Hi Kees,
On Sat, 25 Nov 2017 12:57:04 -0800
Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Nov 22, 2017 at 11:57 PM, Daniel Vetter <daniel@xxxxxxxx> wrote:
> > On Wed, Nov 22, 2017 at 07:21:00PM +0100, Boris Brezillon wrote:
> >> On Wed, 22 Nov 2017 19:13:09 +0100
> >> Daniel Vetter <daniel@xxxxxxxx> wrote:
> >>
> >> > On Wed, Nov 22, 2017 at 6:51 PM, Boris Brezillon
> >> > <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> >> > > Hi Stefan,
> >> > >
> >> > > On Wed, 22 Nov 2017 17:43:35 +0100 (CET)
> >> > > Stefan Wahren <stefan.wahren@xxxxxxxx> wrote:
> >> > >
> >> > >> Hi Boris,
> >> > >> if i boot Raspberry Pi 3 (ARM64, defconfig, linux-next-20171122) with sufficient CMA memory (32 MB), i'll get this warning during boot:
> >> > >>
> >> > >> [ 7.623510] vc4-drm soc:gpu: bound 3f902000.hdmi (ops vc4_hdmi_ops [vc4])
> >> > >> [ 7.632453] vc4-drm soc:gpu: bound 3f806000.vec (ops vc4_vec_ops [vc4])
> >> > >> [ 7.639707] vc4-drm soc:gpu: bound 3f400000.hvs (ops vc4_hvs_ops [vc4])
> >> > >> [ 7.647364] vc4-drm soc:gpu: bound 3f206000.pixelvalve (ops vc4_crtc_ops [vc4])
> >> > >> [ 7.655451] vc4-drm soc:gpu: bound 3f207000.pixelvalve (ops vc4_crtc_ops [vc4])
> >> > >> [ 7.663415] vc4-drm soc:gpu: bound 3f807000.pixelvalve (ops vc4_crtc_ops [vc4])
> >> > >> [ 7.730580] vc4-drm soc:gpu: bound 3fc00000.v3d (ops vc4_v3d_ops [vc4])
> >> > >> [ 7.743965] [drm] Initialized vc4 0.0.0 20140616 for soc:gpu on minor 0
> >> > >> [ 7.750841] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
> >> > >> [ 7.757620] [drm] Driver supports precise vblank timestamp query.
> >> > >> [ 7.811775] ------------[ cut here ]------------
> >> > >> [ 7.811780] refcount_t: increment on 0; use-after-free.
> >> > >> [ 7.811881] WARNING: CPU: 2 PID: 2188 at lib/refcount.c:153 refcount_inc+0x44/0x50
> >> > >> [ 7.811884] Modules linked in: vc4(+) cfg80211 cec drm_kms_helper smsc95xx usbnet drm rfkill brcmutil bcm2835_rng rng_core bcm2835_dma crc32_ce i2c_bcm2835 pwm_bcm2835 ip_tables x_tables ipv6
> >> > >> [ 7.811950] CPU: 2 PID: 2188 Comm: systemd-udevd Not tainted 4.14.0-next-20171122 #1
> >> > >> [ 7.811953] Hardware name: Raspberry Pi 3 Model B Rev 1.2 (DT)
> >> > >> [ 7.811958] task: ffff800036b91c00 task.stack: ffff00000d6f0000
> >> > >> [ 7.811967] pstate: 20000005 (nzCv daif -PAN -UAO)
> >> > >> [ 7.811974] pc : refcount_inc+0x44/0x50
> >> > >> [ 7.811981] lr : refcount_inc+0x44/0x50
> >> > >> [ 7.811984] sp : ffff00000d6f3300
> >> > >> [ 7.811988] x29: ffff00000d6f3300 x28: 0000000000000000
> >> > >> [ 7.811996] x27: 0000000000000003 x26: ffff800037107800
> >> > >> [ 7.812004] x25: 0000000000000001 x24: ffff800035afdc00
> >> > >> [ 7.812012] x23: 0000000000000000 x22: ffff800035dfa600
> >> > >> [ 7.812020] x21: ffff800035afd9b0 x20: ffff800035afd9a4
> >> > >> [ 7.812027] x19: 0000000000000000 x18: 0000000000000000
> >> > >> [ 7.812034] x17: 0000000000000001 x16: 0000000000000019
> >> > >> [ 7.812042] x15: 0000000000000001 x14: 00000000fffffff0
> >> > >> [ 7.812049] x13: ffff0000090ae840 x12: ffff000008fa2d50
> >> > >> [ 7.812057] x11: ffff000008fa2000 x10: ffff0000090ad000
