Re: [PATCH v3] drm/fbdev-generic: prohibit potential out-of-bounds access

From: Sui Jingfeng
Date: Tue Apr 18 2023 - 13:53:45 EST

Hi,

On 2023/4/18 16:32, Daniel Vetter wrote:
On Mon, Apr 17, 2023 at 07:32:19PM +0800, Sui Jingfeng wrote:
The fbdev test of IGT may write after EOF, which lead to out-of-bound
access for the drm drivers using fbdev-generic. For example, on a x86
+ aspeed bmc card platform, with a 1680x1050 resolution display, running
fbdev test if IGT 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 direct reason is that damage rectange computed by
drm_fb_helper_memory_range_to_clip() does not guaranteed to be in-bound.
It is already results in workaround code populate to elsewhere. Another
reason is that exposing a larger buffer size than the actual needed help
to trigger this bug intrinsic in drm_fb_helper_memory_range_to_clip().

Others fbdev emulation solutions write to the GEM buffer directly, they
won't reproduce this bug because the .fb_dirty function callback do not
being hooked, so no chance is given to drm_fb_helper_memory_range_to_clip()
to generate a out-of-bound when drm_fb_helper_sys_write() is called.

This patch break the trigger condition of this bug by shrinking the shadow
buffer size to sizes->surface_height * buffer->fb->pitches[0].

Fixes: '8fbc9af55de0 ("drm/fbdev-generic: Set screen size to size of GEM
buffer")'

Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
---
drivers/gpu/drm/drm_fbdev_generic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

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];
So I read core some more and stumbled over drm_fb_helper_deferred_io().
Which has all the code and comments about this, including limiting.

I think it would be clearer if we fix the issue there, instead of passing
limits around in obscure places that then again get broken?

No, it is more obscure doing that way...


As the size of the shadow screen buffer will be exposed to userspace.

The size 'helper->fb->height * helper->fb->pitches[0]' is a exactly(best) fit,

You are guaranteed to waste at lease one byte by increasing one byte,

and can not store all pixels by decreasing one byte (In the case where `helper->fb->pitches[0] = helper->fb->width * 4`).

It implicitly tell the userspace do not go beyond that boundary.

although userspace program can still choose to write  after EOF,

But it is for test purpose, to test the kernel if it can return a -EFBIG or not.

The thing is,
Thomas both authored the limit checks in drm_fb_helper_deferred_io() and
the patch which broken them again, so clearly this isn't very obvious. I'm
thinking of something like this:


diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index ef4eb8b12766..726dab67c359 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -697,10 +697,7 @@ void drm_fb_helper_deferred_io(struct fb_info *info, struct list_head *pagerefli
* of the screen and account for non-existing scanlines. Hence,
* keep the covered memory area within the screen buffer.
*/
- if (info->screen_size)
- total_size = info->screen_size;
- else
- total_size = info->fix.smem_len;
+ total_size = helper->fb->height * helper->fb->pitches[0];

This is just to mitigate the mistakes already has been made,

because it  do not do a good splitting between the *clip* part and the *damage update* part.

An ideal clipping do not obscure its updating backend with a out-of-bound damage rectangle.

Why did the drm_fb_helper_memory_range_to_clip() can not do a good job in all case

to pass its backend a always meaningful damage rect ?

max_off = min(max_off, total_size);
if (min_off < max_off) {


I think that would make it utmost clear on what we're doing and why.
Otherwise we're just going to re-create the same bug again, like we've
done already :-)

No, we create no bugs, we fix one.

Thanks.

-Daniel

screen_buffer = vzalloc(screen_size);
if (!screen_buffer) {
ret = -ENOMEM;
--
2.25.1