Re: [Qemu-devel] [PATCH v2 1/4] display: ensure qxl log_buf is a nul terminated string

[Daniel P . Berrangé]

On Fri, Jan 18, 2019 at 11:40:29AM -0600, Eric Blake wrote:
> On 1/18/19 11:31 AM, Daniel P. Berrangé wrote:
> > The QXL_IO_LOG command allows the guest to send log messages to the host
> > via a buffer in the QXLRam struct. QEMU prints these to the console if
> > the qxl 'guestdebug' option is set to non-zero. It will also feed them
> > to the trace subsystem if any backends are built-in.
> > 
> > In both cases the log_buf data will get treated as being as a nul
> > terminated string, by the printf '%s' format specifier and / or other
> > code reading the buffer.
> > 
> > QEMU does nothing to guarantee that the log_buf really is nul terminated,
> > so there is potential for out of bounds array access.
> > 
> > This would affect any QEMU which has the log, syslog or ftrace trace
> > backends built into QEMU. It can only be triggered if the 'qxl_io_log'
> > trace event is enabled, however, so they are not vulnerable without
> > specific administrative action to enable this.
> > 
> > It would also affect QEMU if the 'guestdebug' parameter is set to a
> > non-zero value, which again is not the default and requires explicit
> > admin opt-in.
> > 
> > Signed-off-by: Daniel P. Berrangé <address@hidden>
> > ---
> >  hw/display/qxl.c        | 3 ++-
> >  hw/display/trace-events | 2 +-
> >  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> Reviewed-by: Eric Blake <address@hidden>
> 
> > +++ b/hw/display/qxl.c
> > @@ -1763,7 +1763,8 @@ async_common:
> >          qxl_set_mode(d, val, 0);
> >          break;
> >      case QXL_IO_LOG:
> > -        trace_qxl_io_log(d->id, d->ram->log_buf);
> > +        d->ram->log_buf[sizeof(d->ram->log_buf) - 1] = '\0';
> 
> May lose a character from the log, but the improved safety is worth it.

In practice it won't.  The Xorg QXL driver and Windows XDDM DOD driver
don't use this feature at all. The Windows QXL miniport driver uses snprintf
when populating the buffer so will already have truncated if it was going
to overflow.

> 
> > +        trace_qxl_io_log(d->id, (const char *)d->ram->log_buf);
> >          if (d->guestdebug) {
> >              fprintf(stderr, "qxl/guest-%d: %" PRId64 ": %s", d->id,
> >                      qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), 
> > d->ram->log_buf);
> > diff --git a/hw/display/trace-events b/hw/display/trace-events
> > index 5a48c6cb6a..387c6b8931 100644
> > --- a/hw/display/trace-events
> > +++ b/hw/display/trace-events
> > @@ -72,7 +72,7 @@ qxl_interface_update_area_complete_rest(int qid, uint32_t 
> > num_updated_rects) "%d
> >  qxl_interface_update_area_complete_overflow(int qid, int max) "%d max=%d"
> >  qxl_interface_update_area_complete_schedule_bh(int qid, uint32_t 
> > num_dirty) "%d #dirty=%d"
> >  qxl_io_destroy_primary_ignored(int qid, const char *mode) "%d %s"
> > -qxl_io_log(int qid, const uint8_t *log_buf) "%d %s"
> > +qxl_io_log(int qid, const char *log_buf) "%d %s"
> >  qxl_io_read_unexpected(int qid) "%d"
> >  qxl_io_unexpected_vga_mode(int qid, uint64_t addr, uint64_t val, const 
> > char *desc) "%d 0x%"PRIx64"=%"PRIu64" (%s)"
> >  qxl_io_write(int qid, const char *mode, uint64_t addr, const char *aname, 
> > uint64_t val, unsigned size, int async) "%d %s addr=%"PRIu64 " (%s) 
> > val=%"PRIu64" size=%u async=%d"
> > 
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 




Regards,
Daniel
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