Re: [PATCH] hw/display/exynos4210_fimd: Fix potential NULL pointer dereference

[Peter Maydell]

On Sat, 31 Oct 2020 at 02:57, AlexChen <alex.chen@huawei.com> wrote:
>
> On 2020/10/30 22:28, Peter Maydell wrote:
> > On Fri, 30 Oct 2020 at 10:23, AlexChen <alex.chen@huawei.com> wrote:
> >>
> >> In exynos4210_fimd_update(), the pointer s is dereferenced before
> >> being check if it is valid, which may lead to NULL pointer dereference.
> >> So move the assignment to global_width after checking that the s is valid
> >>
> >> Reported-by: Euler Robot <euler.robot@huawei.com>
> >> Signed-off-by: Alex Chen <alex.chen@huawei.com>
> >> ---
> >>  hw/display/exynos4210_fimd.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/hw/display/exynos4210_fimd.c b/hw/display/exynos4210_fimd.c
> >> index 4c16e1f5a0..a1179d2f89 100644
> >> --- a/hw/display/exynos4210_fimd.c
> >> +++ b/hw/display/exynos4210_fimd.c
> >> @@ -1275,12 +1275,12 @@ static void exynos4210_fimd_update(void *opaque)
> >>      bool blend = false;
> >>      uint8_t *host_fb_addr;
> >>      bool is_dirty = false;
> >> -    const int global_width = (s->vidtcon[2] & FIMD_VIDTCON2_SIZE_MASK) + 
> >> 1;
> >>
> >>      if (!s || !s->console || !s->enabled ||
> >>          surface_bits_per_pixel(qemu_console_surface(s->console)) == 0) {
> >>          return;
> >>      }
> >> +    const int global_width = (s->vidtcon[2] & FIMD_VIDTCON2_SIZE_MASK) + 
> >> 1;
> >>      exynos4210_update_resolution(s);
> >>      surface = qemu_console_surface(s->console);
> >
> > Yep, this is a bug, but QEMU's coding style doesn't permit
> > variable declarations in the middle of functions. You need
> > to split the declaration (which stays where it is) and the
> > initialization (which can moved down below the !s test.
> >
> Thans for your review. I have also considered this modification to be 
> incompatible with
> the QEMU's coding style. But the type of global_width is const int which 
> cannot be
> assigned once they are defined.
> Could we define the global_width as int, such as this modification:
>
> diff --git a/hw/display/exynos4210_fimd.c b/hw/display/exynos4210_fimd.c
> index 4c16e1f5a0..34a960a976 100644
> --- a/hw/display/exynos4210_fimd.c
> +++ b/hw/display/exynos4210_fimd.c
> @@ -1275,12 +1275,14 @@ static void exynos4210_fimd_update(void *opaque)
>      bool blend = false;
>      uint8_t *host_fb_addr;
>      bool is_dirty = false;
> -    const int global_width = (s->vidtcon[2] & FIMD_VIDTCON2_SIZE_MASK) + 1;
> +    int global_width;
>
>      if (!s || !s->console || !s->enabled ||
>          surface_bits_per_pixel(qemu_console_surface(s->console)) == 0) {
>          return;
>      }
> +
> +    global_width = (s->vidtcon[2] & FIMD_VIDTCON2_SIZE_MASK) + 1;
>      exynos4210_update_resolution(s);
>      surface = qemu_console_surface(s->console);

Yes, I think that would be fine.

thanks
-- PMM