bpf: Mark dentry->d_inode as trusted_or_null

LSM hooks such as security_path_mknod() and security_inode_rename() have
access to newly allocated negative dentry, which has NULL d_inode.
Therefore, it is necessary to do the NULL pointer check for d_inode.

Also add selftests that checks the verifier enforces the NULL pointer
check.

Signed-off-by: Song Liu <song@kernel.org>
Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20250613052857.1992233-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: liu-song-6

kernel/bpf/verifier.c

@@ -7027,8 +7027,7 @@ BTF_TYPE_SAFE_TRUSTED(struct file) {
 	struct inode *f_inode;
 };
 
-BTF_TYPE_SAFE_TRUSTED(struct dentry) {
-	/* no negative dentry-s in places where bpf can see it */
+BTF_TYPE_SAFE_TRUSTED_OR_NULL(struct dentry) {
 	struct inode *d_inode;
 };
 
@@ -7066,7 +7065,6 @@ static bool type_is_trusted(struct bpf_verifier_env *env,
 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct bpf_iter__task));
 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct linux_binprm));
 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct file));
-	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct dentry));
 
 	return btf_nested_type_is_trusted(&env->log, reg, field_name, btf_id, "__safe_trusted");
 }
@@ -7076,6 +7074,7 @@ static bool type_is_trusted_or_null(struct bpf_verifier_env *env,
 				    const char *field_name, u32 btf_id)
 {
 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED_OR_NULL(struct socket));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED_OR_NULL(struct dentry));
 
 	return btf_nested_type_is_trusted(&env->log, reg, field_name, btf_id,
 					  "__safe_trusted_or_null");

tools/testing/selftests/bpf/progs/verifier_vfs_accept.c

@@ -2,6 +2,7 @@
 /* Copyright (c) 2024 Google LLC. */
 
 #include <vmlinux.h>
+#include <errno.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
@@ -82,4 +83,21 @@ int BPF_PROG(path_d_path_from_file_argument, struct file *file)
 	return 0;
 }
 
+SEC("lsm.s/inode_rename")
+__success
+int BPF_PROG(inode_rename, struct inode *old_dir, struct dentry *old_dentry,
+	     struct inode *new_dir, struct dentry *new_dentry,
+	     unsigned int flags)
+{
+	struct inode *inode = new_dentry->d_inode;
+	ino_t ino;
+
+	if (!inode)
+		return 0;
+	ino = inode->i_ino;
+	if (ino == 0)
+		return -EACCES;
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/verifier_vfs_reject.c

@@ -2,6 +2,7 @@
 /* Copyright (c) 2024 Google LLC. */
 
 #include <vmlinux.h>
+#include <errno.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 #include <linux/limits.h>
@@ -158,4 +159,18 @@ int BPF_PROG(path_d_path_kfunc_non_lsm, struct path *path, struct file *f)
 	return 0;
 }
 
+SEC("lsm.s/inode_rename")
+__failure __msg("invalid mem access 'trusted_ptr_or_null_'")
+int BPF_PROG(inode_rename, struct inode *old_dir, struct dentry *old_dentry,
+	     struct inode *new_dir, struct dentry *new_dentry,
+	     unsigned int flags)
+{
+	struct inode *inode = new_dentry->d_inode;
+	ino_t ino;
+
+	ino = inode->i_ino;
+	if (ino == 0)
+		return -EACCES;
+	return 0;
+}
 char _license[] SEC("license") = "GPL";