Re: [PATCH v2] drm/fbdev-generic: prohibit potential out-of-bounds access
From: Daniel Vetter
Date: Sun Apr 16 2023 - 03:57:31 EST
On Fri, Apr 14, 2023 at 06:58:53PM +0800, Sui Jingfeng wrote:
> 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.
'thoroughly' would be better word to describe that I think.
I think for the other discussions I've covered it all already in the other
thread.
-Daniel
>
>
> >
> > > 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 ?
>
>
