ext4: limit xattr size to INT_MAX

ext4 isn't validating the sizes of xattrs where the value of the xattr
is stored in an external inode.  This is problematic because
->e_value_size is a u32, but ext4_xattr_get() returns an int.  A very
large size is misinterpreted as an error code, which ext4_get_acl()
translates into a bogus ERR_PTR() for which IS_ERR() returns false,
causing a crash.

Fix this by validating that all xattrs are <= INT_MAX bytes.

This issue has been assigned CVE-2018-1095.

https://bugzilla.kernel.org/show_bug.cgi?id=199185
https://bugzilla.redhat.com/show_bug.cgi?id=1560793

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Fixes: e50e5129f384 ("ext4: xattr-in-inode support")

fs/ext4/xattr.c

@@ -195,10 +195,13 @@ ext4_xattr_check_entries(struct ext4_xattr_entry *entry, void *end,
 
 	/* Check the values */
 	while (!IS_LAST_ENTRY(entry)) {
-		if (entry->e_value_size != 0 &&
-		    entry->e_value_inum == 0) {
+		u32 size = le32_to_cpu(entry->e_value_size);
+
+		if (size > INT_MAX)
+			return -EFSCORRUPTED;
+
+		if (size != 0 && entry->e_value_inum == 0) {
 			u16 offs = le16_to_cpu(entry->e_value_offs);
-			u32 size = le32_to_cpu(entry->e_value_size);
 			void *value;
 
 			/*