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Fixed processing of response body chunks ... #105

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Fixed processing of response body chunks ... #105

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5b02ab0
Fix incorrect handling of request/response body data chain of ngx_buf…
turchanov Apr 18, 2018
f238283
A body filter function (ngx_http_modsecurity_body_filter in our case)…
turchanov Apr 19, 2018
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55 changes: 25 additions & 30 deletions src/ngx_http_modsecurity_body_filter.c
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Original file line number Diff line number Diff line change
Expand Up @@ -35,7 +35,6 @@ ngx_http_modsecurity_body_filter_init(void)
ngx_int_t
ngx_http_modsecurity_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
int buffer_fully_loadead = 0;
ngx_chain_t *chain = in;
ngx_http_modsecurity_ctx_t *ctx = NULL;
#if defined(MODSECURITY_SANITY_CHECKS) && (MODSECURITY_SANITY_CHECKS)
Expand Down Expand Up @@ -135,47 +134,43 @@ ngx_http_modsecurity_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
}
#endif

int is_request_processed = 0;
for (; chain != NULL; chain = chain->next)
{
/* XXX: chain->buf->last_buf || chain->buf->last_in_chain */
if (chain->buf->last_buf) {
buffer_fully_loadead = 1;
u_char *data = chain->buf->pos;
int ret;

msc_append_response_body(ctx->modsec_transaction, data, chain->buf->last - data);
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Let's use ngx_buf_size() here.

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ngx_buf_size is defined as

#define ngx_buf_size(b)                                                      \
    (ngx_buf_in_memory(b) ? (off_t) (b->last - b->pos):                      \
                            (b->file_last - b->file_pos))

If somehow we happend to receive a buf which is completely in a file (i.e., buf->in_file == true but ngx_buf_in_memory(buf) == false) then .pos = .last = 0 but .file_last - .file_post > 0, so we will try to dereference a non-zero length block at NULL pointer .pos. But with current .last - .pos we would erroneously assume that we have no data but won't segfault at least.

But from the other hand I did a quick testing with gdb, even when a response body is buffered to a temporary file, nginx supplies a buf which has both .in_file and .temporary set (ngx_buf_in_memory(buf) == true => .pos, .last are valid).
ngx_http_charset_filter_module also references .pos/.last in ngx_http_charset_recode_{to,from}_utf8 without worries . So I think it is guaranteed that response body filters get chain bufs with ngx_buf_in_memory(buf) == true ?

One more thing, I did a quick grep of Nginx sources and see that ngx_buf_size is not used that much, whereas .last - .pos is much more common... maybe we should stick to the latter?

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@turchanov well ok - let's keep .last - .pos.

ret = ngx_http_modsecurity_process_intervention(ctx->modsec_transaction, r);
if (ret > 0) {
return ngx_http_filter_finalize_request(r,
&ngx_http_modsecurity_module, ret);
}
}

if (buffer_fully_loadead == 1)
{
int ret;
ngx_pool_t *old_pool;
/* XXX: chain->buf->last_buf || chain->buf->last_in_chain */
is_request_processed = chain->buf->last_buf;

for (chain = in; chain != NULL; chain = chain->next)
{
u_char *data = chain->buf->start;
if (is_request_processed) {
ngx_pool_t *old_pool;

old_pool = ngx_http_modsecurity_pcre_malloc_init(r->pool);
msc_process_response_body(ctx->modsec_transaction);
ngx_http_modsecurity_pcre_malloc_done(old_pool);

msc_append_response_body(ctx->modsec_transaction, data, chain->buf->end - data);
/* XXX: I don't get how body from modsec being transferred to nginx's buffer. If so - after adjusting of nginx's
XXX: body we can proceed to adjust body size (content-length). see xslt_body_filter() for example */
ret = ngx_http_modsecurity_process_intervention(ctx->modsec_transaction, r);
if (ret > 0) {
return ngx_http_filter_finalize_request(r,
&ngx_http_modsecurity_module, ret);
return ret;
}
}

old_pool = ngx_http_modsecurity_pcre_malloc_init(r->pool);
msc_process_response_body(ctx->modsec_transaction);
ngx_http_modsecurity_pcre_malloc_done(old_pool);
else if (ret < 0) {
return ngx_http_filter_finalize_request(r,
&ngx_http_modsecurity_module, NGX_HTTP_INTERNAL_SERVER_ERROR);

/* XXX: I don't get how body from modsec being transferred to nginx's buffer. If so - after adjusting of nginx's
XXX: body we can proceed to adjust body size (content-length). see xslt_body_filter() for example */
ret = ngx_http_modsecurity_process_intervention(ctx->modsec_transaction, r);
if (ret > 0) {
return ret;
}
else if (ret < 0) {
return ngx_http_filter_finalize_request(r,
&ngx_http_modsecurity_module, NGX_HTTP_INTERNAL_SERVER_ERROR);
}
}
}
else
if (!is_request_processed)
{
dd("buffer was not fully loaded! ctx: %p", ctx);
}
Expand Down
4 changes: 2 additions & 2 deletions src/ngx_http_modsecurity_pre_access.c
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Original file line number Diff line number Diff line change
Expand Up @@ -163,10 +163,10 @@ ngx_http_modsecurity_pre_access_handler(ngx_http_request_t *r)

while (chain && !already_inspected)
{
u_char *data = chain->buf->start;
u_char *data = chain->buf->pos;

msc_append_request_body(ctx->modsec_transaction, data,
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Ditto (it seems like the commit from #104 though, I left the same comment there).

chain->buf->last - chain->buf->pos);
chain->buf->last - data);

if (chain->buf->last_buf) {
break;
Expand Down