Re: [PATCH-for-5.0 v2 1/4] Revert "prevent crash when executing guest-file-read with large count"

[Philippe Mathieu-Daudé]

On 3/31/20 4:12 PM, Daniel P. Berrangé wrote:
> On Tue, Mar 31, 2020 at 04:06:35PM +0200, Philippe Mathieu-Daudé wrote:
> > By using g_try_malloc() instead of g_malloc() the qemu-guest-agent
> > Denial-of-Service attack referred in commit 807e2b6fce is reduced,
> > but still triggerable:
>
> As explained previously, I believe there is *no* denial of service
> attack here. The described scenario is just a user hurting themselves
> by intentionally telling QEMU not to limit the amount of data returned.

Yes. Do you mind updating the BZ, eventually marking as NOTABUG? Then I 
can adapt the patch descriptions.
https://bugzilla.redhat.com/show_bug.cgi?id=1594054

> >    - bisect file size S until g_try_malloc(S) fails,
> >    - use S - 1:
> >      g_try_malloc(S - 1) succeeds, but g_new0() few lines later will
> >      fail.
> >
> >   346     buf = g_try_malloc0(count + 1);
> >   347     if (!buf) {
> >   348         error_setg(errp,
> >   349                    "failed to allocate sufficient memory "
> >   350                    "to complete the requested service");
> >   351         return NULL;
> >   352     }
> >   353     is_ok = ReadFile(fh, buf, count, &read_count, NULL);
> >   354     if (!is_ok) {
> >   355         error_setg_win32(errp, GetLastError(), "failed to read file");
> >   356         slog("guest-file-read failed, handle %" PRId64, handle);
> >   357     } else {
> >   358         buf[read_count] = 0;
> >   359         read_data = g_new0(GuestFileRead, 1);
> >                           ^^^^^^
> >
> > Instead we are going to put a low hard limit on 'count' in the next
> > commits.
> > This reverts commit 807e2b6fce022707418bc8f61c069d91c613b3d2.
> >
> > Suggested-by: Daniel P. Berrangé <address@hidden>
> > Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> > ---
> >   qga/commands-win32.c | 8 +-------
> >   1 file changed, 1 insertion(+), 7 deletions(-)
> >
> > diff --git a/qga/commands-win32.c b/qga/commands-win32.c
> > index b49920e201..46cea7d1d9 100644
> > --- a/qga/commands-win32.c
> > +++ b/qga/commands-win32.c
> > @@ -343,13 +343,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, bool 
> > has_count,
> >       }
> >   
> >       fh = gfh->fh;
> > -    buf = g_try_malloc0(count + 1);
> > -    if (!buf) {
> > -        error_setg(errp,
> > -                   "failed to allocate sufficient memory "
> > -                   "to complete the requested service");
> > -        return NULL;
> > -    }
> > +    buf = g_malloc0(count + 1);
> >       is_ok = ReadFile(fh, buf, count, &read_count, NULL);
> >       if (!is_ok) {
> >           error_setg_win32(errp, GetLastError(), "failed to read file");
> > --
> > 2.21.1
>
> Regards,
> Daniel