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Subject: Re: [PATCH v3] drm/fbdev-generic: prohibit potential out-of-bounds
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To:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@gmail.com>, Li Yi <liyi@loongson.cn>,
        Helge Deller <deller@gmx.de>,
        Lucas De Marchi <lucas.demarchi@intel.com>,
        linux-kernel@vger.kernel.org, linux-fbdev@vger.kernel.org,
        dri-devel@lists.freedesktop.org, loongson-kernel@lists.loongnix.cn
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From:   Sui Jingfeng <15330273260@189.cn>
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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@loongson.cn>
>> ---
>>   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.

> -Daniel
>
>>   	screen_buffer = vzalloc(screen_size);
>>   	if (!screen_buffer) {
>>   		ret = -ENOMEM;
>> -- 
>> 2.25.1
>>