Re: [PATCH v2] drm/fbdev-generic: prohibit potential out-of-bounds access

From: Thomas Zimmermann
Date: Mon Apr 17 2023 - 03:30:05 EST

Hi

Am 14.04.23 um 12:58 schrieb Sui Jingfeng:
Hi,

On 2023/4/14 03:16, Thomas Zimmermann wrote:
Hi,

thanks for the patch. This is effectively a revert of commit 8fbc9af55de0 ("drm/fbdev-generic: Set screen size to size of GEM buffer"). Please add a Fixes tag.

Am 13.04.23 um 20:06 schrieb Sui Jingfeng:
From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>

The crazy fbdev test of IGT may write after EOF, which lead to out-of-bound

Please drop 'crazy'. :)

This is OK.

By using the world 'crazy',

I meant that the test is very good and maybe it is written by professional  peoples

with the guidance by  experienced  engineer. So that even the corner get tested.



access for the drm drivers using fbdev-generic. For example, run fbdev test
on a x86-64+ast2400 platform with 1680x1050 resolution will cause the linux
kernel hang with 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 indirect reason is drm_fb_helper_memory_range_to_clip() generate damage
rectangles which partially or completely go out of the active display area.
The second of argument 'off' is passing from the user-space, this will lead
to the out-of-bound if it is large than (fb_height + 1) * fb_pitches; while
DIV_ROUND_UP() may also controbute to error by 1.

This patch will add code to restrict the damage rect computed go beyond of
the last line of the framebuffer.

Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
---
  drivers/gpu/drm/drm_fb_helper.c     | 16 ++++++++++++----
  drivers/gpu/drm/drm_fbdev_generic.c |  2 +-
  2 files changed, 13 insertions(+), 5 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;

Please scratch all these changes. The current code should work as intended. Only the generic fbdev emulation uses this code and it should really be moved there at some point.


Are you meant  that we should remove all these changes in drivers/gpu/drm/drm_fb_helper.c ?

As Daniel mentioned, there's the discussion in the other thread. I don't want to reopen it here. Just to summarize: I'm not convinced that this should be DRM code because it can be shared with other fbdev drivers.

[...]

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];

This has been bothering me over the weekend. And I think it's because what we want the screen_size to be heigth * pitch, but the mmap'ed memory is still at page granularity. Therefore...

[...]

      screen_buffer = vzalloc(screen_size);

... this line should explicitly allocate multiples of pages. Something like

/* allocate page-size multiples for mmap */
vzalloc(PAGE_ALIGN(screen_size))

It has not been a bug so far because vzalloc() always returns full pages IIRC. It's still worth fixing.

Best regards
Thomas


      if (!screen_buffer) {
          ret = -ENOMEM;


--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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