[PATCH v4 2/7] landlock: Add IOCTL access right
Mickaël Salaün
mic at digikod.net
Thu Nov 16 21:50:56 UTC 2023
On Fri, Nov 03, 2023 at 04:57:12PM +0100, Günther Noack wrote:
> Introduces the LANDLOCK_ACCESS_FS_IOCTL access right
> and increments the Landlock ABI version to 5.
>
> Like the truncate right, these rights are associated with a file
> descriptor at the time of open(2), and get respected even when the
> file descriptor is used outside of the thread which it was originally
> opened in.
>
> A newly enabled Landlock policy therefore does not apply to file
> descriptors which are already open.
>
> If the LANDLOCK_ACCESS_FS_IOCTL right is handled, only a small number
> of safe IOCTL commands will be permitted on newly opened files. The
> permitted IOCTLs can be configured through the ruleset in limited ways
> now. (See documentation for details.)
>
> Noteworthy scenarios which require special attention:
>
> TTY devices support IOCTLs like TIOCSTI and TIOCLINUX, which can be
> used to control shell processes on the same terminal which run at
> different privilege levels, which may make it possible to escape a
> sandbox. Because stdin, stdout and stderr are normally inherited
> rather than newly opened, IOCTLs are usually permitted on them even
> after the Landlock policy is enforced.
>
> Some legitimate file system features, like setting up fscrypt, are
> exposed as IOCTL commands on regular files and directories -- users of
> Landlock are advised to double check that the sandboxed process does
> not need to invoke these IOCTLs.
>
> Known limitations:
>
> The LANDLOCK_ACCESS_FS_IOCTL access right is a coarse-grained control
> over IOCTL commands. Future work will enable a more fine-grained
> access control for IOCTLs.
>
> In the meantime, Landlock users may use path-based restrictions in
> combination with their knowledge about the file system layout to
> control what IOCTLs can be done. Mounting file systems with the nodev
> option can help to distinguish regular files and devices, and give
> guarantees about the affected files, which Landlock alone can not give
> yet.
>
> Signed-off-by: Günther Noack <gnoack at google.com>
> ---
> include/uapi/linux/landlock.h | 31 +++++---
> security/landlock/fs.c | 74 ++++++++++++++++++--
> security/landlock/limits.h | 10 ++-
> security/landlock/ruleset.h | 53 +++++++++++++-
> security/landlock/syscalls.c | 6 +-
> tools/testing/selftests/landlock/base_test.c | 2 +-
> tools/testing/selftests/landlock/fs_test.c | 5 +-
> 7 files changed, 161 insertions(+), 20 deletions(-)
>
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index 25c8d7677539..6d41c059e910 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -128,7 +128,7 @@ struct landlock_net_port_attr {
> * files and directories. Files or directories opened before the sandboxing
> * are not subject to these restrictions.
> *
> - * A file can only receive these access rights:
> + * The following access rights apply only to files:
> *
> * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
> * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
> @@ -138,12 +138,13 @@ struct landlock_net_port_attr {
> * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
> * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
> * :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
> - * ``O_TRUNC``. Whether an opened file can be truncated with
> - * :manpage:`ftruncate(2)` is determined during :manpage:`open(2)`, in the
> - * same way as read and write permissions are checked during
> - * :manpage:`open(2)` using %LANDLOCK_ACCESS_FS_READ_FILE and
> - * %LANDLOCK_ACCESS_FS_WRITE_FILE. This access right is available since the
> - * third version of the Landlock ABI.
> + * ``O_TRUNC``. This access right is available since the third version of the
> + * Landlock ABI.
> + *
> + * Whether an opened file can be truncated with :manpage:`ftruncate(2)` or used
> + * with `ioctl(2)` is determined during :manpage:`open(2)`, in the same way as
> + * read and write permissions are checked during :manpage:`open(2)` using
> + * %LANDLOCK_ACCESS_FS_READ_FILE and %LANDLOCK_ACCESS_FS_WRITE_FILE.
> *
> * A directory can receive access rights related to files or directories. The
> * following access right is applied to the directory itself, and the
> @@ -198,13 +199,26 @@ struct landlock_net_port_attr {
> * If multiple requirements are not met, the ``EACCES`` error code takes
> * precedence over ``EXDEV``.
> *
> + * The following access right applies both to files and directories:
> + *
> + * - %LANDLOCK_ACCESS_FS_IOCTL: Invoke :manpage:`ioctl(2)` commands on an opened
> + * file or directory.
> + *
> + * This access right applies to all :manpage:`ioctl(2)` commands, except of
> + * ``FIOCLEX``, ``FIONCLEX``, ``FIONBIO``, ``FIOASYNC`` and ``FIONREAD``.
> + * These commands continue to be invokable independent of the
> + * %LANDLOCK_ACCESS_FS_IOCTL access right.
> + *
> + * This access right is available since the fourth version of the Landlock
It is now the fifth version. Same for the documentation.
> + * ABI.
> + *
> * .. warning::
> *
> * It is currently not possible to restrict some file-related actions
> * accessible through these syscall families: :manpage:`chdir(2)`,
> * :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
> * :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
> - * :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
> + * :manpage:`fcntl(2)`, :manpage:`access(2)`.
> * Future Landlock evolutions will enable to restrict them.
> */
> /* clang-format off */
> @@ -223,6 +237,7 @@ struct landlock_net_port_attr {
> #define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12)
> #define LANDLOCK_ACCESS_FS_REFER (1ULL << 13)
> #define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14)
> +#define LANDLOCK_ACCESS_FS_IOCTL (1ULL << 15)
> /* clang-format on */
>
> /**
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index bc7c126deea2..aa54970c235f 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -7,12 +7,14 @@
> * Copyright © 2021-2022 Microsoft Corporation
> */
>
> +#include <asm/ioctls.h>
> #include <linux/atomic.h>
> #include <linux/bitops.h>
> #include <linux/bits.h>
> #include <linux/compiler_types.h>
> #include <linux/dcache.h>
> #include <linux/err.h>
> +#include <linux/falloc.h>
> #include <linux/fs.h>
> #include <linux/init.h>
> #include <linux/kernel.h>
> @@ -28,6 +30,7 @@
> #include <linux/types.h>
> #include <linux/wait_bit.h>
> #include <linux/workqueue.h>
> +#include <uapi/linux/fiemap.h>
> #include <uapi/linux/landlock.h>
>
> #include "common.h"
> @@ -147,7 +150,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
> LANDLOCK_ACCESS_FS_EXECUTE | \
> LANDLOCK_ACCESS_FS_WRITE_FILE | \
> LANDLOCK_ACCESS_FS_READ_FILE | \
> - LANDLOCK_ACCESS_FS_TRUNCATE)
> + LANDLOCK_ACCESS_FS_TRUNCATE | \
> + LANDLOCK_ACCESS_FS_IOCTL)
> /* clang-format on */
>
> /*
> @@ -157,6 +161,7 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
> const struct path *const path,
> access_mask_t access_rights)
> {
> + access_mask_t handled;
> int err;
> struct landlock_id id = {
> .type = LANDLOCK_KEY_INODE,
> @@ -169,9 +174,11 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
> if (WARN_ON_ONCE(ruleset->num_layers != 1))
> return -EINVAL;
>
> + handled = landlock_get_fs_access_mask(ruleset, 0);
> + /* Expands the synthetic IOCTL groups. */
> + access_rights |= expand_all_ioctl(handled, access_rights);
> /* Transforms relative access rights to absolute ones. */
> - access_rights |= LANDLOCK_MASK_ACCESS_FS &
> - ~landlock_get_fs_access_mask(ruleset, 0);
> + access_rights |= LANDLOCK_MASK_ACCESS_FS & ~handled;
> id.key.object = get_inode_object(d_backing_inode(path->dentry));
> if (IS_ERR(id.key.object))
> return PTR_ERR(id.key.object);
> @@ -1123,7 +1130,9 @@ static int hook_file_open(struct file *const file)
> {
> layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
> access_mask_t open_access_request, full_access_request, allowed_access;
> - const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
> + const access_mask_t optional_access =
> + LANDLOCK_ACCESS_FS_TRUNCATE | LANDLOCK_ACCESS_FS_IOCTL |
> + IOCTL_CMD_G1 | IOCTL_CMD_G2 | IOCTL_CMD_G3 | IOCTL_CMD_G4;
I think it would be more future-proof to declare a new global const and
use it here for optional_access:
static const access_mask_t ioctl_groups =
IOCTL_CMD_G1 |
IOCTL_CMD_G2 |
IOCTL_CMD_G3 |
IOCTL_CMD_G4;
> const struct landlock_ruleset *const dom = get_current_fs_domain();
>
> if (!dom)
> @@ -1196,6 +1205,62 @@ static int hook_file_truncate(struct file *const file)
> return -EACCES;
> }
>
> +static access_mask_t required_ioctl_access(unsigned int cmd)
> +{
> + switch (cmd) {
> + case FIOQSIZE:
> + return IOCTL_CMD_G1;
> + case FS_IOC_FIEMAP:
> + case FIBMAP:
> + case FIGETBSZ:
> + return IOCTL_CMD_G2;
> + case FIONREAD:
> + case FIDEDUPERANGE:
> + return IOCTL_CMD_G3;
> + case FICLONE:
> + case FICLONERANGE:
> + case FS_IOC_RESVSP:
> + case FS_IOC_RESVSP64:
> + case FS_IOC_UNRESVSP:
> + case FS_IOC_UNRESVSP64:
> + case FS_IOC_ZERO_RANGE:
> + return IOCTL_CMD_G4;
> + case FIOCLEX:
> + case FIONCLEX:
> + case FIONBIO:
> + case FIOASYNC:
> + /*
> + * FIOCLEX, FIONCLEX, FIONBIO and FIOASYNC manipulate the FD's
> + * close-on-exec and the file's buffered-IO and async flags.
> + * These operations are also available through fcntl(2),
> + * and are unconditionally permitted in Landlock.
> + */
> + return 0;
You can move this block at the top.
> + default:
> + /*
> + * Other commands are guarded by the catch-all access right.
> + */
> + return LANDLOCK_ACCESS_FS_IOCTL;
> + }
> +}
> +
> +static int hook_file_ioctl(struct file *file, unsigned int cmd,
> + unsigned long arg)
> +{
> + access_mask_t required_access = required_ioctl_access(cmd);
> + access_mask_t allowed_access = landlock_file(file)->allowed_access;
You can use const for these variables.
> +
> + /*
> + * It is the access rights at the time of opening the file which
> + * determine whether ioctl can be used on the opened file later.
> + *
> + * The access right is attached to the opened file in hook_file_open().
> + */
> + if ((allowed_access & required_access) == required_access)
> + return 0;
Please add a new line.
> + return -EACCES;
> +}
> +
> static struct security_hook_list landlock_hooks[] __ro_after_init = {
> LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
>
> @@ -1218,6 +1283,7 @@ static struct security_hook_list landlock_hooks[] __ro_after_init = {
> LSM_HOOK_INIT(file_alloc_security, hook_file_alloc_security),
> LSM_HOOK_INIT(file_open, hook_file_open),
> LSM_HOOK_INIT(file_truncate, hook_file_truncate),
> + LSM_HOOK_INIT(file_ioctl, hook_file_ioctl),
> };
>
> __init void landlock_add_fs_hooks(void)
> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> index 93c9c6f91556..d0a95169ba3f 100644
> --- a/security/landlock/limits.h
> +++ b/security/landlock/limits.h
> @@ -18,7 +18,15 @@
> #define LANDLOCK_MAX_NUM_LAYERS 16
> #define LANDLOCK_MAX_NUM_RULES U32_MAX
>
> -#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_TRUNCATE
> +#define LANDLOCK_LAST_PUBLIC_ACCESS_FS LANDLOCK_ACCESS_FS_IOCTL
> +#define LANDLOCK_MASK_PUBLIC_ACCESS_FS ((LANDLOCK_LAST_PUBLIC_ACCESS_FS << 1) - 1)
> +
Please add a comment to explain that the semantic of these synthetic
access rights is defined by the required_ioctl_access() helper.
> +#define IOCTL_CMD_G1 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 1)
> +#define IOCTL_CMD_G2 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 2)
> +#define IOCTL_CMD_G3 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 3)
> +#define IOCTL_CMD_G4 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 4)
> +
> +#define LANDLOCK_LAST_ACCESS_FS IOCTL_CMD_G4
> #define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
> #define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS)
> #define LANDLOCK_SHIFT_ACCESS_FS 0
> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> index c7f1526784fd..58d96aff3980 100644
> --- a/security/landlock/ruleset.h
> +++ b/security/landlock/ruleset.h
> @@ -30,7 +30,7 @@
> LANDLOCK_ACCESS_FS_REFER)
> /* clang-format on */
>
> -typedef u16 access_mask_t;
> +typedef u32 access_mask_t;
> /* Makes sure all filesystem access rights can be stored. */
> static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
> /* Makes sure all network access rights can be stored. */
> @@ -256,6 +256,54 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
> refcount_inc(&ruleset->usage);
> }
>
> +/**
> + * expand_ioctl - return the dst flags from either the src flag or the
Return...
> + * LANDLOCK_ACCESS_FS_IOCTL flag, depending on whether the
> + * LANDLOCK_ACCESS_FS_IOCTL and src access rights are handled or not.
> + *
> + * @handled: Handled access rights
> + * @access: The access mask to copy values from
> + * @src: A single access right to copy from in @access.
> + * @dst: One or more access rights to copy to
Please don't add extra spaces, that would avoid to shift all the lines
if we get another name which is longer.
> + *
> + * Returns:
> + * @dst, or 0
> + */
> +static inline access_mask_t expand_ioctl(access_mask_t handled,
> + access_mask_t access,
> + access_mask_t src, access_mask_t dst)
> +{
> + if (!(handled & LANDLOCK_ACCESS_FS_IOCTL))
> + return 0;
> +
> + access_mask_t copy_from = (handled & src) ? src :
> + LANDLOCK_ACCESS_FS_IOCTL;
> + if (access & copy_from)
> + return dst;
Please add a newline after return.
> + return 0;
> +}
> +
> +/**
> + * Returns @access with the synthetic IOCTL group flags enabled if necessary.
I think htmldocs will print a warning with this format.
> + *
> + * @handled: Handled FS access rights.
> + * @access: FS access rights to expand.
> + *
> + * Returns:
> + * @access expanded by the necessary flags for the synthetic IOCTL access rights.
> + */
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