untrusted comment: verify with openbsd-66-base.pub
RWSvK/c+cFe24KyiCX3cI1VcaqktFGIEyrhryDBN4QvPPvY0hsMOAUX5ImldKbNP+eC7TYKZbxxr/GxpXPZb7LEMisQ07f3yEAE=

OpenBSD 6.6 errata 042, September 5, 2020:

Buffer overflow in amdgpu ioctl.

Apply by doing:
   signify -Vep /etc/signify/openbsd-66-base.pub -x 042_amdgpu.patch.sig \
       -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
   KK=`sysctl -n kern.osversion | cut -d# -f1`
   cd /usr/src/sys/arch/`machine`/compile/$KK
   make obj
   make config
   make
   make install

Index: sys/dev/pci/drm/amd/amdgpu/amdgpu_kms.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/drm/amd/amdgpu/amdgpu_kms.c,v
retrieving revision 1.5
diff -u -p -r1.5 amdgpu_kms.c
--- sys/dev/pci/drm/amd/amdgpu/amdgpu_kms.c     13 Aug 2019 22:02:28 -0000      1.5
+++ sys/dev/pci/drm/amd/amdgpu/amdgpu_kms.c     3 Sep 2020 13:51:19 -0000
@@ -609,8 +609,15 @@ static int amdgpu_info_ioctl(struct drm_
                * in the bitfields */
               if (se_num == AMDGPU_INFO_MMR_SE_INDEX_MASK)
                       se_num = 0xffffffff;
+               else if (se_num >= AMDGPU_GFX_MAX_SE)
+                       return -EINVAL;
               if (sh_num == AMDGPU_INFO_MMR_SH_INDEX_MASK)
                       sh_num = 0xffffffff;
+               else if (sh_num >= AMDGPU_GFX_MAX_SH_PER_SE)
+                       return -EINVAL;
+
+               if (info->read_mmr_reg.count > 128)
+                       return -EINVAL;

               regs = kmalloc_array(info->read_mmr_reg.count, sizeof(*regs), GFP_KERNEL);
               if (!regs)