open: introduce openat2(2) syscall

/* Background. */
For a very long time, extending openat(2) with new features has been
incredibly frustrating. This stems from the fact that openat(2) is
possibly the most famous counter-example to the mantra "don't silently
accept garbage from userspace" -- it doesn't check whether unknown flags
are present[1].

This means that (generally) the addition of new flags to openat(2) has
been fraught with backwards-compatibility issues (O_TMPFILE has to be
defined as __O_TMPFILE|O_DIRECTORY|[O_RDWR or O_WRONLY] to ensure old
kernels gave errors, since it's insecure to silently ignore the
flag[2]). All new security-related flags therefore have a tough road to
being added to openat(2).

Userspace also has a hard time figuring out whether a particular flag is
supported on a particular kernel. While it is now possible with
contemporary kernels (thanks to [3]), older kernels will expose unknown
flag bits through fcntl(F_GETFL). Giving a clear -EINVAL during
openat(2) time matches modern syscall designs and is far more
fool-proof.

In addition, the newly-added path resolution restriction LOOKUP flags
(which we would like to expose to user-space) don't feel related to the
pre-existing O_* flag set -- they affect all components of path lookup.
We'd therefore like to add a new flag argument.

Adding a new syscall allows us to finally fix the flag-ignoring problem,
and we can make it extensible enough so that we will hopefully never
need an openat3(2).

/* Syscall Prototype. */
/*
* open_how is an extensible structure (similar in interface to
* clone3(2) or sched_setattr(2)). The size parameter must be set to
* sizeof(struct open_how), to allow for future extensions. All future
* extensions will be appended to open_how, with their zero value
* acting as a no-op default.
*/
struct open_how { /* ... */ };

int openat2(int dfd, const char *pathname,
struct open_how *how, size_t size);

/* Description. */
The initial version of 'struct open_how' contains the following fields:

flags
Used to specify openat(2)-style flags. However, any unknown flag
bits or otherwise incorrect flag combinations (like O_PATH|O_RDWR)
will result in -EINVAL. In addition, this field is 64-bits wide to
allow for more O_ flags than currently permitted with openat(2).

mode
The file mode for O_CREAT or O_TMPFILE.

Must be set to zero if flags does not contain O_CREAT or O_TMPFILE.

resolve
Restrict path resolution (in contrast to O_* flags they affect all
path components). The current set of flags are as follows (at the
moment, all of the RESOLVE_ flags are implemented as just passing
the corresponding LOOKUP_ flag).

RESOLVE_NO_XDEV => LOOKUP_NO_XDEV
RESOLVE_NO_SYMLINKS => LOOKUP_NO_SYMLINKS
RESOLVE_NO_MAGICLINKS => LOOKUP_NO_MAGICLINKS
RESOLVE_BENEATH => LOOKUP_BENEATH
RESOLVE_IN_ROOT => LOOKUP_IN_ROOT

open_how does not contain an embedded size field, because it is of
little benefit (userspace can figure out the kernel open_how size at
runtime fairly easily without it). It also only contains u64s (even
though ->mode arguably should be a u16) to avoid having padding fields
which are never used in the future.

Note that as a result of the new how->flags handling, O_PATH|O_TMPFILE
is no longer permitted for openat(2). As far as I can tell, this has
always been a bug and appears to not be used by userspace (and I've not
seen any problems on my machines by disallowing it). If it turns out
this breaks something, we can special-case it and only permit it for
openat(2) but not openat2(2).

After input from Florian Weimer, the new open_how and flag definitions
are inside a separate header from uapi/linux/fcntl.h, to avoid problems
that glibc has with importing that header.

/* Testing. */
In a follow-up patch there are over 200 selftests which ensure that this
syscall has the correct semantics and will correctly handle several
attack scenarios.

In addition, I've written a userspace library[4] which provides
convenient wrappers around openat2(RESOLVE_IN_ROOT) (this is necessary
because no other syscalls support RESOLVE_IN_ROOT, and thus lots of care
must be taken when using RESOLVE_IN_ROOT'd file descriptors with other
syscalls). During the development of this patch, I've run numerous
verification tests using libpathrs (showing that the API is reasonably
usable by userspace).

/* Future Work. */
Additional RESOLVE_ flags have been suggested during the review period.
These can be easily implemented separately (such as blocking auto-mount
during resolution).

Furthermore, there are some other proposed changes to the openat(2)
interface (the most obvious example is magic-link hardening[5]) which
would be a good opportunity to add a way for userspace to restrict how
O_PATH file descriptors can be re-opened.

Another possible avenue of future work would be some kind of
CHECK_FIELDS[6] flag which causes the kernel to indicate to userspace
which openat2(2) flags and fields are supported by the current kernel
(to avoid userspace having to go through several guesses to figure it
out).

[1]: https://lwn.net/Articles/588444/
[2]: https://lore.kernel.org/lkml/[e-mail]
[3]: commit 629e014bb834 ("fs: completely ignore unknown open flags")
[4]: https://sourceware.org/bugzilla/show_bug.cgi?id=17523
[5]: https://lore.kernel.org/lkml/[e-mail]/
[6]: https://youtu.be/ggD-eb3yPVs

Suggested-by: Christian Brauner <[e-mail]>
Signed-off-by: Aleksa Sarai <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

CREDITS

···
 N: Aleksa Sarai
 E: [e-mail]
 W: https://www.cyphar.com/
-D: `pids` cgroup subsystem
+D: /sys/fs/cgroup/pids
+D: openat2(2)
+S: Sydney, Australia
 
 N: Dipankar Sarma
 E: [e-mail]

MAINTAINERS

···
 F:	include/linux/fs.h
 F:	include/linux/fs_types.h
 F:	include/uapi/linux/fs.h
+F:	include/uapi/linux/openat2.h
 
 FINTEK F75375S HARDWARE MONITOR AND FAN CONTROLLER DRIVER
 M:	Riku Voipio <[e-mail]>

arch/alpha/kernel/syscalls/syscall.tbl

···
 543	common	fspick				sys_fspick
 544	common	pidfd_open			sys_pidfd_open
 # 545 reserved for clone3
+547	common	openat2				sys_openat2

arch/arm/tools/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 435	common	clone3				sys_clone3
+437	common	openat2				sys_openat2

arch/arm64/include/asm/unistd.h

···
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		436
+#define __NR_compat_syscalls		438
 #endif
 
 #define __ARCH_WANT_SYS_CLONE

arch/arm64/include/asm/unistd32.h

···
 __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
 #define __NR_clone3 435
 __SYSCALL(__NR_clone3, sys_clone3)
+#define __NR_openat2 437
+__SYSCALL(__NR_openat2, sys_openat2)
 
 /*
  * Please add new compat syscalls above this comment and update

arch/ia64/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 # 435 reserved for clone3
+437	common	openat2				sys_openat2

arch/m68k/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 # 435 reserved for clone3
+437	common	openat2				sys_openat2

arch/microblaze/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 435	common	clone3				sys_clone3
+437	common	openat2				sys_openat2

arch/mips/kernel/syscalls/syscall_n32.tbl

···
 433	n32	fspick				sys_fspick
 434	n32	pidfd_open			sys_pidfd_open
 435	n32	clone3				__sys_clone3
+437	n32	openat2				sys_openat2

arch/mips/kernel/syscalls/syscall_n64.tbl

···
 433	n64	fspick				sys_fspick
 434	n64	pidfd_open			sys_pidfd_open
 435	n64	clone3				__sys_clone3
+437	n64	openat2				sys_openat2

arch/mips/kernel/syscalls/syscall_o32.tbl

···
 433	o32	fspick				sys_fspick
 434	o32	pidfd_open			sys_pidfd_open
 435	o32	clone3				__sys_clone3
+437	o32	openat2				sys_openat2

arch/parisc/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 435	common	clone3				sys_clone3_wrapper
+437	common	openat2				sys_openat2

arch/powerpc/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 435	nospu	clone3				ppc_clone3
+437	common	openat2				sys_openat2

arch/s390/kernel/syscalls/syscall.tbl

···
 433  common	fspick			sys_fspick			sys_fspick
 434  common	pidfd_open		sys_pidfd_open			sys_pidfd_open
 435  common	clone3			sys_clone3			sys_clone3
+437  common	openat2			sys_openat2			sys_openat2

arch/sh/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 # 435 reserved for clone3
+437	common	openat2				sys_openat2

arch/sparc/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 # 435 reserved for clone3
+437	common	openat2			sys_openat2

