CVE-2024-36027: btrfs: zoned: do not flag ZEROOUT on non-dirty extent buffer

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

btrfs: zoned: do not flag ZEROOUT on non-dirty extent buffer

Btrfs clears the content of an extent buffer marked as EXTENTBUFFERZONEDZEROOUT before the bio submission. This mechanism is introduced to prevent a write hole of an extent buffer, which is once allocated, marked dirty, but turns out unnecessary and cleaned up within one transaction operation.

Currently, btrfsclearbufferdirty() marks the extent buffer as EXTENTBUFFERZONEDZEROOUT, and skips the entry function. If this call happens while the buffer is under IO (with the WRITEBACK flag set, without the DIRTY flag), we can add the ZEROOUT flag and clear the buffer's content just before a bio submission. As a result:

1) it can lead to adding faulty delayed reference item which leads to a FS corrupted (EUCLEAN) error, and

2) it writes out cleared tree node on disk

The former issue is previously discussed in [1]. The corruption happens when it runs a delayed reference update. So, on-disk data is safe.

[1] https://lore.kernel.org/linux-btrfs/3f4f2a0ff1a6c818050434288925bdcf3cd719e5.1709124777.git.naohiro.aota@wdc.com/

The latter one can reach on-disk data. But, as that node is already processed by btrfsclearbufferdirty(), that will be invalidated in the next transaction commit anyway. So, the chance of hitting the corruption is relatively small.

Anyway, we should skip flagging ZEROOUT on a non-DIRTY extent buffer, to keep the content under IO intact.