ext4: add bounds checking to ext4_xattr_find_entry()

Add some paranoia checks to make sure we don't stray beyond the end of
the valid memory region containing ext4 xattr entries while we are
scanning for a match.

Also rename the function to xattr_find_entry() since it is static and
thus only used in fs/ext4/xattr.c

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/xattr.c

@@ -276,18 +276,22 @@ errout:
 	__xattr_check_inode((inode), (header), (end), __func__, __LINE__)
 
 static int
-ext4_xattr_find_entry(struct ext4_xattr_entry **pentry, int name_index,
-		      const char *name, int sorted)
+xattr_find_entry(struct inode *inode, struct ext4_xattr_entry **pentry,
+		 void *end, int name_index, const char *name, int sorted)
 {
-	struct ext4_xattr_entry *entry;
+	struct ext4_xattr_entry *entry, *next;
 	size_t name_len;
 	int cmp = 1;
 
 	if (name == NULL)
 		return -EINVAL;
 	name_len = strlen(name);
-	entry = *pentry;
-	for (; !IS_LAST_ENTRY(entry); entry = EXT4_XATTR_NEXT(entry)) {
+	for (entry = *pentry; !IS_LAST_ENTRY(entry); entry = next) {
+		next = EXT4_XATTR_NEXT(entry);
+		if ((void *) next >= end) {
+			EXT4_ERROR_INODE(inode, "corrupted xattr entries");
+			return -EFSCORRUPTED;
+		}
 		cmp = name_index - entry->e_name_index;
 		if (!cmp)
 			cmp = name_len - entry->e_name_len;
@@ -509,6 +513,7 @@ ext4_xattr_block_get(struct inode *inode, int name_index, const char *name,
 	struct buffer_head *bh = NULL;
 	struct ext4_xattr_entry *entry;
 	size_t size;
+	void *end;
 	int error;
 	struct mb_cache *ea_block_cache = EA_BLOCK_CACHE(inode);
 
@@ -530,7 +535,8 @@ ext4_xattr_block_get(struct inode *inode, int name_index, const char *name,
 		goto cleanup;
 	ext4_xattr_block_cache_insert(ea_block_cache, bh);
 	entry = BFIRST(bh);
-	error = ext4_xattr_find_entry(&entry, name_index, name, 1);
+	end = bh->b_data + bh->b_size;
+	error = xattr_find_entry(inode, &entry, end, name_index, name, 1);
 	if (error)
 		goto cleanup;
 	size = le32_to_cpu(entry->e_value_size);
@@ -579,7 +585,7 @@ ext4_xattr_ibody_get(struct inode *inode, int name_index, const char *name,
 	if (error)
 		goto cleanup;
 	entry = IFIRST(header);
-	error = ext4_xattr_find_entry(&entry, name_index, name, 0);
+	error = xattr_find_entry(inode, &entry, end, name_index, name, 0);
 	if (error)
 		goto cleanup;
 	size = le32_to_cpu(entry->e_value_size);
@@ -1808,8 +1814,8 @@ ext4_xattr_block_find(struct inode *inode, struct ext4_xattr_info *i,
 		bs->s.first = BFIRST(bs->bh);
 		bs->s.end = bs->bh->b_data + bs->bh->b_size;
 		bs->s.here = bs->s.first;
-		error = ext4_xattr_find_entry(&bs->s.here, i->name_index,
-					      i->name, 1);
+		error = xattr_find_entry(inode, &bs->s.here, bs->s.end,
+					 i->name_index, i->name, 1);
 		if (error && error != -ENODATA)
 			goto cleanup;
 		bs->s.not_found = error;
@@ -2168,8 +2174,8 @@ int ext4_xattr_ibody_find(struct inode *inode, struct ext4_xattr_info *i,
 		if (error)
 			return error;
 		/* Find the named attribute. */
-		error = ext4_xattr_find_entry(&is->s.here, i->name_index,
-					      i->name, 0);
+		error = xattr_find_entry(inode, &is->s.here, is->s.end,
+					 i->name_index, i->name, 0);
 		if (error && error != -ENODATA)
 			return error;
 		is->s.not_found = error;