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

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 14.04.23 um 12:58 schrieb 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@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@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 ?

As Daniel mentioned, there's the discussion in the other thread. I don't 
want to reopen it here. Just to summarize: I'm not convinced that this 
should be DRM code because it can be shared with other fbdev drivers.

[...]

>>> 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];

This has been bothering me over the weekend. And I think it's because 
what we want the screen_size to be heigth * pitch,  but the mmap'ed 
memory is still at page granularity. Therefore...

[...]
>>
>>>       screen_buffer = vzalloc(screen_size);

... this line should explicitly allocate multiples of pages. Something like

     /* allocate page-size multiples for mmap */
     vzalloc(PAGE_ALIGN(screen_size))

It has not been a bug so far because vzalloc() always returns full pages 
IIRC. It's still worth fixing.

Best regards
Thomas


>>>       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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