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Subject: Re: [PATCH v5] drm/fbdev-generic: prohibit potential out-of-bounds
 access
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To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>, 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
References: <20230420030500.1578756-1-suijingfeng@loongson.cn>
 <CAMuHMdUZoBZEM73fo8-PitSae8wgv-m6sez3nA8J3RFMAtOyOQ@mail.gmail.com>
From:   Sui Jingfeng <15330273260@189.cn>
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Hi,

On 2023/4/20 17:04, Geert Uytterhoeven wrote:
> Hi Sui,
>
> On Thu, Apr 20, 2023 at 5:09 AM Sui Jingfeng <suijingfeng@loongson.cn> wrote:
>> The fbdev test of IGT may write after EOF, which lead to out-of-bound
>> access for drm drivers hire fbdev-generic. For example, run fbdev test
>> on a x86+ast2400 platform, with 1680x1050 resolution, 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 is because damage rectangles computed by
>> drm_fb_helper_memory_range_to_clip() function does not guaranteed to be
>> bound in the screen's active display area. Possible reasons are:
>>
>> 1) Buffers are allocated in the granularity of page size, for mmap system
>>     call support. The shadow screen buffer consumed by fbdev emulation may
>>     also choosed be page size aligned.
>>
>> 2) The DIV_ROUND_UP() used in drm_fb_helper_memory_range_to_clip()
>>     will introduce off-by-one error.
>>
>> For example, on a 16KB page size system, in order to store a 1920x1080
>> XRGB framebuffer, we need allocate 507 pages. Unfortunately, the size
>> 1920*1080*4 can not be divided exactly by 16KB.
>>
>>   1920 * 1080 * 4 = 8294400 bytes
>>   506 * 16 * 1024 = 8290304 bytes
>>   507 * 16 * 1024 = 8306688 bytes
>>
>>   line_length = 1920*4 = 7680 bytes
>>
>>   507 * 16 * 1024 / 7680 = 1081.6
>>
>>   off / line_length = 507 * 16 * 1024 / 7680 = 1081
>>   DIV_ROUND_UP(507 * 16 * 1024, 7680) will yeild 1082
>>
>> memcpy_toio() typically issue the copy line by line, when copy the last
>> line, out-of-bound access will be happen. Because:
>>
>>   1082 * line_length = 1082 * 7680 = 8309760, and 8309760 > 8306688
>>
>> Note that userspace may stil write to the invisiable area if a larger
>> buffer than width x stride is exposed. But it is not a big issue as
>> long as there still have memory resolve the access if not drafting so
>> far.
>>
>>   - Also limit the y1 (Daniel)
>>   - keep fix patch it to minimal (Daniel)
>>   - screen_size is page size aligned because of it need mmap (Thomas)
>>   - Adding fixes tag (Thomas)
>>
>> Fixes: aa15c677cc34 ("drm/fb-helper: Fix vertical damage clipping")
>>
>> Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
>> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
> Thanks for the update!  This v5 is completely different from the v3
> I tested before, so keeping my Tested-by is not really appropriate...

Thanks for testing. I'm a bit of confident that it will works.

Your tested-by is valuable, really don't want drop this. So I keep it.

> I have retested fbtest with shmob-drm on Armadillo-800-EVA
> (800x480@RG16, i.e. 187.5 pages), and fortunately this version still
> works fine, so
> Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
>
> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds