ext4: don't treat fhandle lookup of ea_inode as FS corruption

A file handle that userspace provides to open_by_handle_at() can
legitimately contain an outdated inode number that has since been reused
for another purpose - that's why the file handle also contains a generation
number.

But if the inode number has been reused for an ea_inode, check_igot_inode()
will notice, __ext4_iget() will go through ext4_error_inode(), and if the
inode was newly created, it will also be marked as bad by iget_failed().
This all happens before the point where the inode generation is checked.

ext4_error_inode() is supposed to only be used on filesystem corruption; it
should not be used when userspace just got unlucky with a stale file
handle. So when this happens, let __ext4_iget() just return an error.

Fixes: b3e6bcb ("ext4: add EA_INODE checking to ext4_iget()")
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241129-ext4-ignore-ea-fhandle-v1-1-e532c0d1cee0@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: thejh

fs/ext4/inode.c

@@ -4732,22 +4732,43 @@ static inline void ext4_inode_set_iversion_queried(struct inode *inode, u64 val)
 		inode_set_iversion_queried(inode, val);
 }
 
-static const char *check_igot_inode(struct inode *inode, ext4_iget_flags flags)
-
+static int check_igot_inode(struct inode *inode, ext4_iget_flags flags,
+			    const char *function, unsigned int line)
 {
+	const char *err_str;
+
 	if (flags & EXT4_IGET_EA_INODE) {
-		if (!(EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL))
-			return "missing EA_INODE flag";
+		if (!(EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL)) {
+			err_str = "missing EA_INODE flag";
+			goto error;
+		}
 		if (ext4_test_inode_state(inode, EXT4_STATE_XATTR) ||
-		    EXT4_I(inode)->i_file_acl)
-			return "ea_inode with extended attributes";
+		    EXT4_I(inode)->i_file_acl) {
+			err_str = "ea_inode with extended attributes";
+			goto error;
+		}
 	} else {
-		if ((EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL))
-			return "unexpected EA_INODE flag";
+		if ((EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL)) {
+			/*
+			 * open_by_handle_at() could provide an old inode number
+			 * that has since been reused for an ea_inode; this does
+			 * not indicate filesystem corruption
+			 */
+			if (flags & EXT4_IGET_HANDLE)
+				return -ESTALE;
+			err_str = "unexpected EA_INODE flag";
+			goto error;
+		}
+	}
+	if (is_bad_inode(inode) && !(flags & EXT4_IGET_BAD)) {
+		err_str = "unexpected bad inode w/o EXT4_IGET_BAD";
+		goto error;
 	}
-	if (is_bad_inode(inode) && !(flags & EXT4_IGET_BAD))
-		return "unexpected bad inode w/o EXT4_IGET_BAD";
-	return NULL;
+	return 0;
+
+error:
+	ext4_error_inode(inode, function, line, 0, err_str);
+	return -EFSCORRUPTED;
 }
 
 struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
@@ -4759,7 +4780,6 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 	struct ext4_inode_info *ei;
 	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
 	struct inode *inode;
-	const char *err_str;
 	journal_t *journal = EXT4_SB(sb)->s_journal;
 	long ret;
 	loff_t size;
@@ -4788,10 +4808,10 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 	if (!inode)
 		return ERR_PTR(-ENOMEM);
 	if (!(inode->i_state & I_NEW)) {
-		if ((err_str = check_igot_inode(inode, flags)) != NULL) {
-			ext4_error_inode(inode, function, line, 0, err_str);
+		ret = check_igot_inode(inode, flags, function, line);
+		if (ret) {
 			iput(inode);
-			return ERR_PTR(-EFSCORRUPTED);
+			return ERR_PTR(ret);
 		}
 		return inode;
 	}
@@ -5073,13 +5093,21 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 		ret = -EFSCORRUPTED;
 		goto bad_inode;
 	}
-	if ((err_str = check_igot_inode(inode, flags)) != NULL) {
-		ext4_error_inode(inode, function, line, 0, err_str);
-		ret = -EFSCORRUPTED;
-		goto bad_inode;
+	ret = check_igot_inode(inode, flags, function, line);
+	/*
+	 * -ESTALE here means there is nothing inherently wrong with the inode,
+	 * it's just not an inode we can return for an fhandle lookup.
+	 */
+	if (ret == -ESTALE) {
+		brelse(iloc.bh);
+		unlock_new_inode(inode);
+		iput(inode);
+		return ERR_PTR(-ESTALE);
 	}
-
+	if (ret)
+		goto bad_inode;
 	brelse(iloc.bh);
+
 	unlock_new_inode(inode);
 	return inode;