arch/x86/entry/syscalls/syscall_32.tbl

···
 433	i386	fspick			sys_fspick			__ia32_sys_fspick
 434	i386	pidfd_open		sys_pidfd_open			__ia32_sys_pidfd_open
 435	i386	clone3			sys_clone3			__ia32_sys_clone3
+437	i386	openat2			sys_openat2			__ia32_sys_openat2

arch/x86/entry/syscalls/syscall_64.tbl

···
 433	common	fspick			__x64_sys_fspick
 434	common	pidfd_open		__x64_sys_pidfd_open
 435	common	clone3			__x64_sys_clone3/ptregs
+437	common	openat2			__x64_sys_openat2
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

arch/xtensa/kernel/syscalls/syscall.tbl

···
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
 435	common	clone3				sys_clone3
+437	common	openat2				sys_openat2

fs/open.c

···
 }
 EXPORT_SYMBOL(open_with_fake_path);
 
-static inline int build_open_flags(int flags, umode_t mode, struct open_flags *op)
+#define WILL_CREATE(flags)	(flags & (O_CREAT | __O_TMPFILE))
+#define O_PATH_FLAGS		(O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC)
+
+static inline struct open_how build_open_how(int flags, umode_t mode)
 {
+	struct open_how how = {
+		.flags = flags & VALID_OPEN_FLAGS,
+		.mode = mode & S_IALLUGO,
+	};
+
+	/* O_PATH beats everything else. */
+	if (how.flags & O_PATH)
+		how.flags &= O_PATH_FLAGS;
+	/* Modes should only be set for create-like flags. */
+	if (!WILL_CREATE(how.flags))
+		how.mode = 0;
+	return how;
+}
+
+static inline int build_open_flags(const struct open_how *how,
+				   struct open_flags *op)
+{
+	int flags = how->flags;
 	int lookup_flags = 0;
 	int acc_mode = ACC_MODE(flags);
 
-	/*
-	 * Clear out all open flags we don't know about so that we don't report
-	 * them in fcntl(F_GETFD) or similar interfaces.
-	 */
-	flags &= VALID_OPEN_FLAGS;
-
-	if (flags & (O_CREAT | __O_TMPFILE))
-		op->mode = (mode & S_IALLUGO) | S_IFREG;
-	else
-		op->mode = 0;
-
 	/* Must never be set by userspace */
-	flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC;
+	flags &= ~(FMODE_NONOTIFY | O_CLOEXEC);
+
+	/*
+	 * Older syscalls implicitly clear all of the invalid flags or argument
+	 * values before calling build_open_flags(), but openat2(2) checks all
+	 * of its arguments.
+	 */
+	if (flags & ~VALID_OPEN_FLAGS)
+		return -EINVAL;
+	if (how->resolve & ~VALID_RESOLVE_FLAGS)
+		return -EINVAL;
+
+	/* Deal with the mode. */
+	if (WILL_CREATE(flags)) {
+		if (how->mode & ~S_IALLUGO)
+			return -EINVAL;
+		op->mode = how->mode | S_IFREG;
+	} else {
+		if (how->mode != 0)
+			return -EINVAL;
+		op->mode = 0;
+	}
+
+	/*
+	 * In order to ensure programs get explicit errors when trying to use
+	 * O_TMPFILE on old kernels, O_TMPFILE is implemented such that it
+	 * looks like (O_DIRECTORY|O_RDWR & ~O_CREAT) to old kernels. But we
+	 * have to require userspace to explicitly set it.
+	 */
+	if (flags & __O_TMPFILE) {
+		if ((flags & O_TMPFILE_MASK) != O_TMPFILE)
+			return -EINVAL;
+		if (!(acc_mode & MAY_WRITE))
+			return -EINVAL;
+	}
+	if (flags & O_PATH) {
+		/* O_PATH only permits certain other flags to be set. */
+		if (flags & ~O_PATH_FLAGS)
+			return -EINVAL;
+		acc_mode = 0;
+	}
 
