ext4: add extra checks to ext4_xattr_block_get()

Add explicit checks in ext4_xattr_block_get() just in case the
e_value_offs and e_value_size fields in the the xattr block are
corrupted in memory after the buffer_verified bit is set on the xattr
block.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/xattr.c

@@ -197,7 +197,7 @@ ext4_xattr_check_entries(struct ext4_xattr_entry *entry, void *end,
 	while (!IS_LAST_ENTRY(entry)) {
 		u32 size = le32_to_cpu(entry->e_value_size);
 
-		if (size > INT_MAX)
+		if (size > EXT4_XATTR_SIZE_MAX)
 			return -EFSCORRUPTED;
 
 		if (size != 0 && entry->e_value_inum == 0) {
@@ -540,8 +540,10 @@ ext4_xattr_block_get(struct inode *inode, int name_index, const char *name,
 	if (error)
 		goto cleanup;
 	size = le32_to_cpu(entry->e_value_size);
+	error = -ERANGE;
+	if (unlikely(size > EXT4_XATTR_SIZE_MAX))
+		goto cleanup;
 	if (buffer) {
-		error = -ERANGE;
 		if (size > buffer_size)
 			goto cleanup;
 		if (entry->e_value_inum) {
@@ -550,8 +552,12 @@ ext4_xattr_block_get(struct inode *inode, int name_index, const char *name,
 			if (error)
 				goto cleanup;
 		} else {
-			memcpy(buffer, bh->b_data +
-			       le16_to_cpu(entry->e_value_offs), size);
+			u16 offset = le16_to_cpu(entry->e_value_offs);
+			void *p = bh->b_data + offset;
+
+			if (unlikely(p + size > end))
+				goto cleanup;
+			memcpy(buffer, p, size);
 		}
 	}
 	error = size;
@@ -589,8 +595,10 @@ ext4_xattr_ibody_get(struct inode *inode, int name_index, const char *name,
 	if (error)
 		goto cleanup;
 	size = le32_to_cpu(entry->e_value_size);
+	error = -ERANGE;
+	if (unlikely(size > EXT4_XATTR_SIZE_MAX))
+		goto cleanup;
 	if (buffer) {
-		error = -ERANGE;
 		if (size > buffer_size)
 			goto cleanup;
 		if (entry->e_value_inum) {
@@ -599,8 +607,12 @@ ext4_xattr_ibody_get(struct inode *inode, int name_index, const char *name,
 			if (error)
 				goto cleanup;
 		} else {
-			memcpy(buffer, (void *)IFIRST(header) +
-			       le16_to_cpu(entry->e_value_offs), size);
+			u16 offset = le16_to_cpu(entry->e_value_offs);
+			void *p = (void *)IFIRST(header) + offset;
+
+			if (unlikely(p + size > end))
+				goto cleanup;
+			memcpy(buffer, p, size);
 		}
 	}
 	error = size;

fs/ext4/xattr.h

@@ -70,6 +70,17 @@ struct ext4_xattr_entry {
 		EXT4_I(inode)->i_extra_isize))
 #define IFIRST(hdr) ((struct ext4_xattr_entry *)((hdr)+1))
 
+/*
+ * XATTR_SIZE_MAX is currently 64k, but for the purposes of checking
+ * for file system consistency errors, we use a somewhat bigger value.
+ * This allows XATTR_SIZE_MAX to grow in the future, but by using this
+ * instead of INT_MAX for certain consistency checks, we don't need to
+ * worry about arithmetic overflows.  (Actually XATTR_SIZE_MAX is
+ * defined in include/uapi/linux/limits.h, so changing it is going
+ * not going to be trivial....)
+ */
+#define EXT4_XATTR_SIZE_MAX (1 << 24)
+
 /*
  * The minimum size of EA value when you start storing it in an external inode
  * size of block - size of header - size of 1 entry - 4 null bytes