From: Eric Dumazet edumazet@google.com
[ Upstream commit 3261400639463a853ba2b3be8bd009c2a8089775 ]
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
Fixes: c07aea3ef4d4 ("mm: add a signature in struct page") Reported-by: syzbot syzkaller@googlegroups.com Signed-off-by: Eric Dumazet edumazet@google.com Cc: Shakeel Butt shakeelb@google.com Reviewed-by: Shakeel Butt shakeelb@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/skbuff.h | 21 +++++++++++++++++++++ net/core/datagram.c | 2 +- net/ipv4/tcp.c | 2 +- 3 files changed, 23 insertions(+), 2 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index be7cc31d58961..cfb889f66c703 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -2291,6 +2291,27 @@ static inline void skb_fill_page_desc(struct sk_buff *skb, int i, skb_shinfo(skb)->nr_frags = i + 1; }
+/** + * skb_fill_page_desc_noacc - initialise a paged fragment in an skb + * @skb: buffer containing fragment to be initialised + * @i: paged fragment index to initialise + * @page: the page to use for this fragment + * @off: the offset to the data with @page + * @size: the length of the data + * + * Variant of skb_fill_page_desc() which does not deal with + * pfmemalloc, if page is not owned by us. + */ +static inline void skb_fill_page_desc_noacc(struct sk_buff *skb, int i, + struct page *page, int off, + int size) +{ + struct skb_shared_info *shinfo = skb_shinfo(skb); + + __skb_fill_page_desc_noacc(shinfo, i, page, off, size); + shinfo->nr_frags = i + 1; +} + void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page, int off, int size, unsigned int truesize);
diff --git a/net/core/datagram.c b/net/core/datagram.c index 15ab9ffb27fe9..28e5f921dcaf4 100644 --- a/net/core/datagram.c +++ b/net/core/datagram.c @@ -677,7 +677,7 @@ int __zerocopy_sg_from_iter(struct sock *sk, struct sk_buff *skb, page_ref_sub(last_head, refs); refs = 0; } - skb_fill_page_desc(skb, frag++, head, start, size); + skb_fill_page_desc_noacc(skb, frag++, head, start, size); } if (refs) page_ref_sub(last_head, refs); diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 0ebef2a5950cd..4f6b897ccf23f 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -1002,7 +1002,7 @@ struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags, skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy); } else { get_page(page); - skb_fill_page_desc(skb, i, page, offset, copy); + skb_fill_page_desc_noacc(skb, i, page, offset, copy); }
if (!(flags & MSG_NO_SHARED_FRAGS))