Re: [PATCH v3] drm/fbdev-generic: prohibit potential out-of-bounds access
From: Daniel Vetter
Date: Wed Apr 19 2023 - 12:31:16 EST
On Thu, Apr 20, 2023 at 12:00:41AM +0800, Sui Jingfeng wrote:
> Hi,
>
> Sorry about reply to you so late,
>
> our downstream (product kernel side) userspace GPU/DC driver
>
> has been tested out a few bugs, I'm asking to fulfill my duty to that part
> all days.
>
> I may slow to reply, but I really love to reply.
>
>
> On 2023/4/19 23:09, Daniel Vetter wrote:
> > On Tue, 18 Apr 2023 at 20:16, Sui Jingfeng <15330273260@xxxxxx> wrote:
> > > Hi,
> > >
> > > On 2023/4/19 01:52, Sui Jingfeng wrote:
> > > > Hi,
> > > >
> > > > On 2023/4/18 16:32, Daniel Vetter wrote:
> > > > > On Mon, Apr 17, 2023 at 07:32:19PM +0800, Sui Jingfeng wrote:
> > > > > > The fbdev test of IGT may write after EOF, which lead to out-of-bound
> > > > > > access for the drm drivers using fbdev-generic. For example, on a x86
> > > > > > + aspeed bmc card platform, with a 1680x1050 resolution display,
> > > > > > running
> > > > > > fbdev test if IGT will cause the linux kernel hang with the following
> > > > > > call trace:
> > > > > >
> > > > > > Oops: 0000 [#1] PREEMPT SMP PTI
> > > > > > [IGT] fbdev: starting subtest eof
> > > > > > Workqueue: events drm_fb_helper_damage_work [drm_kms_helper]
> > > > > > [IGT] fbdev: starting subtest nullptr
> > > > > >
> > > > > > RIP: 0010:memcpy_erms+0xa/0x20
> > > > > > RSP: 0018:ffffa17d40167d98 EFLAGS: 00010246
> > > > > > RAX: ffffa17d4eb7fa80 RBX: ffffa17d40e0aa80 RCX: 00000000000014c0
> > > > > > RDX: 0000000000001a40 RSI: ffffa17d40e0b000 RDI: ffffa17d4eb80000
> > > > > > RBP: ffffa17d40167e20 R08: 0000000000000000 R09: ffff89522ecff8c0
> > > > > > R10: ffffa17d4e4c5000 R11: 0000000000000000 R12: ffffa17d4eb7fa80
> > > > > > R13: 0000000000001a40 R14: 000000000000041a R15: ffffa17d40167e30
> > > > > > FS: 0000000000000000(0000) GS:ffff895257380000(0000)
> > > > > > knlGS:0000000000000000
> > > > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > > CR2: ffffa17d40e0b000 CR3: 00000001eaeca006 CR4: 00000000001706e0
> > > > > > Call Trace:
> > > > > > <TASK>
> > > > > > ? drm_fbdev_generic_helper_fb_dirty+0x207/0x330 [drm_kms_helper]
> > > > > > drm_fb_helper_damage_work+0x8f/0x170 [drm_kms_helper]
> > > > > > process_one_work+0x21f/0x430
> > > > > > worker_thread+0x4e/0x3c0
> > > > > > ? __pfx_worker_thread+0x10/0x10
> > > > > > kthread+0xf4/0x120
> > > > > > ? __pfx_kthread+0x10/0x10
> > > > > > ret_from_fork+0x2c/0x50
> > > > > > </TASK>
> > > > > > CR2: ffffa17d40e0b000
> > > > > > ---[ end trace 0000000000000000 ]---
> > > > > >
> > > > > > The direct reason is that damage rectange computed by
> > > > > > drm_fb_helper_memory_range_to_clip() does not guaranteed to be
> > > > > > in-bound.
> > > > > > It is already results in workaround code populate to elsewhere. Another
> > > > > > reason is that exposing a larger buffer size than the actual needed
> > > > > > help
> > > > > > to trigger this bug intrinsic in drm_fb_helper_memory_range_to_clip().
