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

[Thomas Zimmermann]

Hi

Am 17.04.23 um 13:32 schrieb Sui Jingfeng:
> 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];
Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Thanks a lot for the bugfix.

Best regards
Thomas

>   	screen_buffer = vzalloc(screen_size);
>   	if (!screen_buffer) {
>   		ret = -ENOMEM;

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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