 	/*
 	 * O_SYNC is implemented as __O_SYNC|O_DSYNC.  As many places only
···
 	 */
 	if (flags & __O_SYNC)
 		flags |= O_DSYNC;
-
-	if (flags & __O_TMPFILE) {
-		if ((flags & O_TMPFILE_MASK) != O_TMPFILE)
-			return -EINVAL;
-		if (!(acc_mode & MAY_WRITE))
-			return -EINVAL;
-	} else if (flags & O_PATH) {
-		/*
-		 * If we have O_PATH in the open flag. Then we
-		 * cannot have anything other than the below set of flags
-		 */
-		flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
-		acc_mode = 0;
-	}
 
 	op->open_flag = flags;
 
···
 		lookup_flags |= LOOKUP_DIRECTORY;
 	if (!(flags & O_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
+
+	if (how->resolve & RESOLVE_NO_XDEV)
+		lookup_flags |= LOOKUP_NO_XDEV;
+	if (how->resolve & RESOLVE_NO_MAGICLINKS)
+		lookup_flags |= LOOKUP_NO_MAGICLINKS;
+	if (how->resolve & RESOLVE_NO_SYMLINKS)
+		lookup_flags |= LOOKUP_NO_SYMLINKS;
+	if (how->resolve & RESOLVE_BENEATH)
+		lookup_flags |= LOOKUP_BENEATH;
+	if (how->resolve & RESOLVE_IN_ROOT)
+		lookup_flags |= LOOKUP_IN_ROOT;
+
 	op->lookup_flags = lookup_flags;
 	return 0;
 }
···
 struct file *file_open_name(struct filename *name, int flags, umode_t mode)
 {
 	struct open_flags op;
-	int err = build_open_flags(flags, mode, &op);
-	return err ? ERR_PTR(err) : do_filp_open(AT_FDCWD, name, &op);
+	struct open_how how = build_open_how(flags, mode);
+	int err = build_open_flags(&how, &op);
+	if (err)
+		return ERR_PTR(err);
+	return do_filp_open(AT_FDCWD, name, &op);
 }
 
 /**
···
 			    const char *filename, int flags, umode_t mode)
 {
 	struct open_flags op;
-	int err = build_open_flags(flags, mode, &op);
+	struct open_how how = build_open_how(flags, mode);
+	int err = build_open_flags(&how, &op);
 	if (err)
 		return ERR_PTR(err);
 	return do_file_open_root(dentry, mnt, filename, &op);
 }
 EXPORT_SYMBOL(file_open_root);
 
-long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
+static long do_sys_openat2(int dfd, const char __user *filename,
+			   struct open_how *how)
 {
 	struct open_flags op;
-	int fd = build_open_flags(flags, mode, &op);
+	int fd = build_open_flags(how, &op);
 	struct filename *tmp;
 
 	if (fd)
···
 	if (IS_ERR(tmp))
 		return PTR_ERR(tmp);
 
-	fd = get_unused_fd_flags(flags);
+	fd = get_unused_fd_flags(how->flags);
 	if (fd >= 0) {
 		struct file *f = do_filp_open(dfd, tmp, &op);
 		if (IS_ERR(f)) {
···
 	return fd;
 }
 
+long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
+{
+	struct open_how how = build_open_how(flags, mode);
+	return do_sys_openat2(dfd, filename, &how);
+}
+
+
 SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
 {
-	if (force_o_largefile())
-		flags |= O_LARGEFILE;
-
-	return do_sys_open(AT_FDCWD, filename, flags, mode);
+	return ksys_open(filename, flags, mode);
 }
 
 SYSCALL_DEFINE4(openat, int, dfd, const char __user *, filename, int, flags,
···
 {
 	if (force_o_largefile())
 		flags |= O_LARGEFILE;
-
 	return do_sys_open(dfd, filename, flags, mode);
+}
+
+SYSCALL_DEFINE4(openat2, int, dfd, const char __user *, filename,
+		struct open_how __user *, how, size_t, usize)
+{
+	int err;
+	struct open_how tmp;
+
+	BUILD_BUG_ON(sizeof(struct open_how) < OPEN_HOW_SIZE_VER0);
+	BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_LATEST);
+
+	if (unlikely(usize < OPEN_HOW_SIZE_VER0))
+		return -EINVAL;
+
+	err = copy_struct_from_user(&tmp, sizeof(tmp), how, usize);
+	if (err)
+		return err;
+
+	/* O_LARGEFILE is only allowed for non-O_PATH. */
+	if (!(tmp.flags & O_PATH) && force_o_largefile())
+		tmp.flags |= O_LARGEFILE;
+
+	return do_sys_openat2(dfd, filename, &tmp);
 }
 
