f2fs: check discard support for conventional zones

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

f2fs: check discard support for conventional zones

As the helper function f2fs_bdev_support_discard() shows, f2fs checks if

the target block devices support discard by calling

bdev_max_discard_sectors() and bdev_is_zoned(). This check works well

for most cases, but it does not work for conventional zones on zoned

block devices. F2fs assumes that zoned block devices support discard,

and calls __submit_discard_cmd(). When __submit_discard_cmd() is called

for sequential write required zones, it works fine since

__submit_discard_cmd() issues zone reset commands instead of discard

commands. However, when __submit_discard_cmd() is called for

conventional zones, __blkdev_issue_discard() is called even when the

devices do not support discard.

The inappropriate __blkdev_issue_discard() call was not a problem before

the commit 30f1e7241422 ("block: move discard checks into the ioctl

handler") because __blkdev_issue_discard() checked if the target devices

support discard or not. If not, it returned EOPNOTSUPP. After the

commit, __blkdev_issue_discard() no longer checks it. It always returns

zero and sets NULL to the given bio pointer. This NULL pointer triggers

f2fs_bug_on() in __submit_discard_cmd(). The BUG is recreated with the

commands below at the umount step, where /dev/nullb0 is a zoned null_blk

with 5GB total size, 128MB zone size and 10 conventional zones.

$ mkfs.f2fs -f -m /dev/nullb0

$ mount /dev/nullb0 /mnt

$ for ((i=0;i<5;i++)); do dd if=/dev/zero of=/mnt/test bs=65536 count=1600 conv=fsync; done

$ umount /mnt

To fix the BUG, avoid the inappropriate __blkdev_issue_discard() call.

When discard is requested for conventional zones, check if the device

supports discard or not. If not, return EOPNOTSUPP.