ext4: avoid arithemetic overflow that can trigger a BUG

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

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)) {