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Subject: Re: [PATCH v2] drm/fbdev-generic: prohibit potential out-of-bounds
 access
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To:     Thomas Zimmermann <tzimmermann@suse.de>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sui Jingfeng <suijingfeng@loongson.cn>,
        Li Yi <liyi@loongson.cn>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Helge Deller <deller@gmx.de>,
        Lucas De Marchi <lucas.demarchi@intel.com>
Cc:     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 2023/4/17 15:29, Thomas Zimmermann wrote:
> 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))
>
I just thought about your instruction at here, thanks!

But it is already page size aligned if we don't tough it.

It is guaranteed by the GEM memory manger,

so a previous patch tested by me, is turn out to be a extremely correct?

We exposed a  page size aligned buffer(even though it is larger than 
needed) is actually for mmap ?


> 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;
>>>
>