net: do not sense pfmemalloc status in skb_append_pagefrags()
CVE: CVE-2022-50323
Component: kernel
CWE: CWE-664
Severity: Moderate
CVSS v3: 4.7/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2022-01-01T00:00:00+00:00

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

net: do not sense pfmemalloc status in skb_append_pagefrags()

skb_append_pagefrags() is used by af_unix and udp sendpage()
implementation so far.

In commit 326140063946 ("tcp: TX zerocopy should not sense
pfmemalloc status") we explained why we should not sense
pfmemalloc status for pages owned by user space.

We should also use skb_fill_page_desc_noacc()
in skb_append_pagefrags() to avoid following KCSAN report:

BUG: KCSAN: data-race in lru_add_fn / skb_append_pagefrags

write to 0xffffea00058fc1c8 of 8 bytes by task 17319 on cpu 0:
__list_add include/linux/list.h:73 [inline]
list_add include/linux/list.h:88 [inline]
lruvec_add_folio include/linux/mm_inline.h:323 [inline]
lru_add_fn+0x327/0x410 mm/swap.c:228
folio_batch_move_lru+0x1e1/0x2a0 mm/swap.c:246
lru_add_drain_cpu+0x73/0x250 mm/swap.c:669
lru_add_drain+0x21/0x60 mm/swap.c:773
free_pages_and_swap_cache+0x16/0x70 mm/swap_state.c:311
tlb_batch_pages_flush mm/mmu_gather.c:59 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:256 [inline]
tlb_flush_mmu+0x5b2/0x640 mm/mmu_gather.c:263
tlb_finish_mmu+0x86/0x100 mm/mmu_gather.c:363
exit_mmap+0x190/0x4d0 mm/mmap.c:3098
__mmput+0x27/0x1b0 kernel/fork.c:1185
mmput+0x3d/0x50 kernel/fork.c:1207
copy_process+0x19fc/0x2100 kernel/fork.c:2518
kernel_clone+0x166/0x550 kernel/fork.c:2671
__do_sys_clone kernel/fork.c:2812 [inline]
__se_sys_clone kernel/fork.c:2796 [inline]
__x64_sys_clone+0xc3/0xf0 kernel/fork.c:2796
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffffea00058fc1c8 of 8 bytes by task 17325 on cpu 1:
page_is_pfmemalloc include/linux/mm.h:1817 [inline]
__skb_fill_page_desc include/linux/skbuff.h:2432 [inline]
skb_fill_page_desc include/linux/skbuff.h:2453 [inline]
skb_append_pagefrags+0x210/0x600 net/core/skbuff.c:3974
unix_stream_sendpage+0x45e/0x990 net/unix/af_unix.c:2338
kernel_sendpage+0x184/0x300 net/socket.c:3561
sock_sendpage+0x5a/0x70 net/socket.c:1054
pipe_to_sendpage+0x128/0x160 fs/splice.c:361
splice_from_pipe_feed fs/splice.c:415 [inline]
__splice_from_pipe+0x222/0x4d0 fs/splice.c:559
splice_from_pipe fs/splice.c:594 [inline]
generic_splice_sendpage+0x89/0xc0 fs/splice.c:743
do_splice_from fs/splice.c:764 [inline]
direct_splice_actor+0x80/0xa0 fs/splice.c:931
splice_direct_to_actor+0x305/0x620 fs/splice.c:886
do_splice_direct+0xfb/0x180 fs/splice.c:974
do_sendfile+0x3bf/0x910 fs/read_write.c:1255
__do_sys_sendfile64 fs/read_write.c:1323 [inline]
__se_sys_sendfile64 fs/read_write.c:1309 [inline]
__x64_sys_sendfile64+0x10c/0x150 fs/read_write.c:1309
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x0000000000000000 -> 0xffffea00058fc188

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 17325 Comm: syz-executor.0 Not tainted 6.1.0-rc1-syzkaller-00158-g440b7895c990-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022

Statement: The fix stops inspecting pfmemalloc in skb_append_pagefrags() and uses skb_fill_page_desc_noacc() to avoid a KCSAN-reported data race when building skbs from user pages (sendpage/splice via AF_UNIX/UDP). This race could lead to kernel instability or crashes under specific high-rate workloads but does not provide a credible path to information disclosure or privilege escalation.

Remediation: