[PATCH vfs-7.5.bpf v2 1/3] bpf: Add bpf_path_getattr()

[Anastasios Papagiannis]

Direct inode reads can differ from stat() and related system calls,
especially on overlayfs.

Add a sleepable LSM kfunc to query a trusted path and return struct kstat.
Use vfs_getattr_nosec() to avoid calling security_inode_getattr() again.
Limit the copy to the smaller of the buffer and the kernel structure.

Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@xxxxxxxxx>
---
 fs/bpf_fs_kfuncs.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 35ccd98a4de6..97afc9659f1a 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -96,6 +96,40 @@ __bpf_kfunc int bpf_path_d_path(const struct path *path, char *buf, size_t buf__
 	return len;
 }
 
+/**
+ * bpf_path_getattr - get the attributes of the supplied path
+ * @path: trusted path to get the attributes of
+ * @request_mask: STATX_* flags of the attributes that the caller wants
+ * @query_flags: AT_STATX_SYNC_AS_STAT, AT_STATX_FORCE_SYNC or AT_STATX_DONT_SYNC
+ * @stat__uninit: buffer to return the attributes in
+ * @stat__sz: size of the supplied buffer
+ *
+ * Ask the filesystem for the attributes of *path*. This is vfs_getattr()
+ * without the call into security_inode_getattr().
+ *
+ * This BPF kfunc may only be called from sleepable BPF LSM programs.
+ *
+ * Copies the smaller of @stat__sz and sizeof(struct kstat). Any remaining
+ * buffer bytes are left untouched. On error, the buffer is unchanged.
+ *
+ * Return: Zero on success or a negative error code.
+ */
+__bpf_kfunc int bpf_path_getattr(const struct path *path, u32 request_mask,
+				 u32 query_flags, struct kstat *stat__uninit,
+				 size_t stat__sz)
+{
+	struct kstat stat;
+	int ret;
+
+	ret = vfs_getattr_nosec(path, &stat, request_mask, query_flags);
+	if (ret)
+		return ret;
+
+	/* struct kstat in the program's vmlinux.h may have a different size. */
+	memcpy(stat__uninit, &stat, min(stat__sz, sizeof(stat)));
+	return 0;
+}
+
 static bool match_security_bpf_prefix(const char *name__str)
 {
 	return !strncmp(name__str, XATTR_NAME_BPF_LSM, XATTR_NAME_BPF_LSM_LEN);
@@ -504,6 +538,7 @@ BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
 BTF_ID_FLAGS(func, bpf_get_task_exe_file, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_path_d_path)
+BTF_ID_FLAGS(func, bpf_path_getattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE)
@@ -515,8 +550,9 @@ BTF_ID_FLAGS(func, bpf_sock_read_xattr, KF_RCU)
 #endif
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
 
-/* Side-effecting kfuncs that stay exclusive to LSM programs. */
+/* Kfuncs that stay exclusive to LSM programs. */
 BTF_SET_START(bpf_fs_kfunc_lsm_only_ids)
+BTF_ID(func, bpf_path_getattr)
 BTF_ID(func, bpf_set_dentry_xattr)
 BTF_ID(func, bpf_remove_dentry_xattr)
 BTF_SET_END(bpf_fs_kfunc_lsm_only_ids)
-- 
2.56.0