bpf: fs/xattr: Add BPF kfuncs to set and remove xattrs

Add the following kfuncs to set and remove xattrs from BPF programs:

  bpf_set_dentry_xattr
  bpf_remove_dentry_xattr
  bpf_set_dentry_xattr_locked
  bpf_remove_dentry_xattr_locked

The _locked version of these kfuncs are called from hooks where
dentry->d_inode is already locked. Instead of requiring the user
to know which version of the kfuncs to use, the verifier will pick
the proper kfunc based on the calling hook.

Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20250130213549.3353349-5-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

fs/bpf_fs_kfuncs.c

@@ -2,10 +2,12 @@
 /* Copyright (c) 2024 Google LLC. */
 
 #include <linux/bpf.h>
+#include <linux/bpf_lsm.h>
 #include <linux/btf.h>
 #include <linux/btf_ids.h>
 #include <linux/dcache.h>
 #include <linux/fs.h>
+#include <linux/fsnotify.h>
 #include <linux/file.h>
 #include <linux/mm.h>
 #include <linux/xattr.h>
@@ -168,6 +170,160 @@ __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
 
 __bpf_kfunc_end_defs();
 
+static int bpf_xattr_write_permission(const char *name, struct inode *inode)
+{
+	if (WARN_ON(!inode))
+		return -EINVAL;
+
+	/* Only allow setting and removing security.bpf. xattrs */
+	if (!match_security_bpf_prefix(name))
+		return -EPERM;
+
+	return inode_permission(&nop_mnt_idmap, inode, MAY_WRITE);
+}
+
+/**
+ * bpf_set_dentry_xattr_locked - set a xattr of a dentry
+ * @dentry: dentry to get xattr from
+ * @name__str: name of the xattr
+ * @value_p: xattr value
+ * @flags: flags to pass into filesystem operations
+ *
+ * Set xattr *name__str* of *dentry* to the value in *value_ptr*.
+ *
+ * For security reasons, only *name__str* with prefix "security.bpf."
+ * is allowed.
+ *
+ * The caller already locked dentry->d_inode.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+int bpf_set_dentry_xattr_locked(struct dentry *dentry, const char *name__str,
+				const struct bpf_dynptr *value_p, int flags)
+{
+
+	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
+	struct inode *inode = d_inode(dentry);
+	const void *value;
+	u32 value_len;
+	int ret;
+
+	value_len = __bpf_dynptr_size(value_ptr);
+	value = __bpf_dynptr_data(value_ptr, value_len);
+	if (!value)
+		return -EINVAL;
+
+	ret = bpf_xattr_write_permission(name__str, inode);
+	if (ret)
+		return ret;
+
+	ret = __vfs_setxattr(&nop_mnt_idmap, dentry, inode, name__str,
+			     value, value_len, flags);
+	if (!ret) {
+		fsnotify_xattr(dentry);
+
+		/* This xattr is set by BPF LSM, so we do not call
+		 * security_inode_post_setxattr. Otherwise, we would
+		 * risk deadlocks by calling back to the same kfunc.
+		 *
+		 * This is the same as security_inode_setsecurity().
+		 */
+	}
+	return ret;
+}
+
+/**
+ * bpf_remove_dentry_xattr_locked - remove a xattr of a dentry
+ * @dentry: dentry to get xattr from
+ * @name__str: name of the xattr
+ *
+ * Rmove xattr *name__str* of *dentry*.
+ *
+ * For security reasons, only *name__str* with prefix "security.bpf."
+ * is allowed.
+ *
+ * The caller already locked dentry->d_inode.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+int bpf_remove_dentry_xattr_locked(struct dentry *dentry, const char *name__str)
+{
+	struct inode *inode = d_inode(dentry);
+	int ret;
+
+	ret = bpf_xattr_write_permission(name__str, inode);
+	if (ret)
+		return ret;
+
+	ret = __vfs_removexattr(&nop_mnt_idmap, dentry, name__str);
+	if (!ret) {
+		fsnotify_xattr(dentry);
+
+		/* This xattr is removed by BPF LSM, so we do not call
+		 * security_inode_post_removexattr. Otherwise, we would
+		 * risk deadlocks by calling back to the same kfunc.
+		 */
+	}
+	return ret;
+}
+
+__bpf_kfunc_start_defs();
+
+/**
+ * bpf_set_dentry_xattr - set a xattr of a dentry
+ * @dentry: dentry to get xattr from
+ * @name__str: name of the xattr
+ * @value_p: xattr value
+ * @flags: flags to pass into filesystem operations
+ *
+ * Set xattr *name__str* of *dentry* to the value in *value_ptr*.
+ *
+ * For security reasons, only *name__str* with prefix "security.bpf."
+ * is allowed.
+ *
+ * The caller has not locked dentry->d_inode.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_set_dentry_xattr(struct dentry *dentry, const char *name__str,
+				     const struct bpf_dynptr *value_p, int flags)
+{
+	struct inode *inode = d_inode(dentry);
+	int ret;
+
+	inode_lock(inode);
+	ret = bpf_set_dentry_xattr_locked(dentry, name__str, value_p, flags);
+	inode_unlock(inode);
+	return ret;
+}
+
+/**
+ * bpf_remove_dentry_xattr - remove a xattr of a dentry
+ * @dentry: dentry to get xattr from
+ * @name__str: name of the xattr
+ *
+ * Rmove xattr *name__str* of *dentry*.
+ *
+ * For security reasons, only *name__str* with prefix "security.bpf."
+ * is allowed.
+ *
+ * The caller has not locked dentry->d_inode.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_remove_dentry_xattr(struct dentry *dentry, const char *name__str)
+{
+	struct inode *inode = d_inode(dentry);
+	int ret;
+
+	inode_lock(inode);
+	ret = bpf_remove_dentry_xattr_locked(dentry, name__str);
+	inode_unlock(inode);
+	return ret;
+}
+
+__bpf_kfunc_end_defs();
+
 BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
 BTF_ID_FLAGS(func, bpf_get_task_exe_file,
 	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
@@ -175,6 +331,8 @@ BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
 
 static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
@@ -185,6 +343,37 @@ static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
 	return -EACCES;
 }
 
+/* bpf_[set|remove]_dentry_xattr.* hooks have KF_TRUSTED_ARGS and
+ * KF_SLEEPABLE, so they are only available to sleepable hooks with
+ * dentry arguments.
+ *
+ * Setting and removing xattr requires exclusive lock on dentry->d_inode.
+ * Some hooks already locked d_inode, while some hooks have not locked
+ * d_inode. Therefore, we need different kfuncs for different hooks.
+ * Specifically, hooks in the following list (d_inode_locked_hooks)
+ * should call bpf_[set|remove]_dentry_xattr_locked; while other hooks
+ * should call bpf_[set|remove]_dentry_xattr.
+ */
+BTF_SET_START(d_inode_locked_hooks)
+BTF_ID(func, bpf_lsm_inode_post_removexattr)
+BTF_ID(func, bpf_lsm_inode_post_setattr)
+BTF_ID(func, bpf_lsm_inode_post_setxattr)
+BTF_ID(func, bpf_lsm_inode_removexattr)
+BTF_ID(func, bpf_lsm_inode_rmdir)
+BTF_ID(func, bpf_lsm_inode_setattr)
+BTF_ID(func, bpf_lsm_inode_setxattr)
+BTF_ID(func, bpf_lsm_inode_unlink)
+#ifdef CONFIG_SECURITY_PATH
+BTF_ID(func, bpf_lsm_path_unlink)
+BTF_ID(func, bpf_lsm_path_rmdir)
+#endif /* CONFIG_SECURITY_PATH */
+BTF_SET_END(d_inode_locked_hooks)
+
+bool bpf_lsm_has_d_inode_locked(const struct bpf_prog *prog)
+{
+	return btf_id_set_contains(&d_inode_locked_hooks, prog->aux->attach_btf_id);
+}
+
 static const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
 	.owner = THIS_MODULE,
 	.set = &bpf_fs_kfunc_set_ids,

