[PATCH v3] drm/fbdev-generic: prohibit potential out-of-bounds access

[Geert Uytterhoeven]

On Mon, Apr 17, 2023 at 1:45 PM Sui Jingfeng <suijingfeng at loongson.cn> 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 at loongson.cn>

Thanks, this fixes the crashes when running fbtest on shmob-drm.
Tested-by: Geert Uytterhoeven <geert+renesas at glider.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

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