On Thu, May 25, 2017 at 09:28:31AM -0500, Eric Blake wrote:
> On 05/25/2017 05:19 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.
> >
> > In solving this the code needed some refactoring to ensure we
> > don't pass 'un.sun_path' directly to any APIs which expect
> > NUL-terminated strings, because the path is not required to
> > be terminated.
> >
> > Signed-off-by: Daniel P. Berrange <berra...@redhat.com>
> > ---
> >
>
> > } else {
> > 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);
> > - goto err;
>
> The old code fails early if the generated name is too long,...
>
> > - }
> > + path = pathbuf = g_strdup_printf("%s/qemu-socket-XXXXXX", tmpdir);
> >
> > /*
> > * This dummy fd usage silences the mktemp() unsecure warning.
> > @@ -873,24 +868,32 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
> > * to unlink first and thus re-open the race window. The
> > * worst case possible is bind() failing, i.e. a DoS attack.
> > */
> > - fd = mkstemp(un.sun_path);
> > + fd = mkstemp(pathbuf);
>
> ...while the new code ends up creating a too-long name in the
> file-system anyways,...
Opps, yes, that's not good.
>
> > if (fd < 0) {
> > error_setg_errno(errp, errno,
> > "Failed to make a temporary socket name in
> > %s", tmpdir);
> > goto err;
> > }
> > close(fd);
> > - if (update_addr) {
> > - g_free(saddr->path);
> > - saddr->path = g_strdup(un.sun_path);
> > - }
> > }
> >
> > - if (unlink(un.sun_path) < 0 && errno != ENOENT) {
> > + if (strlen(path) > sizeof(un.sun_path)) {
> > + error_setg(errp, "UNIX socket path '%s' is too long", path);
> > + error_append_hint(errp, "Path must be less than %zu bytes\n",
> > + sizeof(un.sun_path));
>
> The old message mentioned the name that was too long, the new does not.
> That's a potential slight loss in error message quality, particularly
> since you are no longer mentioning a UNIX socket (which may give the
> user a clue why 108 is special), but I can probably live with it (it's
> still pretty obvious from the message that you should investigate what
> is too long).
I'm not sure why you are saying I don't mention the path or that it
is a UNIX socket - I do exactly that right there.
$ ./x86_64-softmmu/qemu-system-x86_64 -monitor
unix:/home/berrange/src/virt/qemu/01234567890123456789012345678901234567890123456789012345678901234567890123456789,server
qemu-system-x86_64: -monitor
unix:/home/berrange/src/virt/qemu/01234567890123456789012345678901234567890123456789012345678901234567890123456789,server:
UNIX socket path
'/home/berrange/src/virt/qemu/01234567890123456789012345678901234567890123456789012345678901234567890123456789'
is too long
Path must be less than 108 bytes
> > + if (unlink(path) < 0 && errno != ENOENT) {
>
> ...but skip the unlink of the just-created too-long name. Oops.
yeah will fix that too
>
> > error_setg_errno(errp, errno,
> > - "Failed to unlink socket %s", un.sun_path);
> > + "Failed to unlink socket %s", path);
> > goto err;
> > }
>
> I think you're okay if you just swap the unlink() with the length check.
> Although it seems rather sys-call heavy to mkstemp/unlink compared to
> just doing a length check first, I do like how your refactoring ended up
> with fewer conditionals rather than splitting the generated name code
> across multiple conditionals.
>
> > @@ -932,9 +942,16 @@ static int unix_connect_saddr(UnixSocketAddress *saddr,
> > qemu_set_nonblock(sock);
> > }
> >
> > + if (strlen(saddr->path) > 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));
>
> Again, potential loss in error message quality.
I don't think so.
Regards,
Daniel
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