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Signed-off-by: G=C3=BCnther Noack --- Documentation/userspace-api/landlock.rst | 76 +++++++++++++++++++----- 1 file changed, 61 insertions(+), 15 deletions(-) diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/users= pace-api/landlock.rst index 838cc27db232..32391247f19a 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -76,7 +76,8 @@ to be explicit about the denied-by-default access rights. LANDLOCK_ACCESS_FS_MAKE_BLOCK | LANDLOCK_ACCESS_FS_MAKE_SYM | LANDLOCK_ACCESS_FS_REFER | - LANDLOCK_ACCESS_FS_TRUNCATE, + LANDLOCK_ACCESS_FS_TRUNCATE | + LANDLOCK_ACCESS_FS_IOCTL_DEV, .handled_access_net =3D LANDLOCK_ACCESS_NET_BIND_TCP | LANDLOCK_ACCESS_NET_CONNECT_TCP, @@ -85,10 +86,10 @@ to be explicit about the denied-by-default access right= s. Because we may not know on which kernel version an application will be executed, it is safer to follow a best-effort security approach. Indeed, = we should try to protect users as much as possible whatever the kernel they a= re -using. To avoid binary enforcement (i.e. either all security features or -none), we can leverage a dedicated Landlock command to get the current ver= sion -of the Landlock ABI and adapt the handled accesses. Let's check if we sho= uld -remove access rights which are only supported in higher versions of the AB= I. +using. + +To be compatible with older Linux versions, we detect the available Landlo= ck ABI +version, and only use the available subset of access rights: =20 .. code-block:: c =20 @@ -114,6 +115,10 @@ remove access rights which are only supported in highe= r versions of the ABI. ruleset_attr.handled_access_net &=3D ~(LANDLOCK_ACCESS_NET_BIND_TCP | LANDLOCK_ACCESS_NET_CONNECT_TCP); + __attribute__((fallthrough)); + case 4: + /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */ + ruleset_attr.handled_access_fs &=3D ~LANDLOCK_ACCESS_FS_IOCTL_DEV; } =20 This enables to create an inclusive ruleset that will contain our rules. @@ -225,6 +230,7 @@ access rights per directory enables to change the locat= ion of such directory without relying on the destination directory access rights (except those t= hat are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER`` documentation). + Having self-sufficient hierarchies also helps to tighten the required acce= ss rights to the minimal set of data. This also helps avoid sinkhole directo= ries, i.e. directories where data can be linked to but not linked from. Howeve= r, @@ -318,18 +324,26 @@ It should also be noted that truncating files does no= t require the system call, this can also be done through :manpage:`open(2)` with the fla= gs ``O_RDONLY | O_TRUNC``. =20 -When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE= `` -right is associated with the newly created file descriptor and will be use= d for -subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavio= r is -similar to opening a file for reading or writing, where permissions are ch= ecked -during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)= ` and +The truncate right is associated with the opened file (see below). + +Rights associated with file descriptors +--------------------------------------- + +When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE= `` and +``LANDLOCK_ACCESS_FS_IOCTL_DEV`` rights is associated with the newly creat= ed +file descriptor and will be used for subsequent truncation and ioctl attem= pts +using :manpage:`ftruncate(2)` and :manpage:`ioctl(2)`. The behavior is si= milar +to opening a file for reading or writing, where permissions are checked du= ring +:manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and :manpage:`write(2)` calls. =20 -As a consequence, it is possible to have multiple open file descriptors fo= r the -same file, where one grants the right to truncate the file and the other d= oes -not. It is also possible to pass such file descriptors between processes, -keeping their Landlock properties, even when these processes do not have a= n -enforced Landlock ruleset. +As a consequence, it is possible that a process has multiple open file +descriptors referring to the same file, but Landlock enforces different th= ings +when operating with these file descriptors. This can happen when a Landlo= ck +ruleset gets enforced and the process keeps file descriptors which were op= ened +both before and after the enforcement. It is also possible to pass such f= ile +descriptors between processes, keeping their Landlock properties, even whe= n some +of the involved processes do not have an enforced Landlock ruleset. =20 Compatibility =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D @@ -458,6 +472,28 @@ Memory usage Kernel memory allocated to create rulesets is accounted and can be restric= ted by the Documentation/admin-guide/cgroup-v1/memory.rst. =20 +IOCTL support +------------- + +The ``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right restricts the use of +:manpage:`ioctl(2)`, but it only applies to *newly opened* device files. = This +means specifically that pre-existing file descriptors like stdin, stdout a= nd +stderr are unaffected. + +Users should be aware that TTY devices have traditionally permitted to con= trol +other processes on the same TTY through the ``TIOCSTI`` and ``TIOCLINUX`` = IOCTL +commands. Both of these require ``CAP_SYS_ADMIN`` on modern Linux systems= , but +the behavior is configurable for ``TIOCSTI``. + +On older systems, it is therefore recommended to close inherited TTY file +descriptors, or to reopen them from ``/proc/self/fd/*`` without the +``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right, if possible. + +Landlock's IOCTL support is coarse-grained at the moment, but may become m= ore +fine-grained in the future. Until then, users are advised to establish th= e +guarantees that they need through the file hierarchy, by only allowing the +``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right on files where it is really require= d. + Previous limitations =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 @@ -495,6 +531,16 @@ bind and connect actions to only a set of allowed port= s thanks to the new ``LANDLOCK_ACCESS_NET_BIND_TCP`` and ``LANDLOCK_ACCESS_NET_CONNECT_TCP`` access rights. =20 +IOCTL (ABI < 5) +--------------- + +IOCTL operations could not be denied before the fifth Landlock ABI, so +:manpage:`ioctl(2)` is always allowed when using a kernel that only suppor= ts an +earlier ABI. + +Starting with the Landlock ABI version 5, it is possible to restrict the u= se of +:manpage:`ioctl(2)` using the new ``LANDLOCK_ACCESS_FS_IOCTL_DEV`` access = right. + .. _kernel_support: =20 Kernel support --=20 2.44.0.278.ge034bb2e1d-goog