[PATCH -v2] ext4: avoid arithemetic overflow that can trigger a BUG

[Theodore Ts'o <tytso@mit.edu>]

A maliciously crafted file system can cause an overflow when the
results of a 64-bit calculation is stored into a 32-bit length
parameter.

https://bugzilla.kernel.org/show_bug.cgi?id=200623

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Wen Xu <wen.xu@gatech.edu>
Cc: stable@vger.kernel.org
---
This is a revised version which checks for overflows when caluclating
first_block and last_block.  This catches more overflow scenarios than
just checking for an overflow when calculating map.m_len.

 fs/ext4/ext4.h  | 3 +++
 fs/ext4/inode.c | 8 ++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 249bcee4d7b2..ac05bd86643a 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -686,6 +686,9 @@ enum {
 /* Max physical block we can address w/o extents */
 #define EXT4_MAX_BLOCK_FILE_PHYS	0xFFFFFFFF
 
+/* Max logical block we can support */
+#define EXT4_MAX_LOGICAL_BLOCK		0xFFFFFFFF
+
 /*
  * Structure of an inode on the disk
  */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 8f6ad7667974..694f31364206 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3412,12 +3412,16 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 {
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	unsigned int blkbits = inode->i_blkbits;
-	unsigned long first_block = offset >> blkbits;
-	unsigned long last_block = (offset + length - 1) >> blkbits;
+	unsigned long first_block, last_block;
 	struct ext4_map_blocks map;
 	bool delalloc = false;
 	int ret;
 
+	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
+		return -EINVAL;
+	first_block = offset >> blkbits;
+	last_block = min_t(loff_t, (offset + length - 1) >> blkbits,
+			   EXT4_MAX_LOGICAL_BLOCK);
 
 	if (flags & IOMAP_REPORT) {
 		if (ext4_has_inline_data(inode)) {
-- 
2.18.0.rc0