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Signed-off-by: G=C3=BCnther Noack --- Documentation/userspace-api/landlock.rst | 121 ++++++++++++++++++++--- 1 file changed, 106 insertions(+), 15 deletions(-) diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/users= pace-api/landlock.rst index 2e3822677061..a6e55912139b 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -75,7 +75,8 @@ 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, .handled_access_net =3D LANDLOCK_ACCESS_NET_BIND_TCP | LANDLOCK_ACCESS_NET_CONNECT_TCP, @@ -84,10 +85,10 @@ 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 @@ -113,6 +114,10 @@ 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 for ABI < 5 */ + ruleset_attr.handled_access_fs &=3D ~LANDLOCK_ACCESS_FS_IOCTL; } =20 This enables to create an inclusive ruleset that will contain our rules. @@ -224,6 +229,7 @@ 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, @@ -317,18 +323,72 @@ 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`` rights is associated with the newly created f= ile +descriptor and will be used for subsequent truncation and ioctl attempts u= sing +:manpage:`ftruncate(2)` and :manpage:`ioctl(2)`. The behavior is similar = to +opening a file for reading or writing, where permissions are checked durin= g +: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. + +Restricting IOCTL commands +-------------------------- + +When the ``LANDLOCK_ACCESS_FS_IOCTL`` right is handled, Landlock will rest= rict +the invocation of IOCTL commands. However, to *allow* these IOCTL command= s +again, some of these IOCTL commands are then granted through other, preexi= sting +access rights. + +For example, consider a program which handles ``LANDLOCK_ACCESS_FS_IOCTL``= and +``LANDLOCK_ACCESS_FS_READ_FILE``. The program *allows* +``LANDLOCK_ACCESS_FS_READ_FILE`` on a file ``foo.log``. + +By virtue of granting this access on the ``foo.log`` file, it is now possi= ble to +use common and harmless IOCTL commands which are useful when reading files= , such +as ``FIONREAD``. + +When both ``LANDLOCK_ACCESS_FS_IOCTL`` and other access rights are +handled in the ruleset, these other access rights may start governing +the use of individual IOCTL commands instead of +``LANDLOCK_ACCESS_FS_IOCTL``. For instance, if both +``LANDLOCK_ACCESS_FS_IOCTL`` and ``LANDLOCK_ACCESS_FS_READ_FILE`` are +handled, allowing ``LANDLOCK_ACCESS_FS_READ_FILE`` will make it +possible to use ``FIONREAD`` and other IOCTL commands. + +The following table illustrates how IOCTL attempts for ``FIONREAD`` are +filtered, depending on how a Landlock ruleset handles and allows the +``LANDLOCK_ACCESS_FS_IOCTL`` and ``LANDLOCK_ACCESS_FS_READ_FILE`` rights: + ++-------------------------+--------------+--------------+--------------+ +| | ``FS_IOCTL`` | ``FS_IOCTL`` | ``FS_IOCTL`` | +| | not handled | handled and | handled and | +| | | allowed | not allowed | ++-------------------------+--------------+--------------+--------------+ +| ``FS_READ_FILE`` | allow | allow | deny | +| not handled | | | | ++-------------------------+ +--------------+--------------+ +| ``FS_READ_FILE`` | | allow | +| handled and allowed | | | ++-------------------------+ +-----------------------------+ +| ``FS_READ_FILE`` | | deny | +| handled and not allowed | | | ++-------------------------+--------------+-----------------------------+ + +The full list of IOCTL commands and the access rights which affect them is +documented below. =20 Compatibility =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D @@ -457,6 +517,27 @@ 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`` right restricts the use of :manpage:`ioct= l(2)`, +but it only applies to newly opened files. This means specifically that +pre-existing file descriptors like stdin, stdout and 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. It is therefore recommended to close inherited TTY file descrip= tors, +or to reopen them from ``/proc/self/fd/*`` without the +``LANDLOCK_ACCESS_FS_IOCTL`` right, if possible. The :manpage:`isatty(3)` +function checks whether a given file descriptor is a TTY. + +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`` right on files where it is really harmless. = In +cases where you can control the mounts, the ``nodev`` mount option can hel= p to +rule out that device files can be accessed. + Previous limitations =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 @@ -494,6 +575,16 @@ 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`` access righ= t. + .. _kernel_support: =20 Kernel support --=20 2.43.0.687.g38aa6559b0-goog