[RFC 1/1] ext4: Fix overflow case for map.m_len in ext4_iomap_begin_*

[Ritesh Harjani]

EXT4_MAX_LOGICAL_BLOCK - map.m_lblk + 1 in case when
map.m_lblk (offset) is 0 could overflow an unsigned int
and become 0.

Fix this.

Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
Reported-by: syzbot+77fa5bdb65cc39711820@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: d3b6f23f7167 ("ext4: move ext4_fiemap to use iomap framework")
---
 fs/ext4/inode.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e416096fc081..d630ec7a9c8e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3424,6 +3424,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	int ret;
 	struct ext4_map_blocks map;
 	u8 blkbits = inode->i_blkbits;
+	loff_t len;
 
 	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
 		return -EINVAL;
@@ -3435,8 +3436,11 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	 * Calculate the first and last logical blocks respectively.
 	 */
 	map.m_lblk = offset >> blkbits;
-	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
+	len = min_t(loff_t, (offset + length - 1) >> blkbits,
 			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
+	if (len > EXT4_MAX_LOGICAL_BLOCK)
+		len = EXT4_MAX_LOGICAL_BLOCK;
+	map.m_len = len;
 
 	if (flags & IOMAP_WRITE)
 		ret = ext4_iomap_alloc(inode, &map, flags);
@@ -3524,6 +3528,7 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
 	bool delalloc = false;
 	struct ext4_map_blocks map;
 	u8 blkbits = inode->i_blkbits;
+	loff_t len
 
 	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
 		return -EINVAL;
@@ -3541,8 +3546,11 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
 	 * Calculate the first and last logical block respectively.
 	 */
 	map.m_lblk = offset >> blkbits;
-	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
+	len = min_t(loff_t, (offset + length - 1) >> blkbits,
 			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
+	if (len > EXT4_MAX_LOGICAL_BLOCK)
+		len = EXT4_MAX_LOGICAL_BLOCK;
+	map.m_len = len;
 
 	/*
 	 * Fiemap callers may call for offset beyond s_bitmap_maxbytes.
-- 
2.21.0