[PATCH] drm/msm/dpu: fix stack smashing in dpu_hw_ctl_setup_blendstage

[Abhinav Kumar]

Hi Dmitry

On 2/23/2023 1:57 AM, Dmitry Baryshkov wrote:
> The rewritten dpu_hw_ctl_setup_blendstage() can lightly smash the stack
> when setting the SSPP_NONE pipe. However it was unnoticed until the
> kernel was tested under AOSP (with some kind of stack protection/check).
> 
> This fixes the following backtrace:
> 
> Unexpected kernel BRK exception at EL1
> Internal error: BRK handler: 00000000f20003e8 [#1] PREEMPT SMP
> Hardware name: Thundercomm Dragonboard 845c (DT)
> pstate: a0400005 (NzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : dpu_hw_ctl_setup_blendstage+0x26c/0x278 [msm]
> lr : _dpu_crtc_blend_setup+0x4b4/0x5a0 [msm]
> sp : ffffffc00bdcb720
> x29: ffffffc00bdcb720 x28: ffffff8085debac0 x27: 0000000000000002
> x26: ffffffd74af18320 x25: ffffff8083af75a0 x24: ffffffc00bdcb878
> x23: 0000000000000001 x22: 0000000000000000 x21: ffffff8085a70000
> x20: ffffff8083012dc0 x19: 0000000000000001 x18: 0000000000000000
> x17: 000000040044ffff x16: 045000f4b5593519 x15: 0000000000000000
> x14: 000000000000000b x13: 0000000000000001 x12: 0000000000000000
> x11: 0000000000000001 x10: ffffffc00bdcb764 x9 : ffffffd74af06a08
> x8 : 0000000000000001 x7 : 0000000000000001 x6 : 0000000000000000
> x5 : ffffffc00bdcb878 x4 : 0000000000000002 x3 : ffffffffffffffff
> x2 : ffffffc00bdcb878 x1 : 0000000000000000 x0 : 0000000000000002
> Call trace:
>   dpu_hw_ctl_setup_blendstage+0x26c/0x278 [msm]
>   _dpu_crtc_blend_setup+0x4b4/0x5a0 [msm]
>   dpu_crtc_atomic_begin+0xd8/0x22c [msm]
>   drm_atomic_helper_commit_planes+0x80/0x208 [drm_kms_helper]
>   msm_atomic_commit_tail+0x134/0x6f0 [msm]
>   commit_tail+0xa4/0x1a4 [drm_kms_helper]
>   drm_atomic_helper_commit+0x170/0x184 [drm_kms_helper]
>   drm_atomic_commit+0xac/0xe8
>   drm_mode_atomic_ioctl+0xbf0/0xdac
>   drm_ioctl_kernel+0xc4/0x178
>   drm_ioctl+0x2c8/0x608
>   __arm64_sys_ioctl+0xa8/0xec
>   invoke_syscall+0x44/0x104
>   el0_svc_common.constprop.0+0x44/0xec
>   do_el0_svc+0x38/0x98
>   el0_svc+0x2c/0xb4
>   el0t_64_sync_handler+0xb8/0xbc
>   el0t_64_sync+0x1a0/0x1a4
> Code: 52800016 52800017 52800018 17ffffc7 (d4207d00)
> 
> Fixes: 4488f71f6373 ("drm/msm/dpu: simplify blend configuration")
> Reported-by: Amit Pundir <amit.pundir at linaro.org>
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> ---
>   drivers/gpu/drm/msm/disp/dpu1/dpu_hw_ctl.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_ctl.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_ctl.c
> index b88a2f3724e6..6c53ea560ffa 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_ctl.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_ctl.c
> @@ -446,7 +446,9 @@ static void dpu_hw_ctl_setup_blendstage(struct dpu_hw_ctl *ctx,
>   			 * CTL_LAYER has 3-bit field (and extra bits in EXT register),
>   			 * all EXT registers has 4-bit fields.
>   			 */
> -			if (cfg->idx == 0) {
> +			if (cfg->idx == -1) {
> +				continue;
> +			} else if (cfg->idx == 0) {
>   				mixercfg[0] |= mix << cfg->shift;
>   				mixercfg[1] |= ext << cfg->ext_shift;
>   			} else {

Since I had not reviewed the change which introduced this, had a question.

The issue here is because the shift and ext_shift are -1 for NONE and 
hence the shift causes overflow?

If that was the issue shouldnt we protect all such cases?

So lets say we use SSPP_RGB0, the multirect_index for it will always be 
-1 as it doesnt support smartDMA. What prevents the same issue from 
hitting in that case? Because you are only checking for idx and not the 
shifts.