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

[Sui Jingfeng]

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