f2fs: read COW data with the original inode during atomic write

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

f2fs: read COW data with the original inode during atomic write

When updating an atomic-write file, f2fs_write_begin() may read the
previously written data back from the COW inode:
prepare_atomic_write_begin() locates the block in the COW inode and sets
use_cow, and the read bio is then built with the COW inode:

f2fs_submit_page_read(use_cow ? F2FS_I(inode)->cow_inode : inode,
...);

and f2fs_grab_read_bio() decides whether to schedule fs-layer decryption
(STEP_DECRYPT) for the bio based on that inode via
fscrypt_inode_uses_fs_layer_crypto().

However, the folio being filled belongs to the original inode
(folio->mapping->host == inode), and the data stored in the COW block was
encrypted (or left as plaintext) using the original inode's context, not
the COW inode's -- see f2fs_encrypt_one_page(), which keys off
fio->page->mapping->host. fscrypt_decrypt_pagecache_blocks() likewise
operates on folio->mapping->host.

The COW inode is created as a tmpfile in the parent directory and inherits
its encryption policy from there. With test_dummy_encryption the newly
created COW inode gets the dummy policy and becomes encrypted, while a
pre-existing regular file -- created before the policy applied, e.g.
already present in the on-disk image -- stays unencrypted. The read
path then sets STEP_DECRYPT based on the encrypted COW inode and calls
fscrypt_decrypt_pagecache_blocks() on a folio whose host (the unencrypted
original inode) has a NULL ->i_crypt_info, dereferencing it:

Oops: general protection fault, probably for non-canonical address ...
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
RIP: 0010:fscrypt_decrypt_pagecache_blocks+0xa0/0x310
Workqueue: f2fs_post_read_wq f2fs_post_read_work
Call Trace:
fscrypt_decrypt_bio+0x1eb/0x340
f2fs_post_read_work+0xba/0x140
process_one_work+0x91c/0x1a40
worker_thread+0x677/0xe90
kthread+0x2bc/0x3a0

The COW inode is only needed to locate the on-disk block, and that block
address is already resolved into @blkaddr by prepare_atomic_write_begin()
via __find_data_block(cow_inode, ...); f2fs_submit_page_read() then reads
from that physical @blkaddr directly, so the inode argument only selects
the post-read crypto context, not which block is fetched. Reading with
@inode therefore returns the same (latest, not-yet-committed) COW data,
while making both the fs-layer decryption decision and the inline crypto
path use the correct (original inode's) key.

With the COW inode no longer used at the read site, the use_cow flag has no
remaining consumer; drop it from f2fs_write_begin() and
prepare_atomic_write_begin().