK_ACCESS_FS_INITIALLY_DENIED, access_mask_t, struct
access_mask, and struct access_masks_all to a dedicated access.h file.
Rename LANDLOCK_ACCESS_FS_INITIALLY_DENIED to
_LANDLOCK_ACCESS_FS_INITIALLY_DENIED to make it clear that it's not part
of UAPI. Add some newlines when appropriate.
This file will be extended with following commits, and it will help to
avoid dependency loops.
Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-6-mic@digikod.net
[mic: Fix rebase conflict because of the new cleanup headers]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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