include/linux/bpf_lsm.h

@@ -48,6 +48,11 @@ void bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog, bpf_func_t *bpf_func)
 
 int bpf_lsm_get_retval_range(const struct bpf_prog *prog,
 			     struct bpf_retval_range *range);
+int bpf_set_dentry_xattr_locked(struct dentry *dentry, const char *name__str,
+				const struct bpf_dynptr *value_p, int flags);
+int bpf_remove_dentry_xattr_locked(struct dentry *dentry, const char *name__str);
+bool bpf_lsm_has_d_inode_locked(const struct bpf_prog *prog);
+
 #else /* !CONFIG_BPF_LSM */
 
 static inline bool bpf_lsm_is_sleepable_hook(u32 btf_id)
@@ -86,6 +91,19 @@ static inline int bpf_lsm_get_retval_range(const struct bpf_prog *prog,
 {
 	return -EOPNOTSUPP;
 }
+static inline int bpf_set_dentry_xattr_locked(struct dentry *dentry, const char *name__str,
+					      const struct bpf_dynptr *value_p, int flags)
+{
+	return -EOPNOTSUPP;
+}
+static inline int bpf_remove_dentry_xattr_locked(struct dentry *dentry, const char *name__str)
+{
+	return -EOPNOTSUPP;
+}
+static inline bool bpf_lsm_has_d_inode_locked(const struct bpf_prog *prog)
+{
+	return false;
+}
 #endif /* CONFIG_BPF_LSM */
 
 #endif /* _LINUX_BPF_LSM_H */

kernel/bpf/verifier.c

@@ -11766,6 +11766,8 @@ enum special_kfunc_type {
 	KF_bpf_iter_num_new,
 	KF_bpf_iter_num_next,
 	KF_bpf_iter_num_destroy,
+	KF_bpf_set_dentry_xattr,
+	KF_bpf_remove_dentry_xattr,
 };
 
 BTF_SET_START(special_kfunc_set)
@@ -11795,6 +11797,10 @@ BTF_ID(func, bpf_wq_set_callback_impl)
 #ifdef CONFIG_CGROUPS
 BTF_ID(func, bpf_iter_css_task_new)
 #endif
+#ifdef CONFIG_BPF_LSM
+BTF_ID(func, bpf_set_dentry_xattr)
+BTF_ID(func, bpf_remove_dentry_xattr)
+#endif
 BTF_SET_END(special_kfunc_set)
 
 BTF_ID_LIST(special_kfunc_list)
@@ -11844,6 +11850,13 @@ BTF_ID(func, bpf_local_irq_restore)
 BTF_ID(func, bpf_iter_num_new)
 BTF_ID(func, bpf_iter_num_next)
 BTF_ID(func, bpf_iter_num_destroy)
+#ifdef CONFIG_BPF_LSM
+BTF_ID(func, bpf_set_dentry_xattr)
+BTF_ID(func, bpf_remove_dentry_xattr)
+#else
+BTF_ID_UNUSED
+BTF_ID_UNUSED
+#endif
 
 static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
 {
@@ -20924,6 +20937,14 @@ static void specialize_kfunc(struct bpf_verifier_env *env,
 		 */
 		env->seen_direct_write = seen_direct_write;
 	}
+
+	if (func_id == special_kfunc_list[KF_bpf_set_dentry_xattr] &&
+	    bpf_lsm_has_d_inode_locked(prog))
+		*addr = (unsigned long)bpf_set_dentry_xattr_locked;
+
+	if (func_id == special_kfunc_list[KF_bpf_remove_dentry_xattr] &&
+	    bpf_lsm_has_d_inode_locked(prog))
+		*addr = (unsigned long)bpf_remove_dentry_xattr_locked;
 }
 
 static void __fixup_collection_insert_kfunc(struct bpf_insn_aux_data *insn_aux,