> > > > > >
> > > > > > Others fbdev emulation solutions write to the GEM buffer directly, they
> > > > > > won't reproduce this bug because the .fb_dirty function callback do not
> > > > > > being hooked, so no chance is given to
> > > > > > drm_fb_helper_memory_range_to_clip()
> > > > > > to generate a out-of-bound when drm_fb_helper_sys_write() is called.
> > > > > >
> > > > > > This patch break the trigger condition of this bug by shrinking the
> > > > > > shadow
> > > > > > buffer size to sizes->surface_height * buffer->fb->pitches[0].
> > > > > >
> > > > > > Fixes: '8fbc9af55de0 ("drm/fbdev-generic: Set screen size to size of
> > > > > > GEM
> > > > > > buffer")'
> > > > > >
> > > > > > Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> > > > > > ---
> > > > > > drivers/gpu/drm/drm_fbdev_generic.c | 2 +-
> > > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_fbdev_generic.c
> > > > > > b/drivers/gpu/drm/drm_fbdev_generic.c
> > > > > > index 8e5148bf40bb..b057cfbba938 100644
> > > > > > --- a/drivers/gpu/drm/drm_fbdev_generic.c
> > > > > > +++ b/drivers/gpu/drm/drm_fbdev_generic.c
> > > > > > @@ -94,7 +94,7 @@ static int
> > > > > > drm_fbdev_generic_helper_fb_probe(struct drm_fb_helper *fb_helper,
> > > > > > fb_helper->buffer = buffer;
> > > > > > fb_helper->fb = buffer->fb;
> > > > > > - screen_size = buffer->gem->size;
> > > > > > + screen_size = sizes->surface_height * buffer->fb->pitches[0];
> > > > > So I read core some more and stumbled over drm_fb_helper_deferred_io().
> > > > > Which has all the code and comments about this, including limiting.
> > > > >
> > > > > I think it would be clearer if we fix the issue there, instead of
> > > > > passing
> > > > > limits around in obscure places that then again get broken?
> > > > No, it is more obscure doing that way...
> > > >
> > > >
> > > > As the size of the shadow screen buffer will be exposed to userspace.
> > > >
> > > > The size 'helper->fb->height * helper->fb->pitches[0]' is a
> > > > exactly(best) fit,
> > > >
> > > > You are guaranteed to waste at lease one byte by increasing one byte,
> > > >
> > > > and can not store all pixels by decreasing one byte (In the case where
> > > > `helper->fb->pitches[0] = helper->fb->width * 4`).
> > > >
> > > > It implicitly tell the userspace do not go beyond that boundary.
> > > >
> > > > although userspace program can still choose to write after EOF,
> > > >
> > > > But it is for test purpose, to test the kernel if it can return a
> > > > -EFBIG or not.
> > > >
> > > > > The thing is,
> > > > > Thomas both authored the limit checks in drm_fb_helper_deferred_io() and
> > > > > the patch which broken them again, so clearly this isn't very
> > > > > obvious. I'm
> > > > > thinking of something like this:
> > > > >
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_fb_helper.c
> > > > > b/drivers/gpu/drm/drm_fb_helper.c
> > > > > index ef4eb8b12766..726dab67c359 100644
> > > > > --- a/drivers/gpu/drm/drm_fb_helper.c
> > > > > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > > > > @@ -697,10 +697,7 @@ void drm_fb_helper_deferred_io(struct fb_info
> > > > > *info, struct list_head *pagerefli
> > > > > * of the screen and account for non-existing scanlines. Hence,
> > > > > * keep the covered memory area within the screen buffer.
> > > > > */
> > > > > - if (info->screen_size)
> > > > > - total_size = info->screen_size;
> > > > > - else
> > > > > - total_size = info->fix.smem_len;
> > > > > + total_size = helper->fb->height * helper->fb->pitches[0];
> > > > This is just to mitigate the mistakes already has been made,
> > > >
> > > > because it do not do a good splitting between the *clip* part and the
> > > > *damage update* part.
> > > >
> > > > An ideal clipping do not obscure its updating backend with a
> > > > out-of-bound damage rectangle.
> > > >
> > > > Why did the drm_fb_helper_memory_range_to_clip() can not do a good job
> > > > in all case
> > > >
> > > > to pass its backend a always meaningful damage rect ?
> > > >
> > > > > max_off = min(max_off, total_size);
> > > > > if (min_off < max_off) {
> > > > >
> > > > >
> > > > > I think that would make it utmost clear on what we're doing and why.
