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 <kra...@redhat.com> (maintainer:ramfb)
> Cc: qemu-sta...@nongnu.org
> Fixes: 995b30179bdc
> Signed-off-by: Laszlo Ersek <ler...@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;
> }
>
Ping.
Laszlo
