Re: [Qemu-devel] [PATCH v2] sockets: improve error reporting if UNIX socket path is too long

[Eric Blake]

On 05/24/2017 11:18 AM, Daniel P. Berrange wrote:
> The 'struct sockaddr_un' only allows 108 bytes for the socket
> path.
> 
> If the user supplies a path, QEMU uses snprintf() to silently
> truncate it when too long. This is undesirable because the user
> will then be unable to connect to the path they asked for.
> 
> If the user doesn't supply a path, QEMU builds one based on
> TMPDIR, but if that leads to an overlong path, it mistakenly
> uses error_setg_errno() with a stale errno value, because
> snprintf() does not set errno on truncation.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
>  util/qemu-sockets.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)


> +    pathlen = saddr->path ? strlen(saddr->path) : 0;
> +    if (pathlen != 0) {
> +        if (pathlen > sizeof(un.sun_path)) {
> +            error_setg(errp, "UNIX socket path '%s' is too long", 
> saddr->path);
> +            error_append_hint(errp, "Path must be less than %zu bytes\n",
> +                              sizeof(un.sun_path));
> +            goto err;
> +        }
> +        strncpy(un.sun_path, saddr->path, sizeof(un.sun_path));

Does NOT nul-terminate if pathlen is exactly 108 - but it looks like
Linux is just fine with a Unix socket name that long.

>      } else {
> +        const char *template = "qemu-socket-XXXXXX";
>          const char *tmpdir = getenv("TMPDIR");
>          tmpdir = tmpdir ? tmpdir : "/tmp";
> -        if (snprintf(un.sun_path, sizeof(un.sun_path), 
> "%s/qemu-socket-XXXXXX",
> -                     tmpdir) >= sizeof(un.sun_path)) {
> -            error_setg_errno(errp, errno,
> -                             "TMPDIR environment variable (%s) too large", 
> tmpdir);
> +        if (snprintf(un.sun_path, sizeof(un.sun_path), "%s/%s",
> +                     tmpdir, template) >= sizeof(un.sun_path)) {
> +            error_setg(errp,
> +                       "TMPDIR environment variable (%s) too large", tmpdir);
> +            error_append_hint(errp, "Path must be less than %zu bytes\n",
> +                              sizeof(un.sun_path) - strlen(template) - 1);

Meanwhile, this branch requires nul-termination (that is, a maximum
generated name of 107), but that's because we then pass un.sun_path to
mkstemp() and g_strdup(), both of which require nul-termination.  It's a
bit odd to see the asymmetry between a user-specified name going all the
way to 108 while generated cannot, but not the end of the world.

You do, however, have one more problem to fix:

    if (unlink(un.sun_path) < 0 && errno != ENOENT) {

That now needs to be 'if (unlink(saddr->path) ...', because un.sun_path
is no longer guaranteed to be NUL-terminated (although saddr->path is).
Likewise for the two error_setg_errno() calls after that point.

With that fixed,

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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