tcp: TX zerocopy should not sense pfmemalloc status

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

tcp: TX zerocopy should not sense pfmemalloc status

We got a recent syzbot report [1] showing a possible misuse

of pfmemalloc page status in TCP zerocopy paths.

Indeed, for pages coming from user space or other layers,

using page_is_pfmemalloc() is moot, and possibly could give

false positives.

There has been attempts to make page_is_pfmemalloc() more robust,

but not using it in the first place in this context is probably better,

removing cpu cycles.

Note to stable teams :

You need to backport 84ce071e38a6 ("net: introduce

__skb_fill_page_desc_noacc") as a prereq.

Race is more probable after commit c07aea3ef4d4

("mm: add a signature in struct page") because page_is_pfmemalloc()

is now using low order bit from page->lru.next, which can change

more often than page->index.

Low order bit should never be set for lru.next (when used as an anchor

in LRU list), so KCSAN report is mostly a false positive.

Backporting to older kernel versions seems not necessary.

[1]

BUG: KCSAN: data-race in lru_add_fn / tcp_build_frag

write to 0xffffea0004a1d2c8 of 8 bytes by task 18600 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:105 [inline]

lru_add_fn+0x440/0x520 mm/swap.c:228

folio_batch_move_lru+0x1e1/0x2a0 mm/swap.c:246

folio_batch_add_and_move mm/swap.c:263 [inline]

folio_add_lru+0xf1/0x140 mm/swap.c:490

filemap_add_folio+0xf8/0x150 mm/filemap.c:948

__filemap_get_folio+0x510/0x6d0 mm/filemap.c:1981

pagecache_get_page+0x26/0x190 mm/folio-compat.c:104

grab_cache_page_write_begin+0x2a/0x30 mm/folio-compat.c:116

ext4_da_write_begin+0x2dd/0x5f0 fs/ext4/inode.c:2988

generic_perform_write+0x1d4/0x3f0 mm/filemap.c:3738

ext4_buffered_write_iter+0x235/0x3e0 fs/ext4/file.c:270

ext4_file_write_iter+0x2e3/0x1210

call_write_iter include/linux/fs.h:2187 [inline]

new_sync_write fs/read_write.c:491 [inline]

vfs_write+0x468/0x760 fs/read_write.c:578

ksys_write+0xe8/0x1a0 fs/read_write.c:631

__do_sys_write fs/read_write.c:643 [inline]

__se_sys_write fs/read_write.c:640 [inline]

__x64_sys_write+0x3e/0x50 fs/read_write.c:640

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 0xffffea0004a1d2c8 of 8 bytes by task 18611 on cpu 1:

page_is_pfmemalloc include/linux/mm.h:1740 [inline]

__skb_fill_page_desc include/linux/skbuff.h:2422 [inline]

skb_fill_page_desc include/linux/skbuff.h:2443 [inline]

tcp_build_frag+0x613/0xb20 net/ipv4/tcp.c:1018

do_tcp_sendpages+0x3e8/0xaf0 net/ipv4/tcp.c:1075

tcp_sendpage_locked net/ipv4/tcp.c:1140 [inline]

tcp_sendpage+0x89/0xb0 net/ipv4/tcp.c:1150

inet_sendpage+0x7f/0xc0 net/ipv4/af_inet.c:833

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:1249

__do_sys_sendfile64 fs/read_write.c:1317 [inline]

__se_sys_sendfile64 fs/read_write.c:1303 [inline]

__x64_sys_sendfile64+0x10c/0x150 fs/read_write.c:1303

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 -> 0xffffea0004a1d288

Reported by Kernel Concurrency Sanitizer on:

CPU: 1 PID: 18611 Comm: syz-executor.4 Not tainted 6.0.0-rc2-syzkaller-00248-ge022620b5d05-dirty #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022