CVE-2024-49880: ext4: fix off by one issue in alloc_flex_gd()

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

ext4: fix off by one issue in allocflexgd()

Wesley reported an issue:

================================================================== EXT4-fs (dm-5): resizing filesystem from 7168 to 786432 blocks ------------[ cut here ]------------ kernel BUG at fs/ext4/resize.c:324! CPU: 9 UID: 0 PID: 3576 Comm: resize2fs Not tainted 6.11.0+ #27 RIP: 0010:ext4resizefs+0x1212/0x12d0 Call Trace: ext4ioctl+0x4e0/0x1800 ext4ioctl+0x12/0x20 x64sysioctl+0x99/0xd0 x64syscall+0x1206/0x20d0 dosyscall64+0x72/0x110 entrySYSCALL64afterhwframe+0x76/0x7e ==================================================================

While reviewing the patch, Honza found that when adjusting resizebg in allocflexgd(), it was possible for flexgd->resizebg to be bigger than flexbgsize.

The reproduction of the problem requires the following:

ogroup = flexbgsize 2 n; osize = (ogroup + 1) groupsize; ngroup: [ogroup + flexbgsize, ogroup + flexbgsize 2) osize = (ngroup + 1) groupsize;

Take n=0,flexbgsize=16 as an example:

last:15 |o---------------|--------------n-| ogroup:0 resize to ngroup:30

The corresponding reproducer is:

img=test.img rm -f $img truncate -s 600M $img mkfs.ext4 -F $img -b 1024 -G 16 8M dev=losetup -f --show $img mkdir -p /tmp/test mount $dev /tmp/test resize2fs $dev 248M

Delete the problematic plus 1 to fix the issue, and add a WARNONONCE() to prevent the issue from happening again.

[ Note: another reproucer which this commit fixes is:

img=test.img rm -f $img truncate -s 25MiB $img mkfs.ext4 -b 4096 -E nodiscard,lazyitableinit=0,lazyjournalinit=0 $img truncate -s 3GiB $img dev=losetup -f --show $img mkdir -p /tmp/test mount $dev /tmp/test resize2fs $dev 3G umount $dev losetup -d $dev

-- TYT ]