| Message ID | 20210202162710.657398-9-mic@digikod.net (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Landlock LSM | expand |
On Tue, Feb 02, 2021 at 05:27:06PM +0100, Mickaël Salaün wrote: > From: Mickaël Salaün <mic@linux.microsoft.com> > > These 3 system calls are designed to be used by unprivileged processes > to sandbox themselves: > * landlock_create_ruleset(2): Creates a ruleset and returns its file > descriptor. > * landlock_add_rule(2): Adds a rule (e.g. file hierarchy access) to a > ruleset, identified by the dedicated file descriptor. > * landlock_restrict_self(2): Enforces a ruleset on the calling thread > and its future children (similar to seccomp). This syscall has the > same usage restrictions as seccomp(2): the caller must have the > no_new_privs attribute set or have CAP_SYS_ADMIN in the current user > namespace. Was looking through the set for this :) thanks. > > All these syscalls have a "flags" argument (not currently used) to > enable extensibility. > > Here are the motivations for these new syscalls: > * A sandboxed process may not have access to file systems, including > /dev, /sys or /proc, but it should still be able to add more > restrictions to itself. > * Neither prctl(2) nor seccomp(2) (which was used in a previous version) > fit well with the current definition of a Landlock security policy. > > All passed structs (attributes) are checked at build time to ensure that > they don't contain holes and that they are aligned the same way for each > architecture. > > See the user and kernel documentation for more details (provided by a > following commit): > * Documentation/userspace-api/landlock.rst > * Documentation/security/landlock.rst > > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: James Morris <jmorris@namei.org> > Cc: Jann Horn <jannh@google.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Serge E. Hallyn <serge@hallyn.com> Acked-by: Serge Hallyn <serge@hallyn.com> > Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> > --- > > Changes since v27: > * Forbid creation of rules with an empty allowed_access value because > they are now ignored (since v26) in path walks. > * Rename landlock_enforce_ruleset_self(2) to landlock_restrict_self(2): > shorter and consistent with the two other syscalls (i.e. verb + direct > object). > * Update ruleset access check according to the new access stack. > * Improve landlock_add_rule(2) documentation. > * Fix comment. > * Remove Reviewed-by Jann Horn because of the above changes. > > Changes since v26: > * Rename landlock_enforce_ruleset_current(2) to > landlock_enforce_ruleset_self(2). "current" makes sense for a kernel > developer, but much less from a user space developer stand point. > "self" is widely used to refer to the current task (e.g. /proc/self). > "current" may refer to temporal properties, which could be added later > to this syscall flags (cf. /proc/self/attr/{current,exec}). > * Simplify build_check_abi(). > * Rename syscall.c to syscalls.c . > * Use less ambiguous comments. > * Fix spelling. > > Changes since v25: > * Revert build_check_abi() as non-inline to trigger a warning if it is > not called. > * Use the new limit names. > > Changes since v24: > * Add Reviewed-by: Jann Horn <jannh@google.com> > * Set build_check_abi() as inline. > > Changes since v23: > * Rewrite get_ruleset_from_fd() to please the 0-DAY CI Kernel Test > Service that reported an uninitialized variable (false positive): > https://lore.kernel.org/linux-security-module/202011101854.zGbWwusK-lkp@intel.com/ > Anyway, it is cleaner like this. > * Add a comment about E2BIG which can be returned by > landlock_enforce_ruleset_current(2) when there is no more room for > another stacked ruleset (i.e. domain). > > Changes since v22: > * Replace security_capable() with ns_capable_noaudit() (suggested by > Jann Horn) and explicitly return EPERM. > * Fix landlock_enforce_ruleset_current(2)'s out_put_creds (spotted by > Jann Horn). > * Add __always_inline to copy_min_struct_from_user() to make its > BUILD_BUG_ON() checks reliable (suggested by Jann Horn). > * Simplify path assignation in get_path_from_fd() (suggested by Jann > Horn). > * Fix spelling (spotted by Jann Horn). > > Changes since v21: > * Fix and improve comments. > > Changes since v20: > * Remove two arguments to landlock_enforce_ruleset(2) (requested by Arnd > Bergmann) and rename it to landlock_enforce_ruleset_current(2): remove > the enum landlock_target_type and the target file descriptor (not used > for now). A ruleset can only be enforced on the current thread. > * Remove the size argument in landlock_add_rule() (requested by Arnd > Bergmann). > * Remove landlock_get_features(2) (suggested by Arnd Bergmann). > * Simplify and rename copy_struct_if_any_from_user() to > copy_min_struct_from_user(). > * Rename "options" to "flags" to allign with current syscalls. > * Rename some types and variables in a more consistent way. > * Fix missing type declarations in syscalls.h . > > Changes since v19: > * Replace the landlock(2) syscall with 4 syscalls (one for each > command): landlock_get_features(2), landlock_create_ruleset(2), > landlock_add_rule(2) and landlock_enforce_ruleset(2) (suggested by > Arnd Bergmann). > https://lore.kernel.org/lkml/56d15841-e2c1-2d58-59b8-3a6a09b23b4a@digikod.net/ > * Return EOPNOTSUPP (instead of ENOPKG) when Landlock is disabled. > * Add two new fields to landlock_attr_features to fit with the new > syscalls: last_rule_type and last_target_type. This enable to easily > identify which types are supported. > * Pack landlock_attr_path_beneath struct because of the removed > ruleset_fd. > * Update documentation and fix spelling. > > Changes since v18: > * Remove useless include. > * Remove LLATTR_SIZE() which was only used to shorten lines. Cf. commit > bdc48fa11e46 ("checkpatch/coding-style: deprecate 80-column warning"). > > Changes since v17: > * Synchronize syscall declaration. > * Fix comment. > > Changes since v16: > * Add a size_attr_features field to struct landlock_attr_features for > self-introspection, and move the access_fs field to be more > consistent. > * Replace __aligned_u64 types of attribute fields with __u16, __s32, > __u32 and __u64, and check at build time that these structures does > not contain hole and that they are aligned the same way (8-bits) on > all architectures. This shrinks the size of the userspace ABI, which > may be appreciated especially for struct landlock_attr_features which > could grow a lot in the future. For instance, struct > landlock_attr_features shrinks from 72 bytes to 32 bytes. This change > also enables to remove 64-bits to 32-bits conversion checks. > * Switch syscall attribute pointer and size arguments to follow similar > syscall argument order (e.g. bpf, clone3, openat2). > * Set LANDLOCK_OPT_* types to 32-bits. > * Allow enforcement of empty ruleset, which enables deny-all policies. > * Fix documentation inconsistency. > > Changes since v15: > * Do not add file descriptors referring to internal filesystems (e.g. > nsfs) in a ruleset. > * Replace is_user_mountable() with in-place clean checks. > * Replace EBADR with EBADFD in get_ruleset_from_fd() and > get_path_from_fd(). > * Remove ruleset's show_fdinfo() for now. > > Changes since v14: > * Remove the security_file_open() check in get_path_from_fd(): an > opened FD should not be restricted here, and even less with this hook. > As a result, it is now allowed to add a path to a ruleset even if the > access to this path is not allowed (without O_PATH). This doesn't > change the fact that enforcing a ruleset can't grant any right, only > remove some rights. The new layer levels add more consistent > restrictions. > * Check minimal landlock_attr_* size/content. This fix the case when > no data was provided and e.g., FD 0 was interpreted as ruleset_fd. > Now this leads to a returned -EINVAL. > * Fix credential double-free error case. > * Complete struct landlock_attr_size with size_attr_enforce. > * Fix undefined reference to syscall when Landlock is not selected. > * Remove f.file->f_path.mnt check (suggested by Al Viro). > * Add build-time checks. > * Move ABI checks from fs.c . > * Constify variables. > * Fix spelling. > * Add comments. > > Changes since v13: > * New implementation, replacing the dependency on seccomp(2) and bpf(2). > --- > include/linux/syscalls.h | 7 + > include/uapi/linux/landlock.h | 53 ++++ > kernel/sys_ni.c | 5 + > security/landlock/Makefile | 2 +- > security/landlock/syscalls.c | 444 ++++++++++++++++++++++++++++++++++ > 5 files changed, 510 insertions(+), 1 deletion(-) > create mode 100644 security/landlock/syscalls.c > > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index 7688bc983de5..6918be404b64 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -68,6 +68,8 @@ union bpf_attr; > struct io_uring_params; > struct clone_args; > struct open_how; > +struct landlock_ruleset_attr; > +enum landlock_rule_type; > > #include <linux/types.h> > #include <linux/aio_abi.h> > @@ -1037,6 +1039,11 @@ asmlinkage long sys_pidfd_send_signal(int pidfd, int sig, > siginfo_t __user *info, > unsigned int flags); > asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags); > +asmlinkage long sys_landlock_create_ruleset(const struct landlock_ruleset_attr __user *attr, > + size_t size, __u32 flags); > +asmlinkage long sys_landlock_add_rule(int ruleset_fd, enum landlock_rule_type rule_type, > + const void __user *rule_attr, __u32 flags); > +asmlinkage long sys_landlock_restrict_self(int ruleset_fd, __u32 flags); > > /* > * Architecture-specific system calls > diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h > index f69877099c8e..d1fc6af3381e 100644 > --- a/include/uapi/linux/landlock.h > +++ b/include/uapi/linux/landlock.h > @@ -9,6 +9,59 @@ > #ifndef _UAPI_LINUX_LANDLOCK_H > #define _UAPI_LINUX_LANDLOCK_H > > +#include <linux/types.h> > + > +/** > + * struct landlock_ruleset_attr - Ruleset definition > + * > + * Argument of sys_landlock_create_ruleset(). This structure can grow in > + * future versions. > + */ > +struct landlock_ruleset_attr { > + /** > + * @handled_access_fs: Bitmask of actions (cf. `Filesystem flags`_) > + * that is handled by this ruleset and should then be forbidden if no > + * rule explicitly allow them. This is needed for backward > + * compatibility reasons. > + */ > + __u64 handled_access_fs; > +}; > + > +/** > + * enum landlock_rule_type - Landlock rule type > + * > + * Argument of sys_landlock_add_rule(). > + */ > +enum landlock_rule_type { > + /** > + * @LANDLOCK_RULE_PATH_BENEATH: Type of a &struct > + * landlock_path_beneath_attr . > + */ > + LANDLOCK_RULE_PATH_BENEATH = 1, > +}; > + > +/** > + * struct landlock_path_beneath_attr - Path hierarchy definition > + * > + * Argument of sys_landlock_add_rule(). > + */ > +struct landlock_path_beneath_attr { > + /** > + * @allowed_access: Bitmask of allowed actions for this file hierarchy > + * (cf. `Filesystem flags`_). > + */ > + __u64 allowed_access; > + /** > + * @parent_fd: File descriptor, open with ``O_PATH``, which identifies > + * the parent directory of a file hierarchy, or just a file. > + */ > + __s32 parent_fd; > + /* > + * This struct is packed to avoid trailing reserved members. > + * Cf. security/landlock/syscalls.c:build_check_abi() > + */ > +} __attribute__((packed)); > + > /** > * DOC: fs_access > * > diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c > index 19aa806890d5..cce430cf2ff2 100644 > --- a/kernel/sys_ni.c > +++ b/kernel/sys_ni.c > @@ -266,6 +266,11 @@ COND_SYSCALL(request_key); > COND_SYSCALL(keyctl); > COND_SYSCALL_COMPAT(keyctl); > > +/* security/landlock/syscalls.c */ > +COND_SYSCALL(landlock_create_ruleset); > +COND_SYSCALL(landlock_add_rule); > +COND_SYSCALL(landlock_restrict_self); > + > /* arch/example/kernel/sys_example.c */ > > /* mm/fadvise.c */ > diff --git a/security/landlock/Makefile b/security/landlock/Makefile > index 92e3d80ab8ed..7bbd2f413b3e 100644 > --- a/security/landlock/Makefile > +++ b/security/landlock/Makefile > @@ -1,4 +1,4 @@ > obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o > > -landlock-y := setup.o object.o ruleset.o \ > +landlock-y := setup.o syscalls.o object.o ruleset.o \ > cred.o ptrace.o fs.o > diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c > new file mode 100644 > index 000000000000..ebb3c126a3c0 > --- /dev/null > +++ b/security/landlock/syscalls.c > @@ -0,0 +1,444 @@ > +// SPDX-License-Identifier: GPL-2.0-only > +/* > + * Landlock LSM - System call implementations and user space interfaces > + * > + * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net> > + * Copyright © 2018-2020 ANSSI > + */ > + > +#include <asm/current.h> > +#include <linux/anon_inodes.h> > +#include <linux/build_bug.h> > +#include <linux/capability.h> > +#include <linux/compiler_types.h> > +#include <linux/dcache.h> > +#include <linux/err.h> > +#include <linux/errno.h> > +#include <linux/fs.h> > +#include <linux/limits.h> > +#include <linux/mount.h> > +#include <linux/path.h> > +#include <linux/sched.h> > +#include <linux/security.h> > +#include <linux/stddef.h> > +#include <linux/syscalls.h> > +#include <linux/types.h> > +#include <linux/uaccess.h> > +#include <uapi/linux/landlock.h> > + > +#include "cred.h" > +#include "fs.h" > +#include "limits.h" > +#include "ruleset.h" > +#include "setup.h" > + > +/** > + * copy_min_struct_from_user - Safe future-proof argument copying > + * > + * Extend copy_struct_from_user() to check for consistent user buffer. > + * > + * @dst: Kernel space pointer or NULL. > + * @ksize: Actual size of the data pointed to by @dst. > + * @ksize_min: Minimal required size to be copied. > + * @src: User space pointer or NULL. > + * @usize: (Alleged) size of the data pointed to by @src. > + */ > +static __always_inline int copy_min_struct_from_user(void *const dst, > + const size_t ksize, const size_t ksize_min, > + const void __user *const src, const size_t usize) > +{ > + /* Checks buffer inconsistencies. */ > + BUILD_BUG_ON(!dst); > + if (!src) > + return -EFAULT; > + > + /* Checks size ranges. */ > + BUILD_BUG_ON(ksize <= 0); > + BUILD_BUG_ON(ksize < ksize_min); > + if (usize < ksize_min) > + return -EINVAL; > + if (usize > PAGE_SIZE) > + return -E2BIG; > + > + /* Copies user buffer and fills with zeros. */ > + return copy_struct_from_user(dst, ksize, src, usize); > +} > + > +/* > + * This function only contains arithmetic operations with constants, leading to > + * BUILD_BUG_ON(). The related code is evaluated and checked at build time, > + * but it is then ignored thanks to compiler optimizations. > + */ > +static void build_check_abi(void) > +{ > + struct landlock_ruleset_attr ruleset_attr; > + struct landlock_path_beneath_attr path_beneath_attr; > + size_t ruleset_size, path_beneath_size; > + > + /* > + * For each user space ABI structures, first checks that there is no > + * hole in them, then checks that all architectures have the same > + * struct size. > + */ > + ruleset_size = sizeof(ruleset_attr.handled_access_fs); > + BUILD_BUG_ON(sizeof(ruleset_attr) != ruleset_size); > + BUILD_BUG_ON(sizeof(ruleset_attr) != 8); > + > + path_beneath_size = sizeof(path_beneath_attr.allowed_access); > + path_beneath_size += sizeof(path_beneath_attr.parent_fd); > + BUILD_BUG_ON(sizeof(path_beneath_attr) != path_beneath_size); > + BUILD_BUG_ON(sizeof(path_beneath_attr) != 12); > +} > + > +/* Ruleset handling */ > + > +static int fop_ruleset_release(struct inode *const inode, > + struct file *const filp) > +{ > + struct landlock_ruleset *ruleset = filp->private_data; > + > + landlock_put_ruleset(ruleset); > + return 0; > +} > + > +static ssize_t fop_dummy_read(struct file *const filp, char __user *const buf, > + const size_t size, loff_t *const ppos) > +{ > + /* Dummy handler to enable FMODE_CAN_READ. */ > + return -EINVAL; > +} > + > +static ssize_t fop_dummy_write(struct file *const filp, > + const char __user *const buf, const size_t size, > + loff_t *const ppos) > +{ > + /* Dummy handler to enable FMODE_CAN_WRITE. */ > + return -EINVAL; > +} > + > +/* > + * A ruleset file descriptor enables to build a ruleset by adding (i.e. > + * writing) rule after rule, without relying on the task's context. This > + * reentrant design is also used in a read way to enforce the ruleset on the > + * current task. > + */ > +static const struct file_operations ruleset_fops = { > + .release = fop_ruleset_release, > + .read = fop_dummy_read, > + .write = fop_dummy_write, > +}; > + > +/** > + * sys_landlock_create_ruleset - Create a new ruleset > + * > + * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of > + * the new ruleset. > + * @size: Size of the pointed &struct landlock_ruleset_attr (needed for > + * backward and forward compatibility). > + * @flags: Must be 0. > + * > + * This system call enables to create a new Landlock ruleset, and returns the > + * related file descriptor on success. > + * > + * Possible returned errors are: > + * > + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; > + * - EINVAL: @flags is not 0, or unknown access, or too small @size; > + * - E2BIG or EFAULT: @attr or @size inconsistencies; > + * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs. > + */ > +SYSCALL_DEFINE3(landlock_create_ruleset, > + const struct landlock_ruleset_attr __user *const, attr, > + const size_t, size, const __u32, flags) > +{ > + struct landlock_ruleset_attr ruleset_attr; > + struct landlock_ruleset *ruleset; > + int err, ruleset_fd; > + > + /* Build-time checks. */ > + build_check_abi(); > + > + if (!landlock_initialized) > + return -EOPNOTSUPP; > + > + /* No flag for now. */ > + if (flags) > + return -EINVAL; > + > + /* Copies raw user space buffer. */ > + err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr), > + offsetofend(typeof(ruleset_attr), handled_access_fs), > + attr, size); > + if (err) > + return err; > + > + /* Checks content (and 32-bits cast). */ > + if ((ruleset_attr.handled_access_fs | LANDLOCK_MASK_ACCESS_FS) != > + LANDLOCK_MASK_ACCESS_FS) > + return -EINVAL; > + > + /* Checks arguments and transforms to kernel struct. */ > + ruleset = landlock_create_ruleset(ruleset_attr.handled_access_fs); > + if (IS_ERR(ruleset)) > + return PTR_ERR(ruleset); > + > + /* Creates anonymous FD referring to the ruleset. */ > + ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops, > + ruleset, O_RDWR | O_CLOEXEC); > + if (ruleset_fd < 0) > + landlock_put_ruleset(ruleset); > + return ruleset_fd; > +} > + > +/* > + * Returns an owned ruleset from a FD. It is thus needed to call > + * landlock_put_ruleset() on the return value. > + */ > +static struct landlock_ruleset *get_ruleset_from_fd(const int fd, > + const fmode_t mode) > +{ > + struct fd ruleset_f; > + struct landlock_ruleset *ruleset; > + > + ruleset_f = fdget(fd); > + if (!ruleset_f.file) > + return ERR_PTR(-EBADF); > + > + /* Checks FD type and access right. */ > + if (ruleset_f.file->f_op != &ruleset_fops) { > + ruleset = ERR_PTR(-EBADFD); > + goto out_fdput; > + } > + if (!(ruleset_f.file->f_mode & mode)) { > + ruleset = ERR_PTR(-EPERM); > + goto out_fdput; > + } > + ruleset = ruleset_f.file->private_data; > + if (WARN_ON_ONCE(ruleset->num_layers != 1)) { > + ruleset = ERR_PTR(-EINVAL); > + goto out_fdput; > + } > + landlock_get_ruleset(ruleset); > + > +out_fdput: > + fdput(ruleset_f); > + return ruleset; > +} > + > +/* Path handling */ > + > +/* > + * @path: Must call put_path(@path) after the call if it succeeded. > + */ > +static int get_path_from_fd(const s32 fd, struct path *const path) > +{ > + struct fd f; > + int err = 0; > + > + BUILD_BUG_ON(!__same_type(fd, > + ((struct landlock_path_beneath_attr *)NULL)->parent_fd)); > + > + /* Handles O_PATH. */ > + f = fdget_raw(fd); > + if (!f.file) > + return -EBADF; > + /* > + * Only allows O_PATH file descriptor: enables to restrict ambient > + * filesystem access without requiring to open and risk leaking or > + * misusing a file descriptor. Forbid internal filesystems (e.g. > + * nsfs), including pseudo filesystems that will never be mountable > + * (e.g. sockfs, pipefs). > + */ > + if (!(f.file->f_mode & FMODE_PATH) || > + (f.file->f_path.mnt->mnt_flags & MNT_INTERNAL) || > + (f.file->f_path.dentry->d_sb->s_flags & SB_NOUSER) || > + d_is_negative(f.file->f_path.dentry) || > + IS_PRIVATE(d_backing_inode(f.file->f_path.dentry))) { > + err = -EBADFD; > + goto out_fdput; > + } > + *path = f.file->f_path; > + path_get(path); > + > +out_fdput: > + fdput(f); > + return err; > +} > + > +/** > + * sys_landlock_add_rule - Add a new rule to a ruleset > + * > + * @ruleset_fd: File descriptor tied to the ruleset that should be extended > + * with the new rule. > + * @rule_type: Identify the structure type pointed to by @rule_attr (only > + * LANDLOCK_RULE_PATH_BENEATH for now). > + * @rule_attr: Pointer to a rule (only of type &struct > + * landlock_path_beneath_attr for now). > + * @flags: Must be 0. > + * > + * This system call enables to define a new rule and add it to an existing > + * ruleset. > + * > + * Possible returned errors are: > + * > + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; > + * - EINVAL: @flags is not 0, or inconsistent access in the rule (i.e. > + * &landlock_path_beneath_attr.allowed_access is not a subset of the rule's > + * accesses); > + * - ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access); > + * - EBADF: @ruleset_fd is not a file descriptor for the current thread, or a > + * member of @rule_attr is not a file descriptor as expected; > + * - EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of > + * @rule_attr is not the expected file descriptor type (e.g. file open > + * without O_PATH); > + * - EPERM: @ruleset_fd has no write access to the underlying ruleset; > + * - EFAULT: @rule_attr inconsistency. > + */ > +SYSCALL_DEFINE4(landlock_add_rule, > + const int, ruleset_fd, const enum landlock_rule_type, rule_type, > + const void __user *const, rule_attr, const __u32, flags) > +{ > + struct landlock_path_beneath_attr path_beneath_attr; > + struct path path; > + struct landlock_ruleset *ruleset; > + int res, err; > + > + if (!landlock_initialized) > + return -EOPNOTSUPP; > + > + /* No flag for now. */ > + if (flags) > + return -EINVAL; > + > + if (rule_type != LANDLOCK_RULE_PATH_BENEATH) > + return -EINVAL; > + > + /* Copies raw user space buffer, only one type for now. */ > + res = copy_from_user(&path_beneath_attr, rule_attr, > + sizeof(path_beneath_attr)); > + if (res) > + return -EFAULT; > + > + /* Gets and checks the ruleset. */ > + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_WRITE); > + if (IS_ERR(ruleset)) > + return PTR_ERR(ruleset); > + > + /* > + * Informs about useless rule: empty allowed_access (i.e. deny rules) > + * are ignored in path walks. > + */ > + if (!path_beneath_attr.allowed_access) { > + err = -ENOMSG; > + goto out_put_ruleset; > + } > + /* > + * Checks that allowed_access matches the @ruleset constraints > + * (ruleset->fs_access_masks[0] is automatically upgraded to 64-bits). > + */ > + if ((path_beneath_attr.allowed_access | ruleset->fs_access_masks[0]) != > + ruleset->fs_access_masks[0]) { > + err = -EINVAL; > + goto out_put_ruleset; > + } > + > + /* Gets and checks the new rule. */ > + err = get_path_from_fd(path_beneath_attr.parent_fd, &path); > + if (err) > + goto out_put_ruleset; > + > + /* Imports the new rule. */ > + err = landlock_append_fs_rule(ruleset, &path, > + path_beneath_attr.allowed_access); > + path_put(&path); > + > +out_put_ruleset: > + landlock_put_ruleset(ruleset); > + return err; > +} > + > +/* Enforcement */ > + > +/** > + * sys_landlock_restrict_self - Enforce a ruleset on the calling thread > + * > + * @ruleset_fd: File descriptor tied to the ruleset to merge with the target. > + * @flags: Must be 0. > + * > + * This system call enables to enforce a Landlock ruleset on the current > + * thread. Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its > + * namespace or is running with no_new_privs. This avoids scenarios where > + * unprivileged tasks can affect the behavior of privileged children. > + * > + * Possible returned errors are: > + * > + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; > + * - EINVAL: @flags is not 0. > + * - EBADF: @ruleset_fd is not a file descriptor for the current thread; > + * - EBADFD: @ruleset_fd is not a ruleset file descriptor; > + * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the > + * current thread is not running with no_new_privs, or it doesn't have > + * CAP_SYS_ADMIN in its namespace. > + * - E2BIG: The maximum number of stacked rulesets is reached for the current > + * thread. > + */ > +SYSCALL_DEFINE2(landlock_restrict_self, > + const int, ruleset_fd, const __u32, flags) > +{ > + struct landlock_ruleset *new_dom, *ruleset; > + struct cred *new_cred; > + struct landlock_cred_security *new_llcred; > + int err; > + > + if (!landlock_initialized) > + return -EOPNOTSUPP; > + > + /* No flag for now. */ > + if (flags) > + return -EINVAL; > + > + /* > + * Similar checks as for seccomp(2), except that an -EPERM may be > + * returned. > + */ > + if (!task_no_new_privs(current) && > + !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN)) > + return -EPERM; > + > + /* Gets and checks the ruleset. */ > + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ); > + if (IS_ERR(ruleset)) > + return PTR_ERR(ruleset); > + > + /* Prepares new credentials. */ > + new_cred = prepare_creds(); > + if (!new_cred) { > + err = -ENOMEM; > + goto out_put_ruleset; > + } > + new_llcred = landlock_cred(new_cred); > + > + /* > + * There is no possible race condition while copying and manipulating > + * the current credentials because they are dedicated per thread. > + */ > + new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset); > + if (IS_ERR(new_dom)) { > + err = PTR_ERR(new_dom); > + goto out_put_creds; > + } > + > + /* Replaces the old (prepared) domain. */ > + landlock_put_ruleset(new_llcred->domain); > + new_llcred->domain = new_dom; > + > + landlock_put_ruleset(ruleset); > + return commit_creds(new_cred); > + > +out_put_creds: > + abort_creds(new_cred); > + > +out_put_ruleset: > + landlock_put_ruleset(ruleset); > + return err; > +} > -- > 2.30.0
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index 7688bc983de5..6918be404b64 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h @@ -68,6 +68,8 @@ union bpf_attr; struct io_uring_params; struct clone_args; struct open_how; +struct landlock_ruleset_attr; +enum landlock_rule_type; #include <linux/types.h> #include <linux/aio_abi.h> @@ -1037,6 +1039,11 @@ asmlinkage long sys_pidfd_send_signal(int pidfd, int sig, siginfo_t __user *info, unsigned int flags); asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags); +asmlinkage long sys_landlock_create_ruleset(const struct landlock_ruleset_attr __user *attr, + size_t size, __u32 flags); +asmlinkage long sys_landlock_add_rule(int ruleset_fd, enum landlock_rule_type rule_type, + const void __user *rule_attr, __u32 flags); +asmlinkage long sys_landlock_restrict_self(int ruleset_fd, __u32 flags); /* * Architecture-specific system calls diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h index f69877099c8e..d1fc6af3381e 100644 --- a/include/uapi/linux/landlock.h +++ b/include/uapi/linux/landlock.h @@ -9,6 +9,59 @@ #ifndef _UAPI_LINUX_LANDLOCK_H #define _UAPI_LINUX_LANDLOCK_H +#include <linux/types.h> + +/** + * struct landlock_ruleset_attr - Ruleset definition + * + * Argument of sys_landlock_create_ruleset(). This structure can grow in + * future versions. + */ +struct landlock_ruleset_attr { + /** + * @handled_access_fs: Bitmask of actions (cf. `Filesystem flags`_) + * that is handled by this ruleset and should then be forbidden if no + * rule explicitly allow them. This is needed for backward + * compatibility reasons. + */ + __u64 handled_access_fs; +}; + +/** + * enum landlock_rule_type - Landlock rule type + * + * Argument of sys_landlock_add_rule(). + */ +enum landlock_rule_type { + /** + * @LANDLOCK_RULE_PATH_BENEATH: Type of a &struct + * landlock_path_beneath_attr . + */ + LANDLOCK_RULE_PATH_BENEATH = 1, +}; + +/** + * struct landlock_path_beneath_attr - Path hierarchy definition + * + * Argument of sys_landlock_add_rule(). + */ +struct landlock_path_beneath_attr { + /** + * @allowed_access: Bitmask of allowed actions for this file hierarchy + * (cf. `Filesystem flags`_). + */ + __u64 allowed_access; + /** + * @parent_fd: File descriptor, open with ``O_PATH``, which identifies + * the parent directory of a file hierarchy, or just a file. + */ + __s32 parent_fd; + /* + * This struct is packed to avoid trailing reserved members. + * Cf. security/landlock/syscalls.c:build_check_abi() + */ +} __attribute__((packed)); + /** * DOC: fs_access * diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c index 19aa806890d5..cce430cf2ff2 100644 --- a/kernel/sys_ni.c +++ b/kernel/sys_ni.c @@ -266,6 +266,11 @@ COND_SYSCALL(request_key); COND_SYSCALL(keyctl); COND_SYSCALL_COMPAT(keyctl); +/* security/landlock/syscalls.c */ +COND_SYSCALL(landlock_create_ruleset); +COND_SYSCALL(landlock_add_rule); +COND_SYSCALL(landlock_restrict_self); + /* arch/example/kernel/sys_example.c */ /* mm/fadvise.c */ diff --git a/security/landlock/Makefile b/security/landlock/Makefile index 92e3d80ab8ed..7bbd2f413b3e 100644 --- a/security/landlock/Makefile +++ b/security/landlock/Makefile @@ -1,4 +1,4 @@ obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o -landlock-y := setup.o object.o ruleset.o \ +landlock-y := setup.o syscalls.o object.o ruleset.o \ cred.o ptrace.o fs.o diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c new file mode 100644 index 000000000000..ebb3c126a3c0 --- /dev/null +++ b/security/landlock/syscalls.c @@ -0,0 +1,444 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Landlock LSM - System call implementations and user space interfaces + * + * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net> + * Copyright © 2018-2020 ANSSI + */ + +#include <asm/current.h> +#include <linux/anon_inodes.h> +#include <linux/build_bug.h> +#include <linux/capability.h> +#include <linux/compiler_types.h> +#include <linux/dcache.h> +#include <linux/err.h> +#include <linux/errno.h> +#include <linux/fs.h> +#include <linux/limits.h> +#include <linux/mount.h> +#include <linux/path.h> +#include <linux/sched.h> +#include <linux/security.h> +#include <linux/stddef.h> +#include <linux/syscalls.h> +#include <linux/types.h> +#include <linux/uaccess.h> +#include <uapi/linux/landlock.h> + +#include "cred.h" +#include "fs.h" +#include "limits.h" +#include "ruleset.h" +#include "setup.h" + +/** + * copy_min_struct_from_user - Safe future-proof argument copying + * + * Extend copy_struct_from_user() to check for consistent user buffer. + * + * @dst: Kernel space pointer or NULL. + * @ksize: Actual size of the data pointed to by @dst. + * @ksize_min: Minimal required size to be copied. + * @src: User space pointer or NULL. + * @usize: (Alleged) size of the data pointed to by @src. + */ +static __always_inline int copy_min_struct_from_user(void *const dst, + const size_t ksize, const size_t ksize_min, + const void __user *const src, const size_t usize) +{ + /* Checks buffer inconsistencies. */ + BUILD_BUG_ON(!dst); + if (!src) + return -EFAULT; + + /* Checks size ranges. */ + BUILD_BUG_ON(ksize <= 0); + BUILD_BUG_ON(ksize < ksize_min); + if (usize < ksize_min) + return -EINVAL; + if (usize > PAGE_SIZE) + return -E2BIG; + + /* Copies user buffer and fills with zeros. */ + return copy_struct_from_user(dst, ksize, src, usize); +} + +/* + * This function only contains arithmetic operations with constants, leading to + * BUILD_BUG_ON(). The related code is evaluated and checked at build time, + * but it is then ignored thanks to compiler optimizations. + */ +static void build_check_abi(void) +{ + struct landlock_ruleset_attr ruleset_attr; + struct landlock_path_beneath_attr path_beneath_attr; + size_t ruleset_size, path_beneath_size; + + /* + * For each user space ABI structures, first checks that there is no + * hole in them, then checks that all architectures have the same + * struct size. + */ + ruleset_size = sizeof(ruleset_attr.handled_access_fs); + BUILD_BUG_ON(sizeof(ruleset_attr) != ruleset_size); + BUILD_BUG_ON(sizeof(ruleset_attr) != 8); + + path_beneath_size = sizeof(path_beneath_attr.allowed_access); + path_beneath_size += sizeof(path_beneath_attr.parent_fd); + BUILD_BUG_ON(sizeof(path_beneath_attr) != path_beneath_size); + BUILD_BUG_ON(sizeof(path_beneath_attr) != 12); +} + +/* Ruleset handling */ + +static int fop_ruleset_release(struct inode *const inode, + struct file *const filp) +{ + struct landlock_ruleset *ruleset = filp->private_data; + + landlock_put_ruleset(ruleset); + return 0; +} + +static ssize_t fop_dummy_read(struct file *const filp, char __user *const buf, + const size_t size, loff_t *const ppos) +{ + /* Dummy handler to enable FMODE_CAN_READ. */ + return -EINVAL; +} + +static ssize_t fop_dummy_write(struct file *const filp, + const char __user *const buf, const size_t size, + loff_t *const ppos) +{ + /* Dummy handler to enable FMODE_CAN_WRITE. */ + return -EINVAL; +} + +/* + * A ruleset file descriptor enables to build a ruleset by adding (i.e. + * writing) rule after rule, without relying on the task's context. This + * reentrant design is also used in a read way to enforce the ruleset on the + * current task. + */ +static const struct file_operations ruleset_fops = { + .release = fop_ruleset_release, + .read = fop_dummy_read, + .write = fop_dummy_write, +}; + +/** + * sys_landlock_create_ruleset - Create a new ruleset + * + * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of + * the new ruleset. + * @size: Size of the pointed &struct landlock_ruleset_attr (needed for + * backward and forward compatibility). + * @flags: Must be 0. + * + * This system call enables to create a new Landlock ruleset, and returns the + * related file descriptor on success. + * + * Possible returned errors are: + * + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; + * - EINVAL: @flags is not 0, or unknown access, or too small @size; + * - E2BIG or EFAULT: @attr or @size inconsistencies; + * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs. + */ +SYSCALL_DEFINE3(landlock_create_ruleset, + const struct landlock_ruleset_attr __user *const, attr, + const size_t, size, const __u32, flags) +{ + struct landlock_ruleset_attr ruleset_attr; + struct landlock_ruleset *ruleset; + int err, ruleset_fd; + + /* Build-time checks. */ + build_check_abi(); + + if (!landlock_initialized) + return -EOPNOTSUPP; + + /* No flag for now. */ + if (flags) + return -EINVAL; + + /* Copies raw user space buffer. */ + err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr), + offsetofend(typeof(ruleset_attr), handled_access_fs), + attr, size); + if (err) + return err; + + /* Checks content (and 32-bits cast). */ + if ((ruleset_attr.handled_access_fs | LANDLOCK_MASK_ACCESS_FS) != + LANDLOCK_MASK_ACCESS_FS) + return -EINVAL; + + /* Checks arguments and transforms to kernel struct. */ + ruleset = landlock_create_ruleset(ruleset_attr.handled_access_fs); + if (IS_ERR(ruleset)) + return PTR_ERR(ruleset); + + /* Creates anonymous FD referring to the ruleset. */ + ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops, + ruleset, O_RDWR | O_CLOEXEC); + if (ruleset_fd < 0) + landlock_put_ruleset(ruleset); + return ruleset_fd; +} + +/* + * Returns an owned ruleset from a FD. It is thus needed to call + * landlock_put_ruleset() on the return value. + */ +static struct landlock_ruleset *get_ruleset_from_fd(const int fd, + const fmode_t mode) +{ + struct fd ruleset_f; + struct landlock_ruleset *ruleset; + + ruleset_f = fdget(fd); + if (!ruleset_f.file) + return ERR_PTR(-EBADF); + + /* Checks FD type and access right. */ + if (ruleset_f.file->f_op != &ruleset_fops) { + ruleset = ERR_PTR(-EBADFD); + goto out_fdput; + } + if (!(ruleset_f.file->f_mode & mode)) { + ruleset = ERR_PTR(-EPERM); + goto out_fdput; + } + ruleset = ruleset_f.file->private_data; + if (WARN_ON_ONCE(ruleset->num_layers != 1)) { + ruleset = ERR_PTR(-EINVAL); + goto out_fdput; + } + landlock_get_ruleset(ruleset); + +out_fdput: + fdput(ruleset_f); + return ruleset; +} + +/* Path handling */ + +/* + * @path: Must call put_path(@path) after the call if it succeeded. + */ +static int get_path_from_fd(const s32 fd, struct path *const path) +{ + struct fd f; + int err = 0; + + BUILD_BUG_ON(!__same_type(fd, + ((struct landlock_path_beneath_attr *)NULL)->parent_fd)); + + /* Handles O_PATH. */ + f = fdget_raw(fd); + if (!f.file) + return -EBADF; + /* + * Only allows O_PATH file descriptor: enables to restrict ambient + * filesystem access without requiring to open and risk leaking or + * misusing a file descriptor. Forbid internal filesystems (e.g. + * nsfs), including pseudo filesystems that will never be mountable + * (e.g. sockfs, pipefs). + */ + if (!(f.file->f_mode & FMODE_PATH) || + (f.file->f_path.mnt->mnt_flags & MNT_INTERNAL) || + (f.file->f_path.dentry->d_sb->s_flags & SB_NOUSER) || + d_is_negative(f.file->f_path.dentry) || + IS_PRIVATE(d_backing_inode(f.file->f_path.dentry))) { + err = -EBADFD; + goto out_fdput; + } + *path = f.file->f_path; + path_get(path); + +out_fdput: + fdput(f); + return err; +} + +/** + * sys_landlock_add_rule - Add a new rule to a ruleset + * + * @ruleset_fd: File descriptor tied to the ruleset that should be extended + * with the new rule. + * @rule_type: Identify the structure type pointed to by @rule_attr (only + * LANDLOCK_RULE_PATH_BENEATH for now). + * @rule_attr: Pointer to a rule (only of type &struct + * landlock_path_beneath_attr for now). + * @flags: Must be 0. + * + * This system call enables to define a new rule and add it to an existing + * ruleset. + * + * Possible returned errors are: + * + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; + * - EINVAL: @flags is not 0, or inconsistent access in the rule (i.e. + * &landlock_path_beneath_attr.allowed_access is not a subset of the rule's + * accesses); + * - ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access); + * - EBADF: @ruleset_fd is not a file descriptor for the current thread, or a + * member of @rule_attr is not a file descriptor as expected; + * - EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of + * @rule_attr is not the expected file descriptor type (e.g. file open + * without O_PATH); + * - EPERM: @ruleset_fd has no write access to the underlying ruleset; + * - EFAULT: @rule_attr inconsistency. + */ +SYSCALL_DEFINE4(landlock_add_rule, + const int, ruleset_fd, const enum landlock_rule_type, rule_type, + const void __user *const, rule_attr, const __u32, flags) +{ + struct landlock_path_beneath_attr path_beneath_attr; + struct path path; + struct landlock_ruleset *ruleset; + int res, err; + + if (!landlock_initialized) + return -EOPNOTSUPP; + + /* No flag for now. */ + if (flags) + return -EINVAL; + + if (rule_type != LANDLOCK_RULE_PATH_BENEATH) + return -EINVAL; + + /* Copies raw user space buffer, only one type for now. */ + res = copy_from_user(&path_beneath_attr, rule_attr, + sizeof(path_beneath_attr)); + if (res) + return -EFAULT; + + /* Gets and checks the ruleset. */ + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_WRITE); + if (IS_ERR(ruleset)) + return PTR_ERR(ruleset); + + /* + * Informs about useless rule: empty allowed_access (i.e. deny rules) + * are ignored in path walks. + */ + if (!path_beneath_attr.allowed_access) { + err = -ENOMSG; + goto out_put_ruleset; + } + /* + * Checks that allowed_access matches the @ruleset constraints + * (ruleset->fs_access_masks[0] is automatically upgraded to 64-bits). + */ + if ((path_beneath_attr.allowed_access | ruleset->fs_access_masks[0]) != + ruleset->fs_access_masks[0]) { + err = -EINVAL; + goto out_put_ruleset; + } + + /* Gets and checks the new rule. */ + err = get_path_from_fd(path_beneath_attr.parent_fd, &path); + if (err) + goto out_put_ruleset; + + /* Imports the new rule. */ + err = landlock_append_fs_rule(ruleset, &path, + path_beneath_attr.allowed_access); + path_put(&path); + +out_put_ruleset: + landlock_put_ruleset(ruleset); + return err; +} + +/* Enforcement */ + +/** + * sys_landlock_restrict_self - Enforce a ruleset on the calling thread + * + * @ruleset_fd: File descriptor tied to the ruleset to merge with the target. + * @flags: Must be 0. + * + * This system call enables to enforce a Landlock ruleset on the current + * thread. Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its + * namespace or is running with no_new_privs. This avoids scenarios where + * unprivileged tasks can affect the behavior of privileged children. + * + * Possible returned errors are: + * + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; + * - EINVAL: @flags is not 0. + * - EBADF: @ruleset_fd is not a file descriptor for the current thread; + * - EBADFD: @ruleset_fd is not a ruleset file descriptor; + * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the + * current thread is not running with no_new_privs, or it doesn't have + * CAP_SYS_ADMIN in its namespace. + * - E2BIG: The maximum number of stacked rulesets is reached for the current + * thread. + */ +SYSCALL_DEFINE2(landlock_restrict_self, + const int, ruleset_fd, const __u32, flags) +{ + struct landlock_ruleset *new_dom, *ruleset; + struct cred *new_cred; + struct landlock_cred_security *new_llcred; + int err; + + if (!landlock_initialized) + return -EOPNOTSUPP; + + /* No flag for now. */ + if (flags) + return -EINVAL; + + /* + * Similar checks as for seccomp(2), except that an -EPERM may be + * returned. + */ + if (!task_no_new_privs(current) && + !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN)) + return -EPERM; + + /* Gets and checks the ruleset. */ + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ); + if (IS_ERR(ruleset)) + return PTR_ERR(ruleset); + + /* Prepares new credentials. */ + new_cred = prepare_creds(); + if (!new_cred) { + err = -ENOMEM; + goto out_put_ruleset; + } + new_llcred = landlock_cred(new_cred); + + /* + * There is no possible race condition while copying and manipulating + * the current credentials because they are dedicated per thread. + */ + new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset); + if (IS_ERR(new_dom)) { + err = PTR_ERR(new_dom); + goto out_put_creds; + } + + /* Replaces the old (prepared) domain. */ + landlock_put_ruleset(new_llcred->domain); + new_llcred->domain = new_dom; + + landlock_put_ruleset(ruleset); + return commit_creds(new_cred); + +out_put_creds: + abort_creds(new_cred); + +out_put_ruleset: + landlock_put_ruleset(ruleset); + return err; +}