CVE-2024-35807

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix corruption during on-line resize

We observed a corruption during on-line resize of a file system that is
larger than 16 TiB with 4k block size. With having more then 2³² blocks
resize_inode is turned off by default by mke2fs. The issue can be
reproduced on a smaller file system for convenience by explicitly
turning off resize_inode. An on-line resize across an 8 GiB boundary (the
size of a meta block group in this setup) then leads to a corruption:

dev=/dev/<some_dev> # should be >= 16 GiB
mkdir -p /corruption
/sbin/mke2fs -t ext4 -b 4096 -O ^resize_inode $dev $((2 * 2**21 - 2**15))
mount -t ext4 $dev /corruption

dd if=/dev/zero bs=4096 of=/corruption/test count=$((2*2**21 - 4*2**15))
sha1sum /corruption/test
# 79d2658b39dcfd77274e435b0934028adafaab11 /corruption/test

/sbin/resize2fs $dev $((2*2**21))
# drop page cache to force reload the block from disk
echo 1 > /proc/sys/vm/drop_caches

sha1sum /corruption/test
# 3c2abc63cbf1a94c9e6977e0fbd72cd832c4d5c3 /corruption/test

2²¹ = 2^15*26 equals 8 GiB whereof 2¹⁵ is the number of blocks per
block group and 2⁶ are the number of block groups that make a meta
block group.

The last checksum might be different depending on how the file is laid
out across the physical blocks. The actual corruption occurs at physical
block 63*2¹⁵ = 2064384 which would be the location of the backup of the
meta block group's block descriptor. During the on-line resize the file
system will be converted to meta_bg starting at s_first_meta_bg which is
2 in the example - meaning all block groups after 16 GiB. However, in
ext4_flex_group_add we might add block groups that are not part of the
first meta block group yet. In the reproducer we achieved this by
substracting the size of a whole block group from the point where the
meta block group would start. This must be considered when updating the
backup block group descriptors to follow the non-meta_bg layout. The fix
is to add a test whether the group to add is already part of the meta
block group or not.