[PATCH v8 4/9] landlock: Add IOCTL access right
Mickaël Salaün
mic at digikod.net
Thu Dec 14 14:28:10 UTC 2023
Christian, what do you think about the following IOCTL groups?
On Thu, Dec 14, 2023 at 11:14:10AM +0100, Mickaël Salaün wrote:
> On Thu, Dec 14, 2023 at 10:26:49AM +0100, Mickaël Salaün wrote:
> > On Fri, Dec 08, 2023 at 04:51:16PM +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.)
> > >
> > > Specifically, when LANDLOCK_ACCESS_FS_IOCTL is handled, granting this
> > > right on a file or directory will *not* permit to do all IOCTL
> > > commands, but only influence the IOCTL commands which are not already
> > > handled through other access rights. The intent is to keep the groups
> > > of IOCTL commands more fine-grained.
> > >
> > > 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 | 58 +++++-
> > > security/landlock/fs.c | 176 ++++++++++++++++++-
> > > security/landlock/fs.h | 2 +
> > > security/landlock/limits.h | 11 +-
> > > security/landlock/ruleset.h | 2 +-
> > > security/landlock/syscalls.c | 19 +-
> > > tools/testing/selftests/landlock/base_test.c | 2 +-
> > > tools/testing/selftests/landlock/fs_test.c | 5 +-
> > > 8 files changed, 253 insertions(+), 22 deletions(-)
> > >
> >
> > > diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> > > index 9ba989ef46a5..81ce41e9e6db 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"
> > > @@ -83,6 +86,145 @@ static const struct landlock_object_underops landlock_fs_underops = {
> > > .release = release_inode
> > > };
> > >
> > > +/* IOCTL helpers */
> > > +
> > > +/*
> > > + * These are synthetic access rights, which are only used within the kernel, but
> > > + * not exposed to callers in userspace. The mapping between these access rights
> > > + * and IOCTL commands is defined in the required_ioctl_access() helper function.
> > > + */
> > > +#define LANDLOCK_ACCESS_FS_IOCTL_GROUP1 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 1)
> > > +#define LANDLOCK_ACCESS_FS_IOCTL_GROUP2 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 2)
> > > +#define LANDLOCK_ACCESS_FS_IOCTL_GROUP3 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 3)
> > > +#define LANDLOCK_ACCESS_FS_IOCTL_GROUP4 (LANDLOCK_LAST_PUBLIC_ACCESS_FS << 4)
> > > +
> > > +/* ioctl_groups - all synthetic access rights for IOCTL command groups */
> > > +/* clang-format off */
> > > +#define IOCTL_GROUPS ( \
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP1 | \
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP2 | \
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP3 | \
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP4)
> > > +/* clang-format on */
> > > +
> > > +static_assert((IOCTL_GROUPS & LANDLOCK_MASK_ACCESS_FS) == IOCTL_GROUPS);
> > > +
> > > +/**
> > > + * required_ioctl_access(): Determine required IOCTL access rights.
> > > + *
> > > + * @cmd: The IOCTL command that is supposed to be run.
> > > + *
> > > + * Returns: The access rights that must be granted on an opened file in order to
> > > + * use the given @cmd.
> > > + */
> > > +static access_mask_t required_ioctl_access(unsigned int cmd)
>
> Please use a verb for functions, something like
> get_required_ioctl_access().
>
> >
> > You can add __attribute_const__ after "static", and also constify cmd.
> >
> > > +{
> > > + switch (cmd) {
> > > + 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;
Could you please add comments for the following IOCTL commands
explaining why they make sense for the related file/dir read/write
mapping? We discussed about that in the ML but it would be much easier
to put that doc here for future changes, and for reviewers to understand
the rationale. Some of this doc is already in the cover letter.
To make this easier to follow, what about renaming the IOCTL groups to
something like this:
* LANDLOCK_ACCESS_FS_IOCTL_GROUP1:
LANDLOCK_ACCESS_FS_IOCTL_GET_SIZE
* LANDLOCK_ACCESS_FS_IOCTL_GROUP2:
LANDLOCK_ACCESS_FS_IOCTL_GET_INNER
* LANDLOCK_ACCESS_FS_IOCTL_GROUP3:
LANDLOCK_ACCESS_FS_IOCTL_READ_FILE
* LANDLOCK_ACCESS_FS_IOCTL_GROUP4:
LANDLOCK_ACCESS_FS_IOCTL_WRITE_FILE
> > > + case FIOQSIZE:
> > > + return LANDLOCK_ACCESS_FS_IOCTL_GROUP1;
> > > + case FS_IOC_FIEMAP:
> > > + case FIBMAP:
> > > + case FIGETBSZ:
Does it make sense to not include FIGETBSZ in
LANDLOCK_ACCESS_FS_IOCTL_GROUP1? I think it's OK like this as previously
explained but I'd like to get confirmation:
https://lore.kernel.org/r/20230904.aiWae8eineo4@digikod.net
> > > + return LANDLOCK_ACCESS_FS_IOCTL_GROUP2;
> > > + case FIONREAD:
> > > + case FIDEDUPERANGE:
> > > + return LANDLOCK_ACCESS_FS_IOCTL_GROUP3;
> > > + case FICLONE:
> > > + case FICLONERANGE:
The FICLONE* commands seems to already check read/write permissions with
generic_file_rw_checks(). Always allowing them should then be OK (and
the current tests should still pass), but we can still keep them here to
make the required access right explicit and test with and without
Landlock restrictions to make sure this is consistent with the VFS
access checks. See
https://lore.kernel.org/r/20230904.aiWae8eineo4@digikod.net
If this is correct, a new test should check that Landlock restrictions
are the same as the VFS checks and then don't impact such IOCTLs.
