curl: Make sslverify=off disable host as well as peer verification.

The sslverify setting is supposed to turn off all TLS certificate
checks in libcurl.  However because of the way we use it, it only
turns off peer certificate authenticity checks
(CURLOPT_SSL_VERIFYPEER).  This patch makes it also turn off the check
that the server name in the certificate is the same as the server
you're connecting to (CURLOPT_SSL_VERIFYHOST).

We can use Google's server at 8.8.8.8 which happens to have a bad TLS
certificate to demonstrate this:

$ ./qemu-img create -q -f qcow2 -b 'json: { "file.sslverify": "off", "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: SSL: no alternative certificate subject name matches target host name '8.8.8.8'
Could not open backing image to determine size.

With this patch applied, qemu-img connects to the server regardless of
the bad certificate:

$ ./qemu-img create -q -f qcow2 -b 'json: { "file.sslverify": "off", "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: The requested URL returned error: 404 Not Found

(The 404 error is expected because 8.8.8.8 is not actually serving a
file called "/foo".)

Of course the default (without sslverify=off) remains to always check
the certificate:

$ ./qemu-img create -q -f qcow2 -b 'json: { "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: SSL: no alternative certificate subject name matches target host name '8.8.8.8'
Could not open backing image to determine size.

Further information about the two settings is available here:

https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html
https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYHOST.html

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
 block/curl.c | 2 ++
 1 file changed, 2 insertions(+)

On Fri, Sep 14, 2018 at 10:56:22AM +0100, Richard W.M. Jones wrote:
> The sslverify setting is supposed to turn off all TLS certificate
> checks in libcurl.  However because of the way we use it, it only
> turns off peer certificate authenticity checks
> (CURLOPT_SSL_VERIFYPEER).  This patch makes it also turn off the check
> that the server name in the certificate is the same as the server
> you're connecting to (CURLOPT_SSL_VERIFYHOST).
> 
> We can use Google's server at 8.8.8.8 which happens to have a bad TLS
> certificate to demonstrate this:
> 
> $ ./qemu-img create -q -f qcow2 -b 'json: { "file.sslverify": "off", "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
> qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: SSL: no alternative certificate subject name matches target host name '8.8.8.8'
> Could not open backing image to determine size.
> 
> With this patch applied, qemu-img connects to the server regardless of
> the bad certificate:
> 
> $ ./qemu-img create -q -f qcow2 -b 'json: { "file.sslverify": "off", "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
> qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: The requested URL returned error: 404 Not Found
> 
> (The 404 error is expected because 8.8.8.8 is not actually serving a
> file called "/foo".)
> 
> Of course the default (without sslverify=off) remains to always check
> the certificate:
> 
> $ ./qemu-img create -q -f qcow2 -b 'json: { "file.driver": "https", "file.url": "https://8.8.8.8/foo" }' /var/tmp/file.qcow2
> qemu-img: /var/tmp/file.qcow2: CURL: Error opening file: SSL: no alternative certificate subject name matches target host name '8.8.8.8'
> Could not open backing image to determine size.
> 
> Further information about the two settings is available here:
> 
> https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html
> https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYHOST.html
> 
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> ---
>  block/curl.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/curl.c b/block/curl.c
> index 229bb84a27..fabb2b4da7 100644
> --- a/block/curl.c
> +++ b/block/curl.c
> @@ -483,6 +483,8 @@ static int curl_init_state(BDRVCURLState *s, CURLState *state)
>          curl_easy_setopt(state->curl, CURLOPT_URL, s->url);
>          curl_easy_setopt(state->curl, CURLOPT_SSL_VERIFYPEER,
>                           (long) s->sslverify);
> +        curl_easy_setopt(state->curl, CURLOPT_SSL_VERIFYHOST,
> +                         s->sslverify ? 2L : 0L);
>          if (s->cookie) {
>              curl_easy_setopt(state->curl, CURLOPT_COOKIE, s->cookie);
>          }
> -- 
> 2.19.0.rc0
> 

Thanks,

Applied to my block branch:

git://github.com/codyprime/qemu-kvm-jtc block

-Jeff