Hello!

On Fri, Jan 26, 2024 at 04:26:00PM +0400, Sergey Kandaurov wrote:

> > On 27 Nov 2023, at 06:50, Maxim Dounin <mdounin@mdounin.ru> wrote:
> >
> > # HG changeset patch
> > # User Maxim Dounin <mdounin@mdounin.ru>
> > # Date 1701049758 -10800
> > # Mon Nov 27 04:49:18 2023 +0300
> > # Node ID faf0b9defc76b8683af466f8a950c2c241382970
> > # Parent a5e39e9d1f4c84dcbe6a2f9e079372a3d63aef0b
> > Upstream: fixed usage of closed sockets with filter finalization.
> >
> > When filter finalization is triggered when working with an upstream server,
> > and error_page redirects request processing to some simple handler,
> > ngx_http_request_finalize() triggers request termination when the response
> > is sent. In particular, via the upstream cleanup handler, nginx will close
> > the upstream connection and the corresponding socket.
> >
> > Still, this can happen to be with ngx_event_pipe() on stack. While
> > the code will set p->downstream_error due to NGX_ERROR returned from the
> > output filter chain by filter finalization, otherwise the error will be
> > ignored till control returns to ngx_http_upstream_process_request().
> > And event pipe might try reading from the (already closed) socket, resulting
> > in "readv() failed (9: Bad file descriptor) while reading upstream" errors
> > (or even segfaults with SSL).
> >
> > Such errors were seen with the following configuration:
> >
> > location /t2 {
> > proxy_pass http://127.0.0.1:8080/big;
> >
> > image_filter_buffer 10m;
> > image_filter resize 150 100;
> > error_page 415 = /empty;
> > }
> >
> > location /empty {
> > return 204;
> > }
> >
> > location /big {
> > # big enough static file
> > }
> >
> > Fix is to set p->upstream_error in ngx_http_upstream_finalize_request(),
> > so the existing checks in ngx_event_pipe_read_upstream() will prevent
> > further reading from the closed upstream connection.
> >
> > Similarly, p->upstream_error is now checked when handling events at
> > ngx_event_pipe() exit, as checking p->upstream->fd is not enough if
> > keepalive upstream connections are being used and the connection was
> > saved to cache during request termination.
> >
>
> Setting p->upstream_error in ngx_http_upstream_finalize_request()
> may look suspicious, because it is used to be set on connection errors
> such as upstream timeout or recv error, or, as a recently introduced
> exception in the fastcgi module, - also when the FastCGI record ends
> prematurely, before receiving all the expected content.
> But technically I think this is quite correct, because we no longer
> want to receive further data, and also (and you mention this in the
> commit log) this repeats closing an upstream connection socket in
> the same place in ngx_http_upstream_finalize_request().
> So I think it should be fine.

The biggest concern I personally see here is with the added
p->upstream_error check at ngx_event_pipe() exit. If there is a
real upstream error, such as when the connection is reset by the
upstream server, and if we want the pipe to be active for some
time (for example, if we want it to continue writing to the
downstream connection), there will be no ngx_handle_read_event()
call. For level-triggered event methods this means that the read
event for the upstream connection will be generated again and
again.

This shouldn't be the problem for existing ngx_event_pipe() uses
though, as p->upstream_error is anyway triggers
ngx_http_upstream_finalize_request().

Still, we can consider introducing a separate flag, such as
p->upstream_closed, or clearing p->upstream, and checking these in
ngx_event_pipe() instead. This probably would be a more clear
solution.

Updated patch below:

# HG changeset patch
# User Maxim Dounin <mdounin@mdounin.ru>
# Date 1706510064 -10800
# Mon Jan 29 09:34:24 2024 +0300
# Node ID 4a91a03dcd8df0652884ed6ebe9f7437ce82fd26
# Parent 7b630f6487068f7cc9dd83762fb4ea39f2f340e9
Upstream: fixed usage of closed sockets with filter finalization.

When filter finalization is triggered when working with an upstream server,
and error_page redirects request processing to some simple handler,
ngx_http_request_finalize() triggers request termination when the response
is sent. In particular, via the upstream cleanup handler, nginx will close
the upstream connection and the corresponding socket.

Still, this can happen to be with ngx_event_pipe() on stack. While
the code will set p->downstream_error due to NGX_ERROR returned from the
output filter chain by filter finalization, otherwise the error will be
ignored till control returns to ngx_http_upstream_process_request().
And event pipe might try reading from the (already closed) socket, resulting
in "readv() failed (9: Bad file descriptor) while reading upstream" errors
(or even segfaults with SSL).

Such errors were seen with the following configuration:

location /t2 {
proxy_pass http://127.0.0.1:8080/big;

image_filter_buffer 10m;
image_filter resize 150 100;
error_page 415 = /empty;
}

location /empty {
return 204;
}

location /big {
# big enough static file
}

Fix is to clear p->upstream in ngx_http_upstream_finalize_request(),
and ensure that p->upstream is checked in ngx_event_pipe_read_upstream()
and when handling events at ngx_event_pipe() exit.

diff --git a/src/event/ngx_event_pipe.c b/src/event/ngx_event_pipe.c
--- a/src/event/ngx_event_pipe.c
+++ b/src/event/ngx_event_pipe.c
@@ -57,7 +57,9 @@ ngx_event_pipe(ngx_event_pipe_t *p, ngx_
do_write = 1;
}

- if (p->upstream->fd != (ngx_socket_t) -1) {
+ if (p->upstream
+ && p->upstream->fd != (ngx_socket_t) -1)
+ {
rev = p->upstream->read;

flags = (rev->eof || rev->error) ? NGX_CLOSE_EVENT : 0;
@@ -108,7 +110,9 @@ ngx_event_pipe_read_upstream(ngx_event_p
ngx_msec_t delay;
ngx_chain_t *chain, *cl, *ln;

- if (p->upstream_eof || p->upstream_error || p->upstream_done) {
+ if (p->upstream_eof || p->upstream_error || p->upstream_done
+ || p->upstream == NULL)
+ {
return NGX_OK;
}

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4561,6 +4561,10 @@ ngx_http_upstream_finalize_request(ngx_h

u->peer.connection = NULL;

+ if (u->pipe) {
+ u->pipe->upstream = NULL;
+ }
+
if (u->pipe && u->pipe->temp_file) {
ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
"http upstream temp fd: %d",

--
Maxim Dounin
http://mdounin.ru/
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