> > > + case FS_IOC_RESVSP:
> > > + case FS_IOC_RESVSP64:
> > > + case FS_IOC_UNRESVSP:
> > > + case FS_IOC_UNRESVSP64:
> > > + case FS_IOC_ZERO_RANGE:
> > > + return LANDLOCK_ACCESS_FS_IOCTL_GROUP4;
> > > + default:
> > > + /*
> > > + * Other commands are guarded by the catch-all access right.
> > > + */
> > > + return LANDLOCK_ACCESS_FS_IOCTL;
> > > + }
> > > +}
We previously talked about allowing all IOCTLs on unix sockets and named
pipes: https://lore.kernel.org/r/ZP7lxmXklksadvz+@google.com
I think the remaining issue with this grouping is that if the VFS
implementation returns -ENOIOCTLCMD, then the IOCTL command can be
forwarded to the device driver (for character or block devices).
For instance, FIONREAD on a character device could translate to unknown
action (on this device), which should then be considered dangerous and
denied unless explicitly allowed with LANDLOCK_ACCESS_FS_IOCTL (but not
any IOCTL_GROUP*).
For instance, FIONREAD on /dev/null should return -ENOTTY, which should
then also be the case if LANDLOCK_ACCESS_FS_IOCTL is allowed (even if
LANDLOCK_ACCESS_FS_READ_FILE is denied). This is also the case for
file_ioctl()'s commands.
One solution to implement this logic would be to add an additional check
in hook_file_ioctl() for specific file types (!S_ISREG or socket or pipe
exceptions) and IOCTL commands.
Christian, is it correct to say that device drivers are not "required"
to follow the same semantic as the VFS's IOCTLs and that (for whatever
reason) collisions may occur? I guess this is not the case for
filesystems, which should implement similar semantic for the same
IOCTLs.
> > > +
> > > +/**
> > > + * expand_ioctl() - Return the dst flags from either the src flag or the
> > > + * %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.
> > > + *
> > > + * Returns: @dst, or 0.
> > > + */
> > > +static access_mask_t expand_ioctl(const access_mask_t handled,
> >
> > static __attribute_const__
> >
> > > + const access_mask_t access,
> > > + const access_mask_t src,
> > > + const access_mask_t dst)
> > > +{
> > > + access_mask_t copy_from;
> > > +
> > > + if (!(handled & LANDLOCK_ACCESS_FS_IOCTL))
> > > + return 0;
> > > +
> > > + copy_from = (handled & src) ? src : LANDLOCK_ACCESS_FS_IOCTL;
> > > + if (access & copy_from)
> > > + return dst;
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +/**
> > > + * landlock_expand_access_fs() - Returns @access with the synthetic IOCTL group
> > > + * flags enabled if necessary.
> > > + *
> > > + * @handled: Handled FS access rights.
> > > + * @access: FS access rights to expand.
> > > + *
> > > + * Returns: @access expanded by the necessary flags for the synthetic IOCTL
> > > + * access rights.
> > > + */
> > > +static access_mask_t landlock_expand_access_fs(const access_mask_t handled,
> >
> > static __attribute_const__
> >
> > > + const access_mask_t access)
> > > +{
> > > + return access |
> > > + expand_ioctl(handled, access, LANDLOCK_ACCESS_FS_WRITE_FILE,
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP1 |
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP2 |
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP4) |
> > > + expand_ioctl(handled, access, LANDLOCK_ACCESS_FS_READ_FILE,
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP1 |
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP2 |
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP3) |
> > > + expand_ioctl(handled, access, LANDLOCK_ACCESS_FS_READ_DIR,
> > > + LANDLOCK_ACCESS_FS_IOCTL_GROUP1);
> > > +}
> > > +
> > > +/**
> > > + * landlock_expand_handled_access_fs() - add synthetic IOCTL access rights to an
> > > + * access mask of handled accesses.
> > > + *
> > > + * @handled: The handled accesses of a ruleset that is being created.
> > > + *
> > > + * Returns: @handled, with the bits for the synthetic IOCTL access rights set,
> > > + * if %LANDLOCK_ACCESS_FS_IOCTL is handled.
> > > + */
> > > +access_mask_t landlock_expand_handled_access_fs(const access_mask_t handled)
> >
> > __attribute_const__ access_mask_t
> >
> > > +{
> > > + return landlock_expand_access_fs(handled, handled);
> > > +}
> > > +
> >
> > > diff --git a/security/landlock/fs.h b/security/landlock/fs.h
> > > index 488e4813680a..c88fe7bda37b 100644
> > > --- a/security/landlock/fs.h
> > > +++ b/security/landlock/fs.h
> > > @@ -92,4 +92,6 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
> > > const struct path *const path,
> > > access_mask_t access_hierarchy);
> > >
> > > +access_mask_t landlock_expand_handled_access_fs(const access_mask_t handled);
> >
> > __attribute_const__ access_mask_t
> >
> > > +
> > > #endif /* _SECURITY_LANDLOCK_FS_H */
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