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systemd/src/resolve/resolved-dnstls-openssl.c
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Joan Bruguera 839a70c353 resolved: Read as much as possible per stream EPOLLIN event 
In commit 2aaf6bb6e9, an issue was fixed where
systemd-resolved could get stuck for multiple seconds waiting for incoming data,
since GnuTLS/OpenSSL can buffer a TLS record, so data could be available, but
no EPOLLIN event would be generated.

To fix this, a somewhat elaborate logic consisting on asking the TLS library
whether it had buffered data, then "faking" an EPOLLIN event was implemented.

However, there is a much simpler solution: Always read as much data as available
(i.e. until we get an event like EAGAIN when trying to read) from the stream
when we get an EPOLLIN event, instead of at most a single packet per event.
This approach does not require asking the TLS library whether it has buffered
data, and the logic is exactly the same for both the TCP and TLS case.

test-resolved-stream is fixed to avoid a latent double free bug.
2022-02-01 19:25:32 +01:00

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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#if !ENABLE_DNS_OVER_TLS || !DNS_OVER_TLS_USE_OPENSSL
#error This source file requires DNS-over-TLS to be enabled and OpenSSL to be available.
#endif
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/x509v3.h>
#include "io-util.h"
#include "openssl-util.h"
#include "resolved-dns-stream.h"
#include "resolved-dnstls.h"
#include "resolved-manager.h"
static int dnstls_flush_write_buffer(DnsStream *stream) {
ssize_t ss;
assert(stream);
assert(stream->encrypted);
if (stream->dnstls_data.buffer_offset < stream->dnstls_data.write_buffer->length) {
assert(stream->dnstls_data.write_buffer->data);
struct iovec iov[1];
iov[0] = IOVEC_MAKE(stream->dnstls_data.write_buffer->data + stream->dnstls_data.buffer_offset,
stream->dnstls_data.write_buffer->length - stream->dnstls_data.buffer_offset);
ss = dns_stream_writev(stream, iov, 1, DNS_STREAM_WRITE_TLS_DATA);
if (ss < 0) {
if (ss == -EAGAIN)
stream->dnstls_events |= EPOLLOUT;
return ss;
} else {
stream->dnstls_data.buffer_offset += ss;
if (stream->dnstls_data.buffer_offset < stream->dnstls_data.write_buffer->length) {
stream->dnstls_events |= EPOLLOUT;
return -EAGAIN;
} else {
BIO_reset(SSL_get_wbio(stream->dnstls_data.ssl));
stream->dnstls_data.buffer_offset = 0;
}
}
}
return 0;
}
int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
_cleanup_(BIO_freep) BIO *rb = NULL, *wb = NULL;
_cleanup_(SSL_freep) SSL *s = NULL;
int error, r;
assert(stream);
assert(stream->manager);
assert(server);
rb = BIO_new_socket(stream->fd, 0);
if (!rb)
return -ENOMEM;
wb = BIO_new(BIO_s_mem());
if (!wb)
return -ENOMEM;
BIO_get_mem_ptr(wb, &stream->dnstls_data.write_buffer);
stream->dnstls_data.buffer_offset = 0;
s = SSL_new(stream->manager->dnstls_data.ctx);
if (!s)
return -ENOMEM;
SSL_set_connect_state(s);
r = SSL_set_session(s, server->dnstls_data.session);
if (r == 0)
return -EIO;
SSL_set_bio(s, TAKE_PTR(rb), TAKE_PTR(wb));
if (server->manager->dns_over_tls_mode == DNS_OVER_TLS_YES) {
X509_VERIFY_PARAM *v;
SSL_set_verify(s, SSL_VERIFY_PEER, NULL);
v = SSL_get0_param(s);
if (server->server_name) {
X509_VERIFY_PARAM_set_hostflags(v, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
if (X509_VERIFY_PARAM_set1_host(v, server->server_name, 0) == 0)
return -ECONNREFUSED;
} else {
const unsigned char *ip;
ip = server->family == AF_INET ? (const unsigned char*) &server->address.in.s_addr : server->address.in6.s6_addr;
if (X509_VERIFY_PARAM_set1_ip(v, ip, FAMILY_ADDRESS_SIZE(server->family)) == 0)
return -ECONNREFUSED;
}
}
if (server->server_name) {
r = SSL_set_tlsext_host_name(s, server->server_name);
if (r <= 0) {
char errbuf[256];
error = ERR_get_error();
ERR_error_string_n(error, errbuf, sizeof(errbuf));
return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to set server name: %s", errbuf);
}
}
ERR_clear_error();
stream->dnstls_data.handshake = SSL_do_handshake(s);
