CVE-2021-47492

In the Linux kernel, the following vulnerability has been resolved: mm,
thp: bail out early in collapse_file for writeback page Currently
collapse_file does not explicitly check PG_writeback, instead,
page_has_private and try_to_release_page are used to filter writeback
pages. This does not work for xfs with blocksize equal to or larger than
pagesize, because in such case xfs has no page->private. This makes
collapse_file bail out early for writeback page. Otherwise, xfs
end_page_writeback will panic as follows. page:fffffe00201bcc80 refcount:0
mapcount:0 mapping:ffff0003f88c86a8 index:0x0 pfn:0x84ef32
aops:xfs_address_space_operations [xfs] ino:30000b7 dentry
name:“libtest.so” flags:
0x57fffe0000008027(locked|referenced|uptodate|active|writeback) raw:
57fffe0000008027 ffff80001b48bc28 ffff80001b48bc28 ffff0003f88c86a8 raw:
0000000000000000 0000000000000000 00000000ffffffff ffff0000c3e9a000 page
dumped because: VM_BUG_ON_PAGE(((unsigned int) page_ref_count(page) + 127u
<= 127u)) page->mem_cgroup:ffff0000c3e9a000 ------------[ cut here
]------------ kernel BUG at include/linux/mm.h:1212! Internal error: Oops -
BUG: 0 [#1] SMP Modules linked in: BUG: Bad page state in process
khugepaged pfn:84ef32 xfs(E) page:fffffe00201bcc80 refcount:0 mapcount:0
mapping:0 index:0x0 pfn:0x84ef32 libcrc32c(E) rfkill(E) aes_ce_blk(E)
crypto_simd(E) … CPU: 25 PID: 0 Comm: swapper/25 Kdump: loaded Tainted:
… pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=–) Call trace:
end_page_writeback+0x1c0/0x214 iomap_finish_page_writeback+0x13c/0x204
iomap_finish_ioend+0xe8/0x19c iomap_writepage_end_bio+0x38/0x50
bio_endio+0x168/0x1ec blk_update_request+0x278/0x3f0
blk_mq_end_request+0x34/0x15c virtblk_request_done+0x38/0x74 [virtio_blk]
blk_done_softirq+0xc4/0x110 __do_softirq+0x128/0x38c
__irq_exit_rcu+0x118/0x150 irq_exit+0x1c/0x30 __handle_domain_irq+0x8c/0xf0
gic_handle_irq+0x84/0x108 el1_irq+0xcc/0x180 arch_cpu_idle+0x18/0x40
default_idle_call+0x4c/0x1a0 cpuidle_idle_call+0x168/0x1e0
do_idle+0xb4/0x104 cpu_startup_entry+0x30/0x9c
secondary_start_kernel+0x104/0x180 Code: d4210000 b0006161 910c8021
94013f4d (d4210000) —[ end trace 4a88c6a074082f8c ]— Kernel panic - not
syncing: Oops - BUG: Fatal exception in interrupt