 #ifdef CONFIG_COMPAT

include/linux/fcntl.h

···
 #ifndef _LINUX_FCNTL_H
 #define _LINUX_FCNTL_H
 
+#include <linux/stat.h>
 #include <uapi/linux/fcntl.h>
 
-/* list of all valid flags for the open/openat flags argument: */
+/* List of all valid flags for the open/openat flags argument: */
 #define VALID_OPEN_FLAGS \
 	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
 	 O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
 	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
 	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
+
+/* List of all valid flags for the how->upgrade_mask argument: */
+#define VALID_UPGRADE_FLAGS \
+	(UPGRADE_NOWRITE | UPGRADE_NOREAD)
+
+/* List of all valid flags for the how->resolve argument: */
+#define VALID_RESOLVE_FLAGS \
+	(RESOLVE_NO_XDEV | RESOLVE_NO_MAGICLINKS | RESOLVE_NO_SYMLINKS | \
+	 RESOLVE_BENEATH | RESOLVE_IN_ROOT)
+
+/* List of all open_how "versions". */
+#define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
 
 #ifndef force_o_largefile
 #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))

include/linux/syscalls.h

···
 union bpf_attr;
 struct io_uring_params;
 struct clone_args;
+struct open_how;
 
 #include <linux/types.h>
 #include <linux/aio_abi.h>
···
 asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group);
 asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
 			   umode_t mode);
+asmlinkage long sys_openat2(int dfd, const char __user *filename,
+			    struct open_how *how, size_t size);
 asmlinkage long sys_close(unsigned int fd);
 asmlinkage long sys_vhangup(void);
 

include/uapi/asm-generic/unistd.h

···
 __SYSCALL(__NR_clone3, sys_clone3)
 #endif
 
+#define __NR_openat2 437
+__SYSCALL(__NR_openat2, sys_openat2)
+
 #undef __NR_syscalls
-#define __NR_syscalls 436
+#define __NR_syscalls 438
 
 /*
  * 32 bit systems traditionally used different

include/uapi/linux/fcntl.h

···
 #define _UAPI_LINUX_FCNTL_H
 
 #include <asm/fcntl.h>
+#include <linux/openat2.h>
 
 #define F_SETLEASE	(F_LINUX_SPECIFIC_BASE + 0)
 #define F_GETLEASE	(F_LINUX_SPECIFIC_BASE + 1)
···
 #define AT_STATX_DONT_SYNC	0x4000	/* - Don't sync attributes with the server */
 
 #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */
-
 
 #endif /* _UAPI_LINUX_FCNTL_H */

include/uapi/linux/openat2.h

···
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_OPENAT2_H
+#define _UAPI_LINUX_OPENAT2_H
+
+#include <linux/types.h>
+
+/*
+ * Arguments for how openat2(2) should open the target path. If only @flags and
+ * @mode are non-zero, then openat2(2) operates very similarly to openat(2).
+ *
+ * However, unlike openat(2), unknown or invalid bits in @flags result in
+ * -EINVAL rather than being silently ignored. @mode must be zero unless one of
+ * {O_CREAT, O_TMPFILE} are set.
+ *
+ * @flags: O_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
+ * @resolve: RESOLVE_* flags.
+ */
+struct open_how {
+	__u64 flags;
+	__u64 mode;
+	__u64 resolve;
+};
+
+/* how->resolve flags for openat2(2). */
+#define RESOLVE_NO_XDEV		0x01 /* Block mount-point crossings
+					(includes bind-mounts). */
+#define RESOLVE_NO_MAGICLINKS	0x02 /* Block traversal through procfs-style
+					"magic-links". */
+#define RESOLVE_NO_SYMLINKS	0x04 /* Block traversal through all symlinks
+					(implies OEXT_NO_MAGICLINKS) */
+#define RESOLVE_BENEATH		0x08 /* Block "lexical" trickery like
+					"..", symlinks, and absolute
+					paths which escape the dirfd. */
+#define RESOLVE_IN_ROOT		0x10 /* Make all jumps to "/" and ".."
+					be scoped inside the dirfd
+					(similar to chroot(2)). */
+
+#endif /* _UAPI_LINUX_OPENAT2_H */