anon_inode: use a proper mode internally

This allows the VFS to not trip over anonymous inodes and we can add
asserts based on the mode into the vfs. When we report it to userspace
we can simply hide the mode to avoid regressions. I've audited all
direct callers of alloc_anon_inode() and only secretmen overrides i_mode
and i_op inode operations but it already uses a regular file.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-1-53a44c20d44e@kernel.org
Fixes: af153bb63a ("vfs: catch invalid modes in may_open()")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@vger.kernel.org # all LTS kernels
Reported-by: syzbot+5d8e79d323a13aa0b248@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/anon_inodes.c

@@ -24,9 +24,43 @@
 
 #include <linux/uaccess.h>
 
+#include "internal.h"
+
 static struct vfsmount *anon_inode_mnt __ro_after_init;
 static struct inode *anon_inode_inode __ro_after_init;
 
+/*
+ * User space expects anonymous inodes to have no file type in st_mode.
+ *
+ * In particular, 'lsof' has this legacy logic:
+ *
+ *	type = s->st_mode & S_IFMT;
+ *	switch (type) {
+ *	  ...
+ *	case 0:
+ *		if (!strcmp(p, "anon_inode"))
+ *			Lf->ntype = Ntype = N_ANON_INODE;
+ *
+ * to detect our old anon_inode logic.
+ *
+ * Rather than mess with our internal sane inode data, just fix it
+ * up here in getattr() by masking off the format bits.
+ */
+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
+		       struct kstat *stat, u32 request_mask,
+		       unsigned int query_flags)
+{
+	struct inode *inode = d_inode(path->dentry);
+
+	generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
+	stat->mode &= ~S_IFMT;
+	return 0;
+}
+
+static const struct inode_operations anon_inode_operations = {
+	.getattr = anon_inode_getattr,
+};
+
 /*
  * anon_inodefs_dname() is called from d_path().
  */
@@ -66,6 +100,7 @@ static struct inode *anon_inode_make_secure_inode(
 	if (IS_ERR(inode))
 		return inode;
 	inode->i_flags &= ~S_PRIVATE;
+	inode->i_op = &anon_inode_operations;
 	error =	security_inode_init_security_anon(inode, &QSTR(name),
 						  context_inode);
 	if (error) {
@@ -313,6 +348,7 @@ static int __init anon_inode_init(void)
 	anon_inode_inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
 	if (IS_ERR(anon_inode_inode))
 		panic("anon_inode_init() inode allocation failed (%ld)\n", PTR_ERR(anon_inode_inode));
+	anon_inode_inode->i_op = &anon_inode_operations;
 
 	return 0;
 }

fs/internal.h

@@ -343,3 +343,6 @@ static inline bool path_mounted(const struct path *path)
 void file_f_owner_release(struct file *file);
 bool file_seek_cur_needs_f_lock(struct file *file);
 int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map);
+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
+		       struct kstat *stat, u32 request_mask,
+		       unsigned int query_flags);

fs/libfs.c

@@ -1647,7 +1647,13 @@ struct inode *alloc_anon_inode(struct super_block *s)
 	 * that it already _is_ on the dirty list.
 	 */
 	inode->i_state = I_DIRTY;
-	inode->i_mode = S_IRUSR | S_IWUSR;
+	/*
+	 * Historically anonymous inodes didn't have a type at all and
+	 * userspace has come to rely on this. Internally they're just
+	 * regular files but S_IFREG is masked off when reporting
+	 * information to userspace.
+	 */
+	inode->i_mode = S_IFREG | S_IRUSR | S_IWUSR;
 	inode->i_uid = current_fsuid();
 	inode->i_gid = current_fsgid();
 	inode->i_flags |= S_PRIVATE;