[PATCH v2] drm/fbdev-generic: prohibit potential out-of-bounds access

[Sui Jingfeng]

Hi,

On 2023/4/14 03:16, Thomas Zimmermann wrote:
> Hi,
>
> thanks for the patch. This is effectively a revert of commit 
> 8fbc9af55de0 ("drm/fbdev-generic: Set screen size to size of GEM 
> buffer"). Please add a Fixes tag.
>
> Am 13.04.23 um 20:06 schrieb Sui Jingfeng:
>> From: Sui Jingfeng <suijingfeng at loongson.cn>
>>
>> The crazy fbdev test of IGT may write after EOF, which lead to 
>> out-of-bound
>
> Please drop 'crazy'. :)

This is OK.

By using the world 'crazy',

I meant that the test is very good and maybe it is written by 
professional  peoples

with the guidance by  experienced  engineer. So that even the corner get 
tested.


>
>> access for the drm drivers using fbdev-generic. For example, run 
>> fbdev test
>> on a x86-64+ast2400 platform with 1680x1050 resolution will cause the 
>> linux
>> kernel hang with 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 indirect reason is drm_fb_helper_memory_range_to_clip() generate 
>> damage
>> rectangles which partially or completely go out of the active display 
>> area.
>> The second of argument 'off' is passing from the user-space, this 
>> will lead
>> to the out-of-bound if it is large than (fb_height + 1) * fb_pitches; 
>> while
>> DIV_ROUND_UP() may also controbute to error by 1.
>>
>> This patch will add code to restrict the damage rect computed go 
>> beyond of
>> the last line of the framebuffer.
>>
>> Signed-off-by: Sui Jingfeng <suijingfeng at loongson.cn>
>> ---
>>   drivers/gpu/drm/drm_fb_helper.c     | 16 ++++++++++++----
>>   drivers/gpu/drm/drm_fbdev_generic.c |  2 +-
>>   2 files changed, 13 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_fb_helper.c 
>> b/drivers/gpu/drm/drm_fb_helper.c
>> index 64458982be40..6bb1b8b27d7a 100644
>> --- a/drivers/gpu/drm/drm_fb_helper.c
>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>> @@ -641,19 +641,27 @@ static void drm_fb_helper_damage(struct 
>> drm_fb_helper *helper, u32 x, u32 y,
>>   static void drm_fb_helper_memory_range_to_clip(struct fb_info 
>> *info, off_t off, size_t len,
>>                              struct drm_rect *clip)
>>   {
>> +    u32 line_length = info->fix.line_length;
>> +    u32 fb_height = info->var.yres;
>>       off_t end = off + len;
>>       u32 x1 = 0;
>> -    u32 y1 = off / info->fix.line_length;
>> +    u32 y1 = off / line_length;
>>       u32 x2 = info->var.xres;
>> -    u32 y2 = DIV_ROUND_UP(end, info->fix.line_length);
>> +    u32 y2 = DIV_ROUND_UP(end, line_length);
>> +
>> +    /* Don't allow any of them beyond the bottom bound of display 
>> area */
>> +    if (y1 > fb_height)
>> +        y1 = fb_height;
>> +    if (y2 > fb_height)
>> +        y2 = fb_height;
>>         if ((y2 - y1) == 1) {
>>           /*
>>            * We've only written to a single scanline. Try to reduce
>>            * the number of horizontal pixels that need an update.
>>            */
>> -        off_t bit_off = (off % info->fix.line_length) * 8;
>> -        off_t bit_end = (end % info->fix.line_length) * 8;
>> +        off_t bit_off = (off % line_length) * 8;
>> +        off_t bit_end = (end % line_length) * 8;
>
> Please scratch all these changes. The current code should work as 
> intended. Only the generic fbdev emulation uses this code and it 
> should really be moved there at some point.


Are you meant  that we should remove all these changes in 
drivers/gpu/drm/drm_fb_helper.c ?


But this changes are helps to prevent the damage box computed go out of 
bound,

the bound of the displayable shadow buffer on multiple display case.

It is the minimum width x height which could be fit in for all 
display/minotor.


For example, one is 1920x1080 monitor, another is 1280x800 monitor.

connected to the motherboard simultaneously.


Then, 1920x1080x4 (suppose we are using the XRGB) scanout buffer will be

allocate by the  GEM backend. But the the actual display area is 1280x800.

This is true at least for my driver on my platform, In this case,

```

    info->var.xres ==1280;

    info->var.yres == 800;

```

If don't restrict this, the damage box computed out of the bound of  (0, 
0) ~ (1280, 800) rectangle.

a 1920x1080 damage box will came out.


When running fbdev test of IGT, the smaller screen display will be OK.

but the larger screen, the area outsize of 1280x800 will also be written.

The background color became completely white from completely black 
before carry out the test,

luckily, linux kernel do not hung, this time.


On multi-screen case, we still need to restrict the damage box computed,

Do not go out of 1280x800,  right?


>
>>             x1 = bit_off / info->var.bits_per_pixel;
>>           x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);
>> 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];
>
> I guess we simply go back to this line. I'd R-b a patch that does 
> exactly this.
>
> But some explanation is in order. Maybe you can add this as a comment 
> to the computation, as it's not obvious:
>
> The value of screen_size should actually be the size of the gem 
> buffer. In a physical framebuffer (i.e., video memory), the size would 
> be a multiple of the page size, but not necessarily a multiple of the 
> screen resolution. There are also pan fbdev's operations, and we could 
> possibly use DRM buffers that are not multiples of the screen width. 
> But the update code requires the use of drm_framebuffer_funcs.dirty, 
> which takes a clipping rectangle and therefore doesn't work well with 
> these odd values for screen_size.
>
> Best regards
> Thomas
>
>>       screen_buffer = vzalloc(screen_size);
>>       if (!screen_buffer) {
>>           ret = -ENOMEM;
>