bpf: Support O_PATH FDs in BPF_OBJ_PIN and BPF_OBJ_GET commands

Current UAPI of BPF_OBJ_PIN and BPF_OBJ_GET commands of bpf() syscall
forces users to specify pinning location as a string-based absolute or
relative (to current working directory) path. This has various
implications related to security (e.g., symlink-based attacks), forces
BPF FS to be exposed in the file system, which can cause races with
other applications.

One of the feedbacks we got from folks working with containers heavily
was that inability to use purely FD-based location specification was an
unfortunate limitation and hindrance for BPF_OBJ_PIN and BPF_OBJ_GET
commands. This patch closes this oversight, adding path_fd field to
BPF_OBJ_PIN and BPF_OBJ_GET UAPI, following conventions established by
*at() syscalls for dirfd + pathname combinations.

This now allows interesting possibilities like working with detached BPF
FS mount (e.g., to perform multiple pinnings without running a risk of
someone interfering with them), and generally making pinning/getting
more secure and not prone to any races and/or security attacks.

This is demonstrated by a selftest added in subsequent patch that takes
advantage of new mount APIs (fsopen, fsconfig, fsmount) to demonstrate
creating detached BPF FS mount, pinning, and then getting BPF map out of
it, all while never exposing this private instance of BPF FS to outside
worlds.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/bpf/20230523170013.728457-4-andrii@kernel.org

include/linux/bpf.h

@@ -2077,8 +2077,8 @@ struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
 struct bpf_link *bpf_link_get_from_fd(u32 ufd);
 struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
 
-int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
-int bpf_obj_get_user(const char __user *pathname, int flags);
+int bpf_obj_pin_user(u32 ufd, int path_fd, const char __user *pathname);
+int bpf_obj_get_user(int path_fd, const char __user *pathname, int flags);
 
 #define BPF_ITER_FUNC_PREFIX "bpf_iter_"
 #define DEFINE_BPF_ITER_FUNC(target, args...)			\

include/uapi/linux/bpf.h

@@ -1272,6 +1272,9 @@ enum {
 
 /* Create a map that will be registered/unregesitered by the backed bpf_link */
 	BPF_F_LINK		= (1U << 13),
+
+/* Get path from provided FD in BPF_OBJ_PIN/BPF_OBJ_GET commands */
+	BPF_F_PATH_FD		= (1U << 14),
 };
 
 /* Flags for BPF_PROG_QUERY. */
@@ -1420,6 +1423,13 @@ union bpf_attr {
 		__aligned_u64	pathname;
 		__u32		bpf_fd;
 		__u32		file_flags;
+		/* Same as dirfd in openat() syscall; see openat(2)
+		 * manpage for details of path FD and pathname semantics;
+		 * path_fd should accompanied by BPF_F_PATH_FD flag set in
+		 * file_flags field, otherwise it should be set to zero;
+		 * if BPF_F_PATH_FD flag is not set, AT_FDCWD is assumed.
+		 */
+		__s32		path_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_PROG_ATTACH/DETACH commands */

kernel/bpf/inode.c

@@ -435,7 +435,7 @@ static int bpf_iter_link_pin_kernel(struct dentry *parent,
 	return ret;
 }
 
-static int bpf_obj_do_pin(const char __user *pathname, void *raw,
+static int bpf_obj_do_pin(int path_fd, const char __user *pathname, void *raw,
 			  enum bpf_type type)
 {
 	struct dentry *dentry;
@@ -444,7 +444,7 @@ static int bpf_obj_do_pin(const char __user *pathname, void *raw,
 	umode_t mode;
 	int ret;
 
-	dentry = user_path_create(AT_FDCWD, pathname, &path, 0);
+	dentry = user_path_create(path_fd, pathname, &path, 0);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 
@@ -477,7 +477,7 @@ out:
 	return ret;
 }
 
-int bpf_obj_pin_user(u32 ufd, const char __user *pathname)
+int bpf_obj_pin_user(u32 ufd, int path_fd, const char __user *pathname)
 {
 	enum bpf_type type;
 	void *raw;
@@ -487,14 +487,14 @@ int bpf_obj_pin_user(u32 ufd, const char __user *pathname)
 	if (IS_ERR(raw))
 		return PTR_ERR(raw);
 
