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

From: Sui Jingfeng
Date: Wed Apr 12 2023 - 13:13:56 EST

Hi,

On 2023/4/11 22:53, Daniel Vetter wrote:
On Sun, Apr 09, 2023 at 09:21:10PM +0800, Sui Jingfeng wrote:
From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>

We should setting the screen buffer size according to the screen's actual
size, rather than the size of the GEM object backing the front framebuffer.
The size of GEM buffer is page size aligned, while the size of active area
of a specific screen is *NOT* necessarily page size aliged. For example,
1680x1050, 1600x900, 1440x900, 800x6000 etc. In those case, the damage rect
computed by drm_fb_helper_memory_range_to_clip() goes out of bottom bounds
of the display.

Run fbdev test of IGT on a x86+ast2400 platform with 1680x1050 resolution
will cause the system 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 ]---

We also add trival code in this patch to restrict the damage rect beyond
the last line of the framebuffer.
Nice catch!
 :)
Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
---
drivers/gpu/drm/drm_fb_helper.c | 2 +-
drivers/gpu/drm/drm_fbdev_generic.c | 2 ++
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 64458982be40..a2b749372759 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -645,7 +645,7 @@ static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off,
u32 x1 = 0;
u32 y1 = off / info->fix.line_length;
u32 x2 = info->var.xres;
- u32 y2 = DIV_ROUND_UP(end, info->fix.line_length);
+ u32 y2 = min_t(u32, DIV_ROUND_UP(end, info->fix.line_length), info->var.yres);
So for additional robustness I think it'd be good if we change the entire
computation here to use drm_framebuffer data and not fb_info data, because
fundamentally that's what the drm kms code consumes. It should all match
anyway, but I think it makes the code more obviously correct.

So in the entire function instead of looking at fb_info->fix we should
probably look at

struct drm_fb_helper *helper = info->par;

And then helper->fb->pitches[0] and helper->fb->height.

If you agree would be great if you can please respin with that (and the
commit message augmented to explain why we do the change)?

Yes, I'm agree.

Thank you for guidance, I will refine this patch with `helper = info->par`.

I will send a v2 when I finished.

if ((y2 - y1) == 1) {
/*
diff --git a/drivers/gpu/drm/drm_fbdev_generic.c b/drivers/gpu/drm/drm_fbdev_generic.c
index 8e5148bf40bb..a6daecb5f640 100644
--- a/drivers/gpu/drm/drm_fbdev_generic.c
+++ b/drivers/gpu/drm/drm_fbdev_generic.c
@@ -95,6 +95,8 @@ static int drm_fbdev_generic_helper_fb_probe(struct drm_fb_helper *fb_helper,
fb_helper->fb = buffer->fb;
screen_size = buffer->gem->size;
I guess you forgot to remove this line here?

Yes, this line should be removed in this patch. I overlooked this, sorry.

Also I'm not understanding
why this matters, I think you're fix only needs the above chunk, not this
one? If I got this right then please drop this part, there's drivers which
only use drm_fb_helper.c but not drm_fbdev_generic.c, and from what I can
tell they all still set the gem buffer size here.

If otoh we need this too, then there's a few more places that need to be
fixed.

I think we need this line, otherwise wrapped around will be happen.

Because I found that the value of variable`y1` will be larger in number than the variable `y2` by 1,

which are computed  in drm_fb_helper_memory_range_to_clip().


This phenomenon will emerged on platforms with large page size or

non page size divisiable display resolution case. Take the LoongArch and Mips as an example,

the default page size is 16KB(to avoid cache alias).  Even with the most frequently used

1920x1080 screen, the screen_size can not be divided exactly.

The total size of the shadow buffer is 1920x1080x4 bytes, 1920x1080x4 / 16384 = 506.25

TTM manage the vram in the term of pages, so TTM will allocate 507 pages for us.

507x16384 = 8306688 bytes.


drm_fb_helper_memory_range_to_clip() will be called when running fbdev eof test in the IGT.

with 8306688 as its second parameter. while 8306688 / (1920x4) = 1081, this cause y1 out of bound.

Simply restrict y2 with a min_t() function yeild 1080 in this case, but y2 - y1 cause *wrap around* here.

because they are both unsigned number.


drm_rect_init() function cast this unsigned int type to int type in end of drm_fb_helper_memory_range_to_clip(),

but the last argument of drm_fb_helper_damage() function is a u32 type,

it cast the return value of  drm_rect_height(&damage_area) back to unsigned type.

Yet, another wrapped around with truncation happened in drm_fb_helper_add_damage_clip()

called by subsequent drm_fb_helper_damage() function.

I finally got reject by drm_fbdev_generic_helper_fb_dirty() with follow code:

```

    /* Call damage handlers only if necessary */
    if (!(clip->x1 < clip->x2 && clip->y1 < clip->y2))
        return 0;

```

On x86-64 platform, because 1920x1080x4 dumb buffer is lucky, it be divided exactly by 4KB(page size).

But other resolution will not as luck as this one. Right, fbdev test will be pasted, but wrap around

happens many time.

Therefore, as long as a larger buffer is allowed to exposed to the user-space.

A chance is given to the user-space,  to go beyond of the bottom bound of the actual active display area.

I not sure if this is intended, I feel it should not be allowable by intuition.

+ screen_size = sizes->surface_height * buffer->fb->pitches[0];
+
screen_buffer = vzalloc(screen_size);
if (!screen_buffer) {
ret = -ENOMEM;
Cheers, Daniel

--
2.25.1