Re: [PATCH v5] drm/fbdev-generic: prohibit potential out-of-bounds access

[Geert Uytterhoeven]

Hi Sui,

On Thu, Apr 20, 2023 at 5:09 AM Sui Jingfeng <suijingfeng@xxxxxxxxxxx> wrote:
> The fbdev test of IGT may write after EOF, which lead to out-of-bound
> access for drm drivers hire fbdev-generic. For example, run fbdev test
> on a x86+ast2400 platform, with 1680x1050 resolution, 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 is because damage rectangles computed by
> drm_fb_helper_memory_range_to_clip() function does not guaranteed to be
> bound in the screen's active display area. Possible reasons are:
>
> 1) Buffers are allocated in the granularity of page size, for mmap system
>    call support. The shadow screen buffer consumed by fbdev emulation may
>    also choosed be page size aligned.
>
> 2) The DIV_ROUND_UP() used in drm_fb_helper_memory_range_to_clip()
>    will introduce off-by-one error.
>
> For example, on a 16KB page size system, in order to store a 1920x1080
> XRGB framebuffer, we need allocate 507 pages. Unfortunately, the size
> 1920*1080*4 can not be divided exactly by 16KB.
>
>  1920 * 1080 * 4 = 8294400 bytes
>  506 * 16 * 1024 = 8290304 bytes
>  507 * 16 * 1024 = 8306688 bytes
>
>  line_length = 1920*4 = 7680 bytes
>
>  507 * 16 * 1024 / 7680 = 1081.6
>
>  off / line_length = 507 * 16 * 1024 / 7680 = 1081
>  DIV_ROUND_UP(507 * 16 * 1024, 7680) will yeild 1082
>
> memcpy_toio() typically issue the copy line by line, when copy the last
> line, out-of-bound access will be happen. Because:
>
>  1082 * line_length = 1082 * 7680 = 8309760, and 8309760 > 8306688
>
> Note that userspace may stil write to the invisiable area if a larger
> buffer than width x stride is exposed. But it is not a big issue as
> long as there still have memory resolve the access if not drafting so
> far.
>
>  - Also limit the y1 (Daniel)
>  - keep fix patch it to minimal (Daniel)
>  - screen_size is page size aligned because of it need mmap (Thomas)
>  - Adding fixes tag (Thomas)
>
> Fixes: aa15c677cc34 ("drm/fb-helper: Fix vertical damage clipping")
>
> Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Tested-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>

Thanks for the update!  This v5 is completely different from the v3
I tested before, so keeping my Tested-by is not really appropriate...

I have retested fbtest with shmob-drm on Armadillo-800-EVA
(800x480@RG16, i.e. 187.5 pages), and fortunately this version still
works fine, so
Tested-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds