Re: [PATCH] hw/display/ramfb: plug slight guest-triggerable leak on mode setting

[Marc-André Lureau]

Hi

On Wed, Sep 27, 2023 at 7:46 PM Laszlo Ersek <lersek@redhat.com> wrote:
>
> On 9/19/23 15:19, Laszlo Ersek wrote:
> > The fw_cfg DMA write callback in ramfb prepares a new display surface in
> > QEMU; this new surface is put to use ("swapped in") upon the next display
> > update. At that time, the old surface (if any) is released.
> >
> > If the guest triggers the fw_cfg DMA write callback at least twice between
> > two adjacent display updates, then the second callback (and further such
> > callbacks) will leak the previously prepared (but not yet swapped in)
> > display surface.
> >
> > The issue can be shown by:
> >
> > (1) starting QEMU with "-trace displaysurface_free", and
> >
> > (2) running the following program in the guest UEFI shell:
> >
> >> #include <Library/ShellCEntryLib.h>           // ShellAppMain()
> >> #include <Library/UefiBootServicesTableLib.h> // gBS
> >> #include <Protocol/GraphicsOutput.h>          // 
> >> EFI_GRAPHICS_OUTPUT_PROTOCOL
> >>
> >> INTN
> >> EFIAPI
> >> ShellAppMain (
> >>   IN UINTN   Argc,
> >>   IN CHAR16  **Argv
> >>   )
> >> {
> >>   EFI_STATUS                    Status;
> >>   VOID                          *Interface;
> >>   EFI_GRAPHICS_OUTPUT_PROTOCOL  *Gop;
> >>   UINT32                        Mode;
> >>
> >>   Status = gBS->LocateProtocol (
> >>                   &gEfiGraphicsOutputProtocolGuid,
> >>                   NULL,
> >>                   &Interface
> >>                   );
> >>   if (EFI_ERROR (Status)) {
> >>     return 1;
> >>   }
> >>
> >>   Gop = Interface;
> >>
> >>   Mode = 1;
> >>   for ( ; ;) {
> >>     Status = Gop->SetMode (Gop, Mode);
> >>     if (EFI_ERROR (Status)) {
> >>       break;
> >>     }
> >>
> >>     Mode = 1 - Mode;
> >>   }
> >>
> >>   return 1;
> >> }
> >
> > The symptom is then that:
> >
> > - only one trace message appears periodically,
> >
> > - the time between adjacent messages keeps increasing -- implying that
> >   some list structure (containing the leaked resources) keeps growing,
> >
> > - the "surface" pointer is ever different.
> >
> >> 18566@1695127471.449586:displaysurface_free surface=0x7f2fcc09a7c0
> >> 18566@1695127471.529559:displaysurface_free surface=0x7f2fcc9dac10
> >> 18566@1695127471.659812:displaysurface_free surface=0x7f2fcc441dd0
> >> 18566@1695127471.839669:displaysurface_free surface=0x7f2fcc0363d0
> >> 18566@1695127472.069674:displaysurface_free surface=0x7f2fcc413a80
> >> 18566@1695127472.349580:displaysurface_free surface=0x7f2fcc09cd00
> >> 18566@1695127472.679783:displaysurface_free surface=0x7f2fcc1395f0
> >> 18566@1695127473.059848:displaysurface_free surface=0x7f2fcc1cae50
> >> 18566@1695127473.489724:displaysurface_free surface=0x7f2fcc42fc50
> >> 18566@1695127473.969791:displaysurface_free surface=0x7f2fcc45dcc0
> >> 18566@1695127474.499708:displaysurface_free surface=0x7f2fcc70b9d0
> >> 18566@1695127475.079769:displaysurface_free surface=0x7f2fcc82acc0
> >> 18566@1695127475.709941:displaysurface_free surface=0x7f2fcc369c00
> >> 18566@1695127476.389619:displaysurface_free surface=0x7f2fcc32b910
> >> 18566@1695127477.119772:displaysurface_free surface=0x7f2fcc0d5a20
> >> 18566@1695127477.899517:displaysurface_free surface=0x7f2fcc086c40
> >> 18566@1695127478.729962:displaysurface_free surface=0x7f2fccc72020
> >> 18566@1695127479.609839:displaysurface_free surface=0x7f2fcc185160
> >> 18566@1695127480.539688:displaysurface_free surface=0x7f2fcc23a7e0
> >> 18566@1695127481.519759:displaysurface_free surface=0x7f2fcc3ec870
> >> 18566@1695127482.549930:displaysurface_free surface=0x7f2fcc634960
> >> 18566@1695127483.629661:displaysurface_free surface=0x7f2fcc26b140
> >> 18566@1695127484.759987:displaysurface_free surface=0x7f2fcc321700
> >> 18566@1695127485.940289:displaysurface_free surface=0x7f2fccaad100
> >
> > We figured this wasn't a CVE-worthy problem, as only small amounts of
> > memory were leaked (the framebuffer itself is mapped from guest RAM, QEMU
> > only allocates administrative structures), plus libvirt restricts QEMU
> > memory footprint anyway, thus the guest can only DoS itself.
> >
> > Plug the leak, by releasing the last prepared (not yet swapped in) display
> > surface, if any, in the fw_cfg DMA write callback.
> >
> > Regarding the "reproducer", with the fix in place, the log is flooded with
> > trace messages (one per fw_cfg write), *and* the trace message alternates
> > between just two "surface" pointer values (i.e., nothing is leaked, the
> > allocator flip-flops between two objects in effect).
> >
> > This issue appears to date back to the introducion of ramfb (995b30179bdc,
> > "hw/display: add ramfb, a simple boot framebuffer living in guest ram",
> > 2018-06-18).
> >
> > Cc: Gerd Hoffmann <kraxel@redhat.com> (maintainer:ramfb)
> > Cc: qemu-stable@nongnu.org
> > Fixes: 995b30179bdc
> > Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> > ---
> >  hw/display/ramfb.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/hw/display/ramfb.c b/hw/display/ramfb.c
> > index 79b9754a5820..c2b002d53480 100644
> > --- a/hw/display/ramfb.c
> > +++ b/hw/display/ramfb.c
> > @@ -97,6 +97,7 @@ static void ramfb_fw_cfg_write(void *dev, off_t offset, 
> > size_t len)
> >
> >      s->width = width;
> >      s->height = height;
> > +    qemu_free_displaysurface(s->ds);
> >      s->ds = surface;
> >  }

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

Incidentally I found the same issue:
https://patchew.org/QEMU/20230920082634.3349487-1-marcandre.lureau@redhat.com/


fwiw, my migration support patch is still unreviewed:
https://patchew.org/QEMU/20230920082651.3349712-1-marcandre.lureau@redhat.com/

-- 
Marc-André Lureau