> > > > > Otherwise we're just going to re-create the same bug again, like we've
> > > > > done already :-)
> > > > No, we create no bugs, we fix one.
> > > >
> > > > Thanks.
> > > >
> > > But honestly I do not have strong feel toward this, I just type what I'm
> > > understand without seeing you resend a V3.
> > >
> > > It's OK in overall, I will help to test this tomorrow. :-)
> > Apologies for making you jump around all the time and doing different
> > versions of the same bugfix :-/
>
> No, I do not mind. I'm wondering if you are testing me.
Nah I'm really not any clearer on this than you :-/
> > I think this one here is ok to merge,
>
> NO, to be honest, this version is not ok.
>
> I have just tested it on LoongArch . It does not prevent out-of-bound on
> LoongArch.
>
> bellow is the call trace when running the fbdev test of IGT.
>
>
> [ 369.628841] Console: switching to colour dummy device 80x25
> [ 369.634440] [IGT] fbdev: executing
> [ 369.654684] [IGT] fbdev: starting subtest info
> [ 369.659173] [IGT] fbdev: starting subtest pan
> [ 369.722093] [IGT] fbdev: starting subtest read
> [ 369.737795] [IGT] fbdev: starting subtest unaligned-read
> [ 369.745695] [IGT] fbdev: starting subtest write
> [ 369.753154] CPU 3 Unable to handle kernel paging request at virtual address ffff800034bd0000, era == 9000000000223d5c, ra == ffff8000020d75a8
> [ 369.774570] [IGT] fbdev: starting subtest unaligned-write
> [ 369.779960] Oops[#1]:
> [ 369.782215] CPU: 3 PID: 504 Comm: kworker/3:3 Not tainted 6.3.0-rc5+ #377
> [ 369.782219] Hardware name: Loongson Loongson-3A5000-HV-7A2000-1w-V0.1-EVB/Loongson-LS3A5000-7A2000-1w-EVB-V1.21, BIOS Loongson-UDK2018-V4.0.05383-beta10 1
> [ 369.782223] Workqueue: events drm_fb_helper_damage_work [drm_kms_helper]
> [ 369.802706] $ 0 : 0000000000000000 ffff8000020d75a8 90000001367fc000 90000001367ffcf0
> [ 369.809368] $ 4 : ffff80003a3ec000 ffff800034bcee00 0000000000001e00 9000000001160000
> [ 369.825275] $ 8 : ffff80003a3ebfff 0000000000000040 0000000000000000 ffff80000214f20c
> [ 369.825280] $12 : ffff80003a3ecc00 ffff800034bd0000 0000000000000000 0000000000001e00
> [ 369.841188] $16 : 9000000107490800 9000000001a24000 9000000001a28000 00000e00207ec000
> [ 369.849152] $20 : fffffffffe000000 900000010684cec0 0000000000000439 90000001367ffd90
> [ 369.849165] $24 : 9000000106bc5200 ffff8000020de000 ffff800034bcee00 0000000000001e00
> [ 369.865072] $28 : 90000001076d4400 0000000000000001 900000010553c000 ffff80003a3eae00
> [ 369.865078] era : 9000000000223d5c __memcpy_toio+0x4c/0x90
> [ 369.878651] ra : ffff8000020d75a8 drm_fbdev_generic_helper_fb_dirty+0x1cc/0x300 [drm_kms_helper]
> [ 369.878762] [IGT] fbdev: starting subtest eof
> [ 369.887679] CSR crmd: 000000b0
> [ 369.887680] CSR prmd: 00000004
> [ 369.887682] CSR euen: 00000000
> [ 369.887683] CSR ecfg: 00071c1c
> [ 369.901340] CSR estat: 00010000
> [ 369.901342] ExcCode : 1 (SubCode 0)
> [ 369.901344] BadVA : ffff800034bd0000
> [ 369.914659] PrId : 0014c011 (Loongson-64bit)
> [ 369.914660] Modules linked in: uas usb_storage snd_seq_dummy snd_seq snd_seq_device ip_set rfkill nf_tables nfnetlink vfat fat loongson ttm acpi_ipmi drm_kms_helper syscopyarea sysfillrect ipmi_si ipmi_devintf sysimgblt ipmi_msghandler fuse efivarfs
> [ 369.919013] Process kworker/3:3 (pid: 504, threadinfo=00000000a1234af0, task=000000004e2cde6f)
> [ 369.949519] Stack : 9000000106bc5318 900000010684cec0 9000000107afd470 ffff800039c00000