-	ret = bpf_obj_do_pin(pathname, raw, type);
+	ret = bpf_obj_do_pin(path_fd, pathname, raw, type);
 	if (ret != 0)
 		bpf_any_put(raw, type);
 
 	return ret;
 }
 
-static void *bpf_obj_do_get(const char __user *pathname,
+static void *bpf_obj_do_get(int path_fd, const char __user *pathname,
 			    enum bpf_type *type, int flags)
 {
 	struct inode *inode;
@@ -502,7 +502,7 @@ static void *bpf_obj_do_get(const char __user *pathname,
 	void *raw;
 	int ret;
 
-	ret = user_path_at(AT_FDCWD, pathname, LOOKUP_FOLLOW, &path);
+	ret = user_path_at(path_fd, pathname, LOOKUP_FOLLOW, &path);
 	if (ret)
 		return ERR_PTR(ret);
 
@@ -526,7 +526,7 @@ out:
 	return ERR_PTR(ret);
 }
 
-int bpf_obj_get_user(const char __user *pathname, int flags)
+int bpf_obj_get_user(int path_fd, const char __user *pathname, int flags)
 {
 	enum bpf_type type = BPF_TYPE_UNSPEC;
 	int f_flags;
@@ -537,7 +537,7 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
 	if (f_flags < 0)
 		return f_flags;
 
-	raw = bpf_obj_do_get(pathname, &type, f_flags);
+	raw = bpf_obj_do_get(path_fd, pathname, &type, f_flags);
 	if (IS_ERR(raw))
 		return PTR_ERR(raw);
 

kernel/bpf/syscall.c

@@ -2697,23 +2697,38 @@ free_prog:
 	return err;
 }
 
-#define BPF_OBJ_LAST_FIELD file_flags
+#define BPF_OBJ_LAST_FIELD path_fd
 
 static int bpf_obj_pin(const union bpf_attr *attr)
 {
-	if (CHECK_ATTR(BPF_OBJ) || attr->file_flags != 0)
+	int path_fd;
+
+	if (CHECK_ATTR(BPF_OBJ) || attr->file_flags & ~BPF_F_PATH_FD)
 		return -EINVAL;
 
-	return bpf_obj_pin_user(attr->bpf_fd, u64_to_user_ptr(attr->pathname));
+	/* path_fd has to be accompanied by BPF_F_PATH_FD flag */
+	if (!(attr->file_flags & BPF_F_PATH_FD) && attr->path_fd)
+		return -EINVAL;
+
+	path_fd = attr->file_flags & BPF_F_PATH_FD ? attr->path_fd : AT_FDCWD;
+	return bpf_obj_pin_user(attr->bpf_fd, path_fd,
+				u64_to_user_ptr(attr->pathname));
 }
 
 static int bpf_obj_get(const union bpf_attr *attr)
 {
+	int path_fd;
+
 	if (CHECK_ATTR(BPF_OBJ) || attr->bpf_fd != 0 ||
-	    attr->file_flags & ~BPF_OBJ_FLAG_MASK)
+	    attr->file_flags & ~(BPF_OBJ_FLAG_MASK | BPF_F_PATH_FD))
 		return -EINVAL;
 
-	return bpf_obj_get_user(u64_to_user_ptr(attr->pathname),
+	/* path_fd has to be accompanied by BPF_F_PATH_FD flag */
+	if (!(attr->file_flags & BPF_F_PATH_FD) && attr->path_fd)
+		return -EINVAL;
+
+	path_fd = attr->file_flags & BPF_F_PATH_FD ? attr->path_fd : AT_FDCWD;
+	return bpf_obj_get_user(path_fd, u64_to_user_ptr(attr->pathname),
 				attr->file_flags);
 }
 

tools/include/uapi/linux/bpf.h

@@ -1272,6 +1272,9 @@ enum {
 
 /* Create a map that will be registered/unregesitered by the backed bpf_link */
 	BPF_F_LINK		= (1U << 13),
+
+/* Get path from provided FD in BPF_OBJ_PIN/BPF_OBJ_GET commands */
+	BPF_F_PATH_FD		= (1U << 14),
 };
 
 /* Flags for BPF_PROG_QUERY. */
@@ -1420,6 +1423,13 @@ union bpf_attr {
 		__aligned_u64	pathname;
 		__u32		bpf_fd;
 		__u32		file_flags;
+		/* Same as dirfd in openat() syscall; see openat(2)
+		 * manpage for details of path FD and pathname semantics;
+		 * path_fd should accompanied by BPF_F_PATH_FD flag set in
+		 * file_flags field, otherwise it should be set to zero;
+		 * if BPF_F_PATH_FD flag is not set, AT_FDCWD is assumed.
+		 */
+		__s32		path_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_PROG_ATTACH/DETACH commands */