if (stream->dnstls_data.handshake <= 0) {
error = SSL_get_error(s, stream->dnstls_data.handshake);
if (!IN_SET(error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
char errbuf[256];
ERR_error_string_n(error, errbuf, sizeof(errbuf));
return log_debug_errno(SYNTHETIC_ERRNO(ECONNREFUSED),
"Failed to invoke SSL_do_handshake: %s", errbuf);
}
}
stream->encrypted = true;
stream->dnstls_data.ssl = TAKE_PTR(s);
r = dnstls_flush_write_buffer(stream);
if (r < 0 && r != -EAGAIN) {
SSL_free(TAKE_PTR(stream->dnstls_data.ssl));
return r;
}
return 0;
}
void dnstls_stream_free(DnsStream *stream) {
assert(stream);
assert(stream->encrypted);
if (stream->dnstls_data.ssl)
SSL_free(stream->dnstls_data.ssl);
}
int dnstls_stream_on_io(DnsStream *stream, uint32_t revents) {
int error, r;
assert(stream);
assert(stream->encrypted);
assert(stream->dnstls_data.ssl);
/* Flush write buffer when requested by OpenSSL */
if ((revents & EPOLLOUT) && (stream->dnstls_events & EPOLLOUT)) {
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
}
if (stream->dnstls_data.shutdown) {
ERR_clear_error();
r = SSL_shutdown(stream->dnstls_data.ssl);
if (r == 0) {
stream->dnstls_events = 0;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return -EAGAIN;
} else if (r < 0) {
error = SSL_get_error(stream->dnstls_data.ssl, r);
if (IN_SET(error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
stream->dnstls_events = error == SSL_ERROR_WANT_READ ? EPOLLIN : EPOLLOUT;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return -EAGAIN;
} else if (error == SSL_ERROR_SYSCALL) {
if (errno > 0)
log_debug_errno(errno, "Failed to invoke SSL_shutdown, ignoring: %m");
} else {
char errbuf[256];
ERR_error_string_n(error, errbuf, sizeof(errbuf));
log_debug("Failed to invoke SSL_shutdown, ignoring: %s", errbuf);
}
}
stream->dnstls_events = 0;
stream->dnstls_data.shutdown = false;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
dns_stream_unref(stream);
return DNSTLS_STREAM_CLOSED;
} else if (stream->dnstls_data.handshake <= 0) {
ERR_clear_error();
stream->dnstls_data.handshake = SSL_do_handshake(stream->dnstls_data.ssl);
if (stream->dnstls_data.handshake <= 0) {
error = SSL_get_error(stream->dnstls_data.ssl, stream->dnstls_data.handshake);
if (IN_SET(error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
stream->dnstls_events = error == SSL_ERROR_WANT_READ ? EPOLLIN : EPOLLOUT;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return -EAGAIN;
} else {
char errbuf[256];
ERR_error_string_n(error, errbuf, sizeof(errbuf));
return log_debug_errno(SYNTHETIC_ERRNO(ECONNREFUSED),
"Failed to invoke SSL_do_handshake: %s",
errbuf);
}
}
stream->dnstls_events = 0;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
}
return 0;
}
int dnstls_stream_shutdown(DnsStream *stream, int error) {
int ssl_error, r;
SSL_SESSION *s;
assert(stream);
assert(stream->encrypted);
assert(stream->dnstls_data.ssl);
if (stream->server) {
s = SSL_get1_session(stream->dnstls_data.ssl);
if (s) {
if (stream->server->dnstls_data.session)
SSL_SESSION_free(stream->server->dnstls_data.session);
stream->server->dnstls_data.session = s;
}
}
if (error == ETIMEDOUT) {
ERR_clear_error();
r = SSL_shutdown(stream->dnstls_data.ssl);
if (r == 0) {
if (!stream->dnstls_data.shutdown) {
stream->dnstls_data.shutdown = true;
dns_stream_ref(stream);
}
stream->dnstls_events = 0;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return -EAGAIN;
} else if (r < 0) {
ssl_error = SSL_get_error(stream->dnstls_data.ssl, r);
if (IN_SET(ssl_error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
stream->dnstls_events = ssl_error == SSL_ERROR_WANT_READ ? EPOLLIN : EPOLLOUT;
r = dnstls_flush_write_buffer(stream);
if (r < 0 && r != -EAGAIN)
return r;
if (!stream->dnstls_data.shutdown) {
stream->dnstls_data.shutdown = true;
dns_stream_ref(stream);
}
return -EAGAIN;
} else if (ssl_error == SSL_ERROR_SYSCALL) {
if (errno > 0)
log_debug_errno(errno, "Failed to invoke SSL_shutdown, ignoring: %m");
} else {
char errbuf[256];
ERR_error_string_n(ssl_error, errbuf, sizeof(errbuf));
log_debug("Failed to invoke SSL_shutdown, ignoring: %s", errbuf);
}
}
stream->dnstls_events = 0;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
}
return 0;
}
static ssize_t dnstls_stream_write(DnsStream *stream, const char *buf, size_t count) {
int error, r;
ssize_t ss;
ERR_clear_error();
ss = r = SSL_write(stream->dnstls_data.ssl, buf, count);
if (r <= 0) {
error = SSL_get_error(stream->dnstls_data.ssl, r);
if (IN_SET(error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
stream->dnstls_events = error == SSL_ERROR_WANT_READ ? EPOLLIN : EPOLLOUT;
ss = -EAGAIN;
} else if (error == SSL_ERROR_ZERO_RETURN) {
stream->dnstls_events = 0;
ss = 0;
} else {
char errbuf[256];
ERR_error_string_n(error, errbuf, sizeof(errbuf));
log_debug("Failed to invoke SSL_write: %s", errbuf);
stream->dnstls_events = 0;
ss = -EPIPE;
}
} else
stream->dnstls_events = 0;
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return ss;
}
ssize_t dnstls_stream_writev(DnsStream *stream, const struct iovec *iov, size_t iovcnt) {
_cleanup_free_ char *buf = NULL;
size_t count;
assert(stream);
assert(stream->encrypted);
assert(stream->dnstls_data.ssl);
assert(iov);
assert(IOVEC_TOTAL_SIZE(iov, iovcnt) > 0);
if (iovcnt == 1)
return dnstls_stream_write(stream, iov[0].iov_base, iov[0].iov_len);
/* As of now, OpenSSL can not accumulate multiple writes, so join into a
single buffer. Suboptimal, but better than multiple SSL_write calls. */
count = IOVEC_TOTAL_SIZE(iov, iovcnt);
buf = new(char, count);
for (size_t i = 0, pos = 0; i < iovcnt; pos += iov[i].iov_len, i++)
memcpy(buf + pos, iov[i].iov_base, iov[i].iov_len);
return dnstls_stream_write(stream, buf, count);
}
ssize_t dnstls_stream_read(DnsStream *stream, void *buf, size_t count) {
int error, r;
ssize_t ss;
assert(stream);
assert(stream->encrypted);
assert(stream->dnstls_data.ssl);
assert(buf);
ERR_clear_error();
ss = r = SSL_read(stream->dnstls_data.ssl, buf, count);
if (r <= 0) {
error = SSL_get_error(stream->dnstls_data.ssl, r);
if (IN_SET(error, SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE)) {
/* If we receive SSL_ERROR_WANT_READ here, there are two possible scenarios:
* OpenSSL needs to renegotiate (so we want to get an EPOLLIN event), or
* There is no more application data is available, so we can just return
And apparently there's no nice way to distinguish between the two.
To handle this, never set EPOLLIN and just continue as usual.
If OpenSSL really wants to read due to renegotiation, it will tell us
again on SSL_write (at which point we will request EPOLLIN force a read);
or we will just eventually read data anyway while we wait for a packet */
stream->dnstls_events = error == SSL_ERROR_WANT_READ ? 0 : EPOLLOUT;
ss = -EAGAIN;
} else if (error == SSL_ERROR_ZERO_RETURN) {
stream->dnstls_events = 0;
ss = 0;
} else {
char errbuf[256];
ERR_error_string_n(error, errbuf, sizeof(errbuf));
log_debug("Failed to invoke SSL_read: %s", errbuf);
stream->dnstls_events = 0;
ss = -EPIPE;
}
} else
stream->dnstls_events = 0;
/* flush write buffer in cache of renegotiation */
r = dnstls_flush_write_buffer(stream);
if (r < 0)
return r;
return ss;
}
void dnstls_server_free(DnsServer *server) {
assert(server);
if (server->dnstls_data.session)
SSL_SESSION_free(server->dnstls_data.session);
}
int dnstls_manager_init(Manager *manager) {
int r;
assert(manager);
ERR_load_crypto_strings();
SSL_load_error_strings();
manager->dnstls_data.ctx = SSL_CTX_new(TLS_client_method());
if (!manager->dnstls_data.ctx)
return -ENOMEM;
r = SSL_CTX_set_min_proto_version(manager->dnstls_data.ctx, TLS1_2_VERSION);
if (r == 0)
return -EIO;
(void) SSL_CTX_set_options(manager->dnstls_data.ctx, SSL_OP_NO_COMPRESSION);
r = SSL_CTX_set_default_verify_paths(manager->dnstls_data.ctx);
if (r == 0)
return log_warning_errno(SYNTHETIC_ERRNO(EIO),
"Failed to load system trust store: %s",
ERR_error_string(ERR_get_error(), NULL));
return 0;
}
void dnstls_manager_free(Manager *manager) {
assert(manager);
if (manager->dnstls_data.ctx)
SSL_CTX_free(manager->dnstls_data.ctx);
}
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