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

[AlexChen <alex.chen@huawei.com>]

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);