> >> > >> [ 7.812064] x9 : 0000000000000000 x8 : ffff0000090b5c2f
> >> > >> [ 7.812072] x7 : 0000000000000000 x6 : 000000000015ee00
> >> > >> [ 7.812079] x5 : 0000000000000000 x4 : 0000000000000000
> >> > >> [ 7.812087] x3 : ffffffffffffffff x2 : 0000800030047000
> >> > >> [ 7.812094] x1 : ffff800036b91c00 x0 : 000000000000002b
> >> > >> [ 7.812102] Call trace:
> >> > >> [ 7.812109] refcount_inc+0x44/0x50
> >> > >> [ 7.812226] vc4_bo_inc_usecnt+0x84/0x88 [vc4]
> >> > >> [ 7.812310] vc4_prepare_fb+0x40/0xf0 [vc4]
> >> > >> [ 7.812460] drm_atomic_helper_prepare_planes+0x54/0xf0 [drm_kms_helper]
> >> > >> [ 7.812543] vc4_atomic_commit+0x88/0x130 [vc4]
> >> > >> [ 7.812868] drm_atomic_commit+0x48/0x68 [drm]
> >> > >> [ 7.813002] restore_fbdev_mode_atomic+0x1d8/0x1e8 [drm_kms_helper]
> >> > >> [ 7.813113] restore_fbdev_mode+0x28/0x160 [drm_kms_helper]
> >> > >> [ 7.813223] drm_fb_helper_restore_fbdev_mode_unlocked.part.24+0x28/0x90 [drm_kms_helper]
> >> > >> [ 7.813331] drm_fb_helper_set_par+0x54/0xa8 [drm_kms_helper]
> >> > >> [ 7.813346] fbcon_init+0x4e8/0x538
> >> > >> [ 7.813357] visual_init+0xb4/0x108
> >> > >> [ 7.813366] do_bind_con_driver+0x1b8/0x3a0
> >> > >> [ 7.813373] do_take_over_console+0x150/0x1d0
> >> > >> [ 7.813380] do_fbcon_takeover+0x6c/0xf0
> >> > >> [ 7.813387] fbcon_event_notify+0x8fc/0x928
> >> > >> [ 7.813399] notifier_call_chain+0x50/0x90
> >> > >> [ 7.813406] __blocking_notifier_call_chain+0x4c/0x90
> >> > >> [ 7.813413] blocking_notifier_call_chain+0x14/0x20
> >> > >> [ 7.813420] fb_notifier_call_chain+0x1c/0x28
> >> > >> [ 7.813426] register_framebuffer+0x1d0/0x2d8
> >> > >> [ 7.813533] __drm_fb_helper_initial_config_and_unlock+0x1e8/0x350 [drm_kms_helper]
> >> > >> [ 7.813639] drm_fb_helper_initial_config+0x40/0x58 [drm_kms_helper]
> >> > >> [ 7.813747] drm_fbdev_cma_init_with_funcs+0x88/0x158 [drm_kms_helper]
> >> > >> [ 7.813855] drm_fbdev_cma_init+0x14/0x28 [drm_kms_helper]
> >> > >> [ 7.813943] vc4_kms_load+0xa4/0xf0 [vc4]
> >> > >> [ 7.814026] vc4_drm_bind+0x100/0x168 [vc4]
> >> > >> [ 7.814038] try_to_bring_up_master+0x144/0x1a8
> >> > >> [ 7.814044] component_master_add_with_match+0x9c/0xe0
> >> > >> [ 7.814128] vc4_platform_drm_probe+0xb4/0xf0 [vc4]
> >> > >> [ 7.814137] platform_drv_probe+0x58/0xc0
> >> > >> [ 7.814146] driver_probe_device+0x224/0x308
> >> > >> [ 7.814153] __driver_attach+0xac/0xb0
> >> > >> [ 7.814161] bus_for_each_dev+0x64/0xa0
> >> > >> [ 7.814169] driver_attach+0x20/0x28
> >> > >> [ 7.814177] bus_add_driver+0x108/0x228
> >> > >> [ 7.814184] driver_register+0x60/0xf8
> >> > >> [ 7.814190] __platform_driver_register+0x40/0x48
> >> > >> [ 7.814274] vc4_drm_register+0x38/0x1000 [vc4]
> >> > >> [ 7.814283] do_one_initcall+0x38/0x120
> >> > >> [ 7.814295] do_init_module+0x58/0x1b8
> >> > >> [ 7.814304] load_module+0x1fa8/0x2280
> >> > >> [ 7.814311] SyS_finit_module+0xc0/0xd0
> >> > >> [ 7.814318] __sys_trace_return+0x0/0x4
> >> > >> [ 7.814325] ---[ end trace 3348554eb91e19a1 ]---
> >> > >
> >> > > Looks like I didn't test this code with CONFIG_REFCOUNT_FULL enabled :-/.
