"value":"In the Linux kernel, the following vulnerability has been resolved:\n\next4: fix corruption during on-line resize\n\nWe observed a corruption during on-line resize of a file system that is\nlarger than 16 TiB with 4k block size. With having more then 2^32 blocks\nresize_inode is turned off by default by mke2fs. The issue can be\nreproduced on a smaller file system for convenience by explicitly\nturning off resize_inode. An on-line resize across an 8 GiB boundary (the\nsize of a meta block group in this setup) then leads to a corruption:\n\n dev=/dev/<some_dev> # should be >= 16 GiB\n mkdir -p /corruption\n /sbin/mke2fs -t ext4 -b 4096 -O ^resize_inode $dev $((2 * 2**21 - 2**15))\n mount -t ext4 $dev /corruption\n\n dd if=/dev/zero bs=4096 of=/corruption/test count=$((2*2**21 - 4*2**15))\n sha1sum /corruption/test\n # 79d2658b39dcfd77274e435b0934028adafaab11 /corruption/test\n\n /sbin/resize2fs $dev $((2*2**21))\n # drop page cache to force reload the block from disk\n echo 1 > /proc/sys/vm/drop_caches\n\n sha1sum /corruption/test\n # 3c2abc63cbf1a94c9e6977e0fbd72cd832c4d5c3 /corruption/test\n\n2^21 = 2^15*2^6 equals 8 GiB whereof 2^15 is the number of blocks per\nblock group and 2^6 are the number of block groups that make a meta\nblock group.\n\nThe last checksum might be different depending on how the file is laid\nout across the physical blocks. The actual corruption occurs at physical\nblock 63*2^15 = 2064384 which would be the location of the backup of the\nmeta block group's block descriptor. During the on-line resize the file\nsystem will be converted to meta_bg starting at s_first_meta_bg which is\n2 in the example - meaning all block groups after 16 GiB. However, in\next4_flex_group_add we might add block groups that are not part of the\nfirst meta block group yet. In the reproducer we achieved this by\nsubstracting the size of a whole block group from the point where the\nmeta block group would start. This must be considered when updating the\nbackup block group descriptors to follow the non-meta_bg layout. The fix\nis to add a test whether the group to add is already part of the meta\nblock group or not."
