Re: [PATCH v2] drm/amdgpu: Avoid possible buffer overflow

From: Su Hui
Date: Mon Aug 21 2023 - 21:39:47 EST


On 2023/8/21 17:31, Christian König wrote:
Am 21.08.23 um 09:37 schrieb Su Hui:
smatch error:
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1257 amdgpu_discovery_reg_base_init() error:
testing array offset 'adev->vcn.num_vcn_inst' after use.

change the assignment order to avoid buffer overflow.

Fixes: c40bdfb2ffa4 ("drm/amdgpu: fix incorrect VCN revision in SRIOV")
Signed-off-by: Su Hui <suhui@xxxxxxxxxxxx>
---
changes in v2:
  - fix the error about ip->revision (thanks to Christophe JAILLET).
  drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c
index 8e1cfc87122d..b07bfd106a9b 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c
@@ -1250,11 +1250,10 @@ static int amdgpu_discovery_reg_base_init(struct amdgpu_device *adev)
                   *     0b10 : encode is disabled
                   *     0b01 : decode is disabled
                   */
- adev->vcn.vcn_config[adev->vcn.num_vcn_inst] =
-                    ip->revision & 0xc0;
-                ip->revision &= ~0xc0;
                  if (adev->vcn.num_vcn_inst <
                      AMDGPU_MAX_VCN_INSTANCES) {
+ adev->vcn.vcn_config[adev->vcn.num_vcn_inst] =
+                        ip->revision & 0xc0;
                      adev->vcn.num_vcn_inst++;
                      adev->vcn.inst_mask |=
                          (1U << ip->instance_number);
@@ -1265,6 +1264,7 @@ static int amdgpu_discovery_reg_base_init(struct amdgpu_device *adev)
                          adev->vcn.num_vcn_inst + 1,
                          AMDGPU_MAX_VCN_INSTANCES);
                  }
+                ip->revision &= ~0xc0;

That doesn't looks correct either. The assignment is intentionally outside of the "if".

See "adev->vcn.vcn_config[adev->vcn.num_vcn_inst] = ip->revision & 0xc0;" is always valid.

Hi,

if "adev->vcn.vcn_config[adev->vcn.num_vcn_inst] = ip->revision & 0xc0;" is always valid, then

"adev->vcn.num_vcn_inst< AMDGPU_MAX_VCN_INSTANCES " is always true. So the below judgement has

no sense.

                  if (adev->vcn.num_vcn_inst <
                      AMDGPU_MAX_VCN_INSTANCES) {

On the contrary, if we need this judgement, then "adev->vcn.vcn_config[adev->vcn.num_vcn_inst] = ip->revision & 0xc0;"is not

always valid, because "adev->vcn.num_vcn_inst >= AMDGPU_MAX_VCN_INSTANCES" can be true, which cause buffer overflow.

So I think this patch has some sense if I don't make some mistakes.

Su Hui


We just avoid incrementing num_vcn_inst when we already have to many.

Regards,
Christian.


              }
              if (le16_to_cpu(ip->hw_id) == SDMA0_HWID ||
                  le16_to_cpu(ip->hw_id) == SDMA1_HWID ||