> >> > >
> >> > > Anyway, can you try to apply the following diff and let me know if it
> >> > > fixes the problem?
> >> > >
> >> > > Thanks,
> >> > >
> >> > > Boris
> >> > >
> >> > > --->8---
> >> > > diff --git a/drivers/gpu/drm/vc4/vc4_bo.c b/drivers/gpu/drm/vc4/vc4_bo.c
> >> > > index 4ae45d7dac42..2decc8e2c79f 100644
> >> > > --- a/drivers/gpu/drm/vc4/vc4_bo.c
> >> > > +++ b/drivers/gpu/drm/vc4/vc4_bo.c
> >> > > @@ -637,7 +637,8 @@ int vc4_bo_inc_usecnt(struct vc4_bo *bo)
> >> > > mutex_lock(&bo->madv_lock);
> >> > > switch (bo->madv) {
> >> > > case VC4_MADV_WILLNEED:
> >> > > - refcount_inc(&bo->usecnt);
> >> > > + if (!refcount_inc_not_zero(&bo->usecnt))
> >> > > + refcount_set(&bo->usecnt, 1);
> >> >
> >> > This is racy.
> >>
> >> Well, in this case it's not, because this section is protected by a
> >> mutex.
> >>
> >> > You need a refcount_inc_allow_zero (if that exists).
> >>
> >> I searched for something like that, but it seems that there's no such
> >> helper.
> >
> > In that case I think the right thing is to switch to atomic_t. That means
> > no overflow protection, but if we don't have a "this can be 0, I know what
> > we're doing" style refcount, then I don't think it's suitable.
> >
> > Adding Kees to confirm or clarify or ...
>
> Adding Elena for more details. IIRC there have been two cases:
>
> 1) refcount == 0 doesn't mean the object has been released. The
> solution tends to be a +1 to the count everywhere.
>
> 2) refcount == 0 means the object was released, but the refcount gets
> reused for the next allocation. The solution tends to be to use
> refcount_set() in the new initialization instead of refcount_inc().
>
> It's not clear to me if either apply here, though.
I'd say #1, since ->usecnt is not controlling the VC4-BO object
lifetime. This being said, I'm not sure the +1 trick is really
appropriate. Indeed, we're using the
refcount_inc_not_zero()/refcount_dec_not_one() helpers to remove/put
the object from/in a
"mem-pointed-by-obj-can-be-freed-under-mem-pressure" pool (note the
object itself is not freed, just the memory portion that is pointed by
this object). If I apply +1 everywhere then I have no way to handle the
0 -> 1 and 1 -> 0 transitions differently (these become 1 -> 2 and 2 ->
1 transitions, and there's no functions called refcount_dec_not_two()
and refcount_inc_not_one()).
Anyway, if refcount_inc_allow_zero() is not option, maybe I should just
use an atomic_t object (as suggested by Daniel) or a plain unsigned
integer protected by the ->madv.lock (as suggested by Eric). Of course,
that means losing built-in counter-overflow checking and reworking a bit
the VC4-MADV logic, but that's not such a big problem.
Regards,
Boris
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