[PATCH] QUIC: better sockaddr initialization

[Alejandro Colomar]

Hi Maxim,

On 5/21/23 03:42, Maxim Dounin wrote:
> # HG changeset patch
> # User Maxim Dounin <mdounin at mdounin.ru>
> # Date 1684633125 -10800
> #      Sun May 21 04:38:45 2023 +0300
> # Node ID 68fa4b86ed46138dd1a8fcf2cfd80206de068bec
> # Parent  235d482ef6bc8c40a956b2413865d42c94e0fc05
> QUIC: better sockaddr initialization.
> 
> The qsock->sockaddr field is a ngx_sockaddr_t union, and therefore can hold
> any sockaddr (and union members, such qsock->sockaddr.sockaddr, can be used
> to access appropriate variant of the sockaddr).  It is better to set it via
> qsock->sockaddr itself though, and not qsock->sockaddr.sockaddr, so static
> analyzers won't complain about out-of-bounds access.

Correct.  The previous code was UB, due to memcpy(3) writing to the
'struct sockaddr' member.  By writing to sockaddr, you were only
allowed to alias via other members the sa_family_t field, but no
others.

Only if you had written to a 'struct sockaddr_storage' (which nginx
doesn't use in the union), you'd have guarantees that it's storage is
accessible via any other sockaddr_* structures.  This broke with
aliasing rules, but the wording in POSIX has been improved for the
upcoming Issue 8 to allow this.

See:
<https://austingroupbugs.net/view.php?id=1641>
<https://lore.kernel.org/linux-man/20230330171310.12330-1-alx@kernel.org/T>
<https://lore.kernel.org/linux-man/20230421202718.21831-1-alx@kernel.org/T>

So, for doing things right, when multiple sockaddr_* structures are
involved, you need to write to a sockaddr_storage, or write to a union
containing all of the structures you're interested in.  Otherwise,
the only field that you can safely alias from any other sockaddr_*
is the sa_family_t one, and aliasing anything else is UB.


> 
> Prodded by Coverity (CID 1530403).
> 
> diff --git a/src/event/quic/ngx_event_quic_udp.c b/src/event/quic/ngx_event_quic_udp.c
> --- a/src/event/quic/ngx_event_quic_udp.c
> +++ b/src/event/quic/ngx_event_quic_udp.c
> @@ -183,7 +183,7 @@ ngx_quic_recvmsg(ngx_event_t *ev)
>  
>              qsock = ngx_quic_get_socket(c);
>  
> -            ngx_memcpy(&qsock->sockaddr.sockaddr, sockaddr, socklen);
> +            ngx_memcpy(&qsock->sockaddr, sockaddr, socklen);

Yes, since you write to the union, you can later use any of the
members.

Reviewed-by: Alejandro Colomar <alx at nginx.com>

Cheers,
Alex

>              qsock->socklen = socklen;
>  
>              c->udp->buffer = &buf;
> _______________________________________________
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-- 
<http://www.alejandro-colomar.es/>
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