ext4: don't read out of bounds when checking for in-inode xattrs

With i_extra_isize equal to or close to the available space, it was
possible for us to read past the end of the inode when trying to detect
or validate in-inode xattrs.  Fix this by checking for the needed extra
space first.

This patch shouldn't have any noticeable effect on
non-corrupted/non-malicious filesystems.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

fs/ext4/inode.c

@@ -4527,7 +4527,9 @@ static inline void ext4_iget_extra_inode(struct inode *inode,
 {
 	__le32 *magic = (void *)raw_inode +
 			EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize;
-	if (*magic == cpu_to_le32(EXT4_XATTR_MAGIC)) {
+	if (EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize + sizeof(__le32) <=
+	    EXT4_INODE_SIZE(inode->i_sb) &&
+	    *magic == cpu_to_le32(EXT4_XATTR_MAGIC)) {
 		ext4_set_inode_state(inode, EXT4_STATE_XATTR);
 		ext4_find_inline_data_nolock(inode);
 	} else

fs/ext4/xattr.c

@@ -231,13 +231,12 @@ static int
 __xattr_check_inode(struct inode *inode, struct ext4_xattr_ibody_header *header,
 			 void *end, const char *function, unsigned int line)
 {
-	struct ext4_xattr_entry *entry = IFIRST(header);
 	int error = -EFSCORRUPTED;
 
-	if (((void *) header >= end) ||
+	if (end - (void *)header < sizeof(*header) + sizeof(u32) ||
 	    (header->h_magic != cpu_to_le32(EXT4_XATTR_MAGIC)))
 		goto errout;
-	error = ext4_xattr_check_names(entry, end, entry);
+	error = ext4_xattr_check_names(IFIRST(header), end, IFIRST(header));
 errout:
 	if (error)
 		__ext4_error_inode(inode, function, line, 0,