> [ 369.949535] [IGT] fbdev: starting subtest nullptr
> [ 369.957476] 0000000000000001 0000000000000000 0000000000000000 76e0ff420f8eaeab
> [ 369.957481] 9000000001712000 fffffffffffffffb 9000000100161080 0000000000000003
> [ 369.970105] 900000000980ba05 900000000980ba00 9000000106bc5200 9000000106bc52d4
> [ 369.978062] ffff8000020de000 9000000106bc52d8 9000000110664b40 ffff8000020d78e4
> [ 369.986018] 043a078000000000 76e0ff420f8eaeab 900000000980be00 9000000009807400
> [ 370.001926] 0000000000000000 9000000110664b40 9000000106bc52d8 9000000000256dbc
> [ 370.001931] 900000000170e000 9000000107afce00 0000000000000001 900000000170e000
> [ 370.009888] 9000000009807428 9000000110664b70 9000000009807400 900000000025737c
> [ 370.025797] 9000000100161080 9000000001148080 9000000107afce00 0000000000000003
> [ 370.026543] [IGT] fbdev: exiting, ret=0
> [ 370.033753] ...
> [ 370.033756] Call Trace:
> [ 370.033757] [<9000000000223d5c>] __memcpy_toio+0x4c/0x90
> [ 370.047681] [<ffff8000020d75a8>] drm_fbdev_generic_helper_fb_dirty+0x1cc/0x300 [drm_kms_helper]
> [ 370.056354] [<ffff8000020d78e4>] drm_fb_helper_damage_work+0xa4/0x1d0 [drm_kms_helper]
> [ 370.056381] [<9000000000256dbc>] process_one_work+0x1ec/0x35c
> [ 370.056385] [<900000000025737c>] worker_thread+0x88/0x428
> [ 370.056387] [<900000000025f4bc>] kthread+0x114/0x120
> [ 370.056392] [<90000000002215a8>] ret_from_kernel_thread+0xc/0xa4
> [ 370.056395]
> [ 370.056396] Code: 00410def 0010bc8c 001500ad <260001ae> 02c02084 02c021ad 29ffe08e 5ffff184 03401cc6
> [ 370.056406]
> [ 370.056421] fbcon_init: detected unhandled fb_set_par error, error code -16
> [ 370.056482] ---[ end trace 0000000000000000 ]---
> [ 370.066620] Console: switching to colour frame buffer device 240x67
>
>
> > I just thought when looking at
> > the history that we revert the exact patch without any other changes
> > or comments,
> Other part of that patch(except this line) may still useful, at least for
> cleanup purpose.
> > and usually that means someone will come up with the same
> > cleanup idea again, and then we'll have a bug again. So maybe a
> > comment or a WARN_ON or something else would be good.
>
> A WARN_ON is acceptable.
>
> > I guess we could also do your patch, but put a WARN_ON that the
> > computed total_size is never bigger than the drm_fb size into
> > drm_fb_helper_deferred_io()? That would also make sure that this bug
> > doesn't get resurrected again.
>
> Best to merge V2 [1] of this series, that is what I am really fixed.
>
> Maybe somebody can help to refine it, to add a better description about this
> question and so on.
>
> [1] https://patchwork.freedesktop.org/patch/532143/?series=116454&rev=1
Ok, I guess this really is the safest one. For that patch, do we need the
change to screen_size in drm_fbdev_generic_helper_fb_probe()? I'm still
not entirely clear. If it works without that change I think that's clearer
for a minimal bugfix, if so can you send that out as v4 please?
Also please Cc: Geert on whatever you're resubmitting, so he can test too
and we can make sure it's still fixing the shmob issue he's seeing.
And finally please include a link to this discussion here with a note that
just reverting the screen_size changes is not enough:
https://lore.kernel.org/dri-devel/ad44df29-3241-0d9e-e708-b0338bf3c623@xxxxxx/
Thanks a lot!
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch