From: John Fastabend john.fastabend@gmail.com
[ Upstream commit c9c89dcd872ea33327673fcb97398993a1f22736 ]
If copy_page_to_iter() fails or even partially completes, but with fewer bytes copied than expected we currently reset sg.start and return EFAULT. This proves problematic if we already copied data into the user buffer before we return an error. Because we leave the copied data in the user buffer and fail to unwind the scatterlist so kernel side believes data has been copied and user side believes data has _not_ been received.
Expected behavior should be to return number of bytes copied and then on the next read we need to return the error assuming its still there. This can happen if we have a copy length spanning multiple scatterlist elements and one or more complete before the error is hit.
The error is rare enough though that my normal testing with server side programs, such as nginx, httpd, envoy, etc., I have never seen this. The only reliable way to reproduce that I've found is to stream movies over my browser for a day or so and wait for it to hang. Not very scientific, but with a few extra WARN_ON()s in the code the bug was obvious.
When we review the errors from copy_page_to_iter() it seems we are hitting a page fault from copy_page_to_iter_iovec() where the code checks fault_in_pages_writeable(buf, copy) where buf is the user buffer. It also seems typical server applications don't hit this case.
The other way to try and reproduce this is run the sockmap selftest tool test_sockmap with data verification enabled, but it doesn't reproduce the fault. Perhaps we can trigger this case artificially somehow from the test tools. I haven't sorted out a way to do that yet though.
Fixes: 604326b41a6fb ("bpf, sockmap: convert to generic sk_msg interface") Signed-off-by: John Fastabend john.fastabend@gmail.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Reviewed-by: Jakub Sitnicki jakub@cloudflare.com Link: https://lore.kernel.org/bpf/160556566659.73229.15694973114605301063.stgit@jo... Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_bpf.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-)
diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c index ad9f382027311..efd098b00104b 100644 --- a/net/ipv4/tcp_bpf.c +++ b/net/ipv4/tcp_bpf.c @@ -47,8 +47,8 @@ int __tcp_bpf_recvmsg(struct sock *sk, struct sk_psock *psock, { struct iov_iter *iter = &msg->msg_iter; int peek = flags & MSG_PEEK; - int i, ret, copied = 0; struct sk_msg *msg_rx; + int i, copied = 0;
msg_rx = list_first_entry_or_null(&psock->ingress_msg, struct sk_msg, list); @@ -69,11 +69,9 @@ int __tcp_bpf_recvmsg(struct sock *sk, struct sk_psock *psock, page = sg_page(sge); if (copied + copy > len) copy = len - copied; - ret = copy_page_to_iter(page, sge->offset, copy, iter); - if (ret != copy) { - msg_rx->sg.start = i; - return -EFAULT; - } + copy = copy_page_to_iter(page, sge->offset, copy, iter); + if (!copy) + return copied ? copied : -EFAULT;
copied += copy; if (likely(!peek)) { @@ -88,6 +86,11 @@ int __tcp_bpf_recvmsg(struct sock *sk, struct sk_psock *psock, put_page(page); } } else { + /* Lets not optimize peek case if copy_page_to_iter + * didn't copy the entire length lets just break. + */ + if (copy != sge->length) + return copied; sk_msg_iter_var_next(i); }