diff mbox series

[v8,9/9] landlock: Document Landlock's file truncation support

Message ID 20221001154908.49665-10-gnoack3000@gmail.com (mailing list archive)
State Handled Elsewhere
Headers show
Series landlock: truncate support | expand

Commit Message

Günther Noack Oct. 1, 2022, 3:49 p.m. UTC
Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.

Adapt the backwards compatibility example and discussion to remove the
truncation flag where needed.

Point out potential surprising behaviour related to truncate.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 Documentation/userspace-api/landlock.rst | 66 +++++++++++++++++++++---
 1 file changed, 59 insertions(+), 7 deletions(-)

Comments

Mickaël Salaün Oct. 5, 2022, 6:57 p.m. UTC | #1
On 01/10/2022 17:49, Günther Noack wrote:
> Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.
> 
> Adapt the backwards compatibility example and discussion to remove the
> truncation flag where needed.
> 
> Point out potential surprising behaviour related to truncate.
> 
> Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> ---
>   Documentation/userspace-api/landlock.rst | 66 +++++++++++++++++++++---
>   1 file changed, 59 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index b8ea59493964..44d6f598b63d 100644
> --- a/Documentation/userspace-api/landlock.rst
> +++ b/Documentation/userspace-api/landlock.rst
> @@ -8,7 +8,7 @@ Landlock: unprivileged access control
>   =====================================
>   
>   :Author: Mickaël Salaün
> -:Date: May 2022
> +:Date: October 2022
>   
>   The goal of Landlock is to enable to restrict ambient rights (e.g. global
>   filesystem access) for a set of processes.  Because Landlock is a stackable
> @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights.
>               LANDLOCK_ACCESS_FS_MAKE_FIFO |
>               LANDLOCK_ACCESS_FS_MAKE_BLOCK |
>               LANDLOCK_ACCESS_FS_MAKE_SYM |
> -            LANDLOCK_ACCESS_FS_REFER,
> +            LANDLOCK_ACCESS_FS_REFER |
> +            LANDLOCK_ACCESS_FS_TRUNCATE,
>       };
>   
>   Because we may not know on which kernel version an application will be
> @@ -69,16 +70,27 @@ should try to protect users as much as possible whatever the kernel they are
>   using.  To avoid binary enforcement (i.e. either all security features or
>   none), we can leverage a dedicated Landlock command to get the current version
>   of the Landlock ABI and adapt the handled accesses.  Let's check if we should
> -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported
> -starting with the second version of the ABI.
> +remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE``
> +access rights, which are only supported starting with the second and third
> +version of the ABI.
>   
>   .. code-block:: c
>   
>       int abi;
>   
>       abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
> -    if (abi < 2) {
> +    if (abi < 0) {
> +        perror("The running kernel does not enable to use Landlock");

Please insert in a dedicated line this comment: /* Degrades gracefully 
if Landlock is not handled. */


> +        return 0;  /* Degrade gracefully if Landlock is not handled. */
> +    }
> +    switch (abi) {
> +    case 1:
> +        /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
>           ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
> +        __attribute__((fallthrough));
> +    case 2:
> +        /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
> +        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
>       }
>   
>   This enables to create an inclusive ruleset that will contain our rules.
> @@ -127,8 +139,8 @@ descriptor.
>   
>   It may also be required to create rules following the same logic as explained
>   for the ruleset creation, by filtering access rights according to the Landlock
> -ABI version.  In this example, this is not required because
> -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule.
> +ABI version.  In this example, this is not required because all of the requested
> +``allowed_access`` rights are already available in ABI 1.
>   
>   We now have a ruleset with one rule allowing read access to ``/usr`` while
>   denying all other handled accesses for the filesystem.  The next step is to
> @@ -251,6 +263,37 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
>   process, a sandboxed process should have a subset of the target process rules,
>   which means the tracee must be in a sub-domain of the tracer.
>   
> +Truncating files
> +----------------
> +
> +The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and
> +``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes
> +overlap in non-intuitive ways.  It is recommended to always specify both of
> +these together.
> +
> +A particularly surprising example is :manpage:`creat(2)`.  The name suggests
> +that this system call requires the rights to create and write files.  However,
> +it also requires the truncate right if an existing file under the same name is
> +already present.
> +
> +It should also be noted that truncating files does not require the
> +``LANDLOCK_ACCESS_FS_WRITE_FILE`` right.  Apart from the :manpage:`truncate(2)`
> +system call, this can also be done through :manpage:`open(2)` with the flags
> +``O_RDONLY | O_TRUNC``.
> +
> +When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE``
> +right is associated with the newly created file descriptor and will be used for
> +subsequent truncation attempts using :manpage:`ftruncate(2)`.  The behavior is
> +similar to opening a file for reading or writing, where permissions are checked
> +during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and
> +:manpage:`write(2)` calls.
> +
> +As a consequence, it is possible to have multiple open file descriptors for the
> +same file, where one grants the right to truncate the file and the other does
> +not.  It is also possible to pass such file descriptors between processes,
> +keeping their Landlock properties, even when these processes do not have an
> +enforced Landlock ruleset.
> +
>   Compatibility
>   =============
>   
> @@ -397,6 +440,15 @@ Starting with the Landlock ABI version 2, it is now possible to securely
>   control renaming and linking thanks to the new `LANDLOCK_ACCESS_FS_REFER`
>   access right.
>   
> +File truncation (ABI < 3)
> +-------------------------
> +
> +File truncation could not be denied before the third Landlock ABI, so it is
> +always allowed when using a kernel that only supports the first or second ABI.
> +
> +Starting with the Landlock ABI version 3, it is now possible to securely control
> +truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
> +
>   .. _kernel_support:
>   
>   Kernel support
Günther Noack Oct. 8, 2022, 8:49 a.m. UTC | #2
On Wed, Oct 05, 2022 at 08:57:37PM +0200, Mickaël Salaün wrote:
> 
> On 01/10/2022 17:49, Günther Noack wrote:
> > Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.
> > 
> > Adapt the backwards compatibility example and discussion to remove the
> > truncation flag where needed.
> > 
> > Point out potential surprising behaviour related to truncate.
> > 
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > ---
> >   Documentation/userspace-api/landlock.rst | 66 +++++++++++++++++++++---
> >   1 file changed, 59 insertions(+), 7 deletions(-)
> > 
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index b8ea59493964..44d6f598b63d 100644
> > --- a/Documentation/userspace-api/landlock.rst
> > +++ b/Documentation/userspace-api/landlock.rst
> > @@ -8,7 +8,7 @@ Landlock: unprivileged access control
> >   =====================================
> >   :Author: Mickaël Salaün
> > -:Date: May 2022
> > +:Date: October 2022
> >   The goal of Landlock is to enable to restrict ambient rights (e.g. global
> >   filesystem access) for a set of processes.  Because Landlock is a stackable
> > @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights.
> >               LANDLOCK_ACCESS_FS_MAKE_FIFO |
> >               LANDLOCK_ACCESS_FS_MAKE_BLOCK |
> >               LANDLOCK_ACCESS_FS_MAKE_SYM |
> > -            LANDLOCK_ACCESS_FS_REFER,
> > +            LANDLOCK_ACCESS_FS_REFER |
> > +            LANDLOCK_ACCESS_FS_TRUNCATE,
> >       };
> >   Because we may not know on which kernel version an application will be
> > @@ -69,16 +70,27 @@ should try to protect users as much as possible whatever the kernel they are
> >   using.  To avoid binary enforcement (i.e. either all security features or
> >   none), we can leverage a dedicated Landlock command to get the current version
> >   of the Landlock ABI and adapt the handled accesses.  Let's check if we should
> > -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported
> > -starting with the second version of the ABI.
> > +remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE``
> > +access rights, which are only supported starting with the second and third
> > +version of the ABI.
> >   .. code-block:: c
> >       int abi;
> >       abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
> > -    if (abi < 2) {
> > +    if (abi < 0) {
> > +        perror("The running kernel does not enable to use Landlock");
> 
> Please insert in a dedicated line this comment: /* Degrades gracefully if
> Landlock is not handled. */

Done, moved the comment to a dedicated line and added the "s".

> > +        return 0;  /* Degrade gracefully if Landlock is not handled. */
> > +    }
> > +    switch (abi) {
> > +    case 1:
> > +        /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
> >           ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
> > +        __attribute__((fallthrough));
> > +    case 2:
> > +        /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
> > +        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
> >       }
> >   This enables to create an inclusive ruleset that will contain our rules.
> > @@ -127,8 +139,8 @@ descriptor.
> >   It may also be required to create rules following the same logic as explained
> >   for the ruleset creation, by filtering access rights according to the Landlock
> > -ABI version.  In this example, this is not required because
> > -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule.
> > +ABI version.  In this example, this is not required because all of the requested
> > +``allowed_access`` rights are already available in ABI 1.
> >   We now have a ruleset with one rule allowing read access to ``/usr`` while
> >   denying all other handled accesses for the filesystem.  The next step is to
> > @@ -251,6 +263,37 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
> >   process, a sandboxed process should have a subset of the target process rules,
> >   which means the tracee must be in a sub-domain of the tracer.
> > +Truncating files
> > +----------------
> > +
> > +The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and
> > +``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes
> > +overlap in non-intuitive ways.  It is recommended to always specify both of
> > +these together.
> > +
> > +A particularly surprising example is :manpage:`creat(2)`.  The name suggests
> > +that this system call requires the rights to create and write files.  However,
> > +it also requires the truncate right if an existing file under the same name is
> > +already present.
> > +
> > +It should also be noted that truncating files does not require the
> > +``LANDLOCK_ACCESS_FS_WRITE_FILE`` right.  Apart from the :manpage:`truncate(2)`
> > +system call, this can also be done through :manpage:`open(2)` with the flags
> > +``O_RDONLY | O_TRUNC``.
> > +
> > +When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE``
> > +right is associated with the newly created file descriptor and will be used for
> > +subsequent truncation attempts using :manpage:`ftruncate(2)`.  The behavior is
> > +similar to opening a file for reading or writing, where permissions are checked
> > +during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and
> > +:manpage:`write(2)` calls.
> > +
> > +As a consequence, it is possible to have multiple open file descriptors for the
> > +same file, where one grants the right to truncate the file and the other does
> > +not.  It is also possible to pass such file descriptors between processes,
> > +keeping their Landlock properties, even when these processes do not have an
> > +enforced Landlock ruleset.
> > +
> >   Compatibility
> >   =============
> > @@ -397,6 +440,15 @@ Starting with the Landlock ABI version 2, it is now possible to securely
> >   control renaming and linking thanks to the new `LANDLOCK_ACCESS_FS_REFER`
> >   access right.
> > +File truncation (ABI < 3)
> > +-------------------------
> > +
> > +File truncation could not be denied before the third Landlock ABI, so it is
> > +always allowed when using a kernel that only supports the first or second ABI.
> > +
> > +Starting with the Landlock ABI version 3, it is now possible to securely control
> > +truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
> > +
> >   .. _kernel_support:
> >   Kernel support

--
diff mbox series

Patch

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index b8ea59493964..44d6f598b63d 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -8,7 +8,7 @@  Landlock: unprivileged access control
 =====================================
 
 :Author: Mickaël Salaün
-:Date: May 2022
+:Date: October 2022
 
 The goal of Landlock is to enable to restrict ambient rights (e.g. global
 filesystem access) for a set of processes.  Because Landlock is a stackable
@@ -60,7 +60,8 @@  the need to be explicit about the denied-by-default access rights.
             LANDLOCK_ACCESS_FS_MAKE_FIFO |
             LANDLOCK_ACCESS_FS_MAKE_BLOCK |
             LANDLOCK_ACCESS_FS_MAKE_SYM |
-            LANDLOCK_ACCESS_FS_REFER,
+            LANDLOCK_ACCESS_FS_REFER |
+            LANDLOCK_ACCESS_FS_TRUNCATE,
     };
 
 Because we may not know on which kernel version an application will be
@@ -69,16 +70,27 @@  should try to protect users as much as possible whatever the kernel they are
 using.  To avoid binary enforcement (i.e. either all security features or
 none), we can leverage a dedicated Landlock command to get the current version
 of the Landlock ABI and adapt the handled accesses.  Let's check if we should
-remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported
-starting with the second version of the ABI.
+remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE``
+access rights, which are only supported starting with the second and third
+version of the ABI.
 
 .. code-block:: c
 
     int abi;
 
     abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
-    if (abi < 2) {
+    if (abi < 0) {
+        perror("The running kernel does not enable to use Landlock");
+        return 0;  /* Degrade gracefully if Landlock is not handled. */
+    }
+    switch (abi) {
+    case 1:
+        /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
         ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+        __attribute__((fallthrough));
+    case 2:
+        /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
+        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
     }
 
 This enables to create an inclusive ruleset that will contain our rules.
@@ -127,8 +139,8 @@  descriptor.
 
 It may also be required to create rules following the same logic as explained
 for the ruleset creation, by filtering access rights according to the Landlock
-ABI version.  In this example, this is not required because
-`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule.
+ABI version.  In this example, this is not required because all of the requested
+``allowed_access`` rights are already available in ABI 1.
 
 We now have a ruleset with one rule allowing read access to ``/usr`` while
 denying all other handled accesses for the filesystem.  The next step is to
@@ -251,6 +263,37 @@  To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
 process, a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
 
+Truncating files
+----------------
+
+The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and
+``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes
+overlap in non-intuitive ways.  It is recommended to always specify both of
+these together.
+
+A particularly surprising example is :manpage:`creat(2)`.  The name suggests
+that this system call requires the rights to create and write files.  However,
+it also requires the truncate right if an existing file under the same name is
+already present.
+
+It should also be noted that truncating files does not require the
+``LANDLOCK_ACCESS_FS_WRITE_FILE`` right.  Apart from the :manpage:`truncate(2)`
+system call, this can also be done through :manpage:`open(2)` with the flags
+``O_RDONLY | O_TRUNC``.
+
+When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE``
+right is associated with the newly created file descriptor and will be used for
+subsequent truncation attempts using :manpage:`ftruncate(2)`.  The behavior is
+similar to opening a file for reading or writing, where permissions are checked
+during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and
+:manpage:`write(2)` calls.
+
+As a consequence, it is possible to have multiple open file descriptors for the
+same file, where one grants the right to truncate the file and the other does
+not.  It is also possible to pass such file descriptors between processes,
+keeping their Landlock properties, even when these processes do not have an
+enforced Landlock ruleset.
+
 Compatibility
 =============
 
@@ -397,6 +440,15 @@  Starting with the Landlock ABI version 2, it is now possible to securely
 control renaming and linking thanks to the new `LANDLOCK_ACCESS_FS_REFER`
 access right.
 
+File truncation (ABI < 3)
+-------------------------
+
+File truncation could not be denied before the third Landlock ABI, so it is
+always allowed when using a kernel that only supports the first or second ABI.
+
+Starting with the Landlock ABI version 3, it is now possible to securely control
+truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
+
 .. _kernel_support:
 
 Kernel support