Re: [Qemu-devel] [PATCH v4 09/24] ui/vnc: Use gcrypto_random_bytes for start_auth_vnc

[Laurent Vivier <lvivier@redhat.com>]

On 06/05/2019 19:33, Richard Henderson wrote:
> Use a better interface for random numbers than rand().
> Fail gracefully if for some reason we cannot use the crypto system.
> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> v2: Use qcrypto_random_bytes, not qemu_getrandom, as there is
>      no need for deterministic results for this interface.
> v3: Fail gracefully in the event qcrypto_random_bytes fails.
> ---
>   ui/vnc.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 785edf3af1..d83f4a6ff9 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -43,6 +43,7 @@
>   #include "crypto/hash.h"
>   #include "crypto/tlscredsanon.h"
>   #include "crypto/tlscredsx509.h"
> +#include "crypto/random.h"
>   #include "qom/object_interfaces.h"
>   #include "qemu/cutils.h"
>   #include "io/dns-resolver.h"
> @@ -2547,16 +2548,6 @@ static void authentication_failed(VncState *vs)
>       vnc_client_error(vs);
>   }
>   
> -static void make_challenge(VncState *vs)
> -{
> -    int i;
> -
> -    srand(time(NULL)+getpid()+getpid()*987654+rand());
> -
> -    for (i = 0 ; i < sizeof(vs->challenge) ; i++)
> -        vs->challenge[i] = (int) (256.0*rand()/(RAND_MAX+1.0));
> -}
> -
>   static int protocol_client_auth_vnc(VncState *vs, uint8_t *data, size_t len)
>   {
>       unsigned char response[VNC_AUTH_CHALLENGE_SIZE];
> @@ -2628,7 +2619,16 @@ reject:
>   
>   void start_auth_vnc(VncState *vs)
>   {
> -    make_challenge(vs);
> +    Error *err = NULL;
> +
> +    if (qcrypto_random_bytes(vs->challenge, sizeof(vs->challenge), &err)) {
> +        trace_vnc_auth_fail(vs, vs->auth, "cannot get random bytes",
> +                            error_get_pretty(err));
> +        error_free(err);
> +        authentication_failed(vs);
> +        return;
> +    }
> +

This part is weird for me: if auth fails we send "vnc_write_u32(vs, 1)" 
but if it succeeds we send the challenge. There is no success value to 
send (like "vnc_write_u32(vs, 0)") ?

>       /* Send client a 'random' challenge */
>       vnc_write(vs, vs->challenge, sizeof(vs->challenge));
>       vnc_flush(vs);
>