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

[Dmitry Baryshkov]

Hi Abhinav,

On Thu, 23 Feb 2023 at 21:17, Abhinav Kumar <quic_abhinavk at quicinc.com> wrote:
>
> 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?

This change protects all the 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.

Because for the RGB0 / rect-2 the cfg->idx will also be -1 (and
shift/ext_shift will be 0).


-- 
With best wishes
Dmitry