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Date: Thu, 20 Apr 2023 13:10:35 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Sui Jingfeng <15330273260@....cn>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
David Airlie <airlied@...il.com>,
Daniel Vetter <daniel@...ll.ch>,
Geert Uytterhoeven <geert+renesas@...der.be>,
Li Yi <liyi@...ngson.cn>, Helge Deller <deller@....de>,
Lucas De Marchi <lucas.demarchi@...el.com>
Cc: loongson-kernel@...ts.loongnix.cn, linux-fbdev@...r.kernel.org,
linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v5] drm/fbdev-generic: prohibit potential out-of-bounds
access
Hi
Am 20.04.23 um 12:04 schrieb Sui Jingfeng:
> Hi
>
> On 2023/4/20 15:07, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 20.04.23 um 09:04 schrieb Thomas Zimmermann:
>>> Hi,
>>>
>>> this patch looks to me good and I'd like to merge it, if no one objects.
>>
>> Rereading it, I might have been too eager. What happened to the
>> setting of screen_size = buffer->gem->size ? It is not relevant?
>>
> Short answer is that it is not relevant.
>
> As long as the computed damage rectangle is sane, it's OK to allocate a
> bit more than needed.
>
> I think it's turn out to be *correct*, if not extremely.
>
> Because it is page size aligned, writing to invisible area for some
> case is not a serve issue.
>
> It also guarantee that the size of shadow screen buffer is exactly the
> same size with its GEM counterpart.
That's good enough for me. :)
>
>
> Otherwise I have to answer the question
>
> What will happen if the 'screen_size' is not page_size aligned and mmap
> will mapping in the granularity of pages ?
You need to map at page granularity. If screen_size is not page-size
aligned, there's this trailing buffer that is accessible, but cannot be
displayed. But userspace has no direct way of knowing that, so let's
ignore that problem for now.
Best regards
Thomas
>
>
> I see efifb also align the buffer going to be mapped with page size.
>
>
>> Best regards
>> Thomas
>>
>>>
>>> In the near future, after i915 has switched to struct drm_client, I
>>> intend to move DRM's deferred-I/O helpers into fbdev-generic and
>>> i915. Those are the two users, but they are fairly different. They
>>> can then both have something tailored towards their needs.
>>>
>>> Best regards
>>> Thomas
>>>
>>> Am 20.04.23 um 05:05 schrieb Sui Jingfeng:
>>>> 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@...ngson.cn>
>>>> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
>>>> Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>>> Link:
>>>> https://lore.kernel.org/dri-devel/ad44df29-3241-0d9e-e708-b0338bf3c623@189.cn/
>>>> ---
>>>> drivers/gpu/drm/drm_fb_helper.c | 16 ++++++++++++----
>>>> 1 file changed, 12 insertions(+), 4 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;
>>>> x1 = bit_off / info->var.bits_per_pixel;
>>>> x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);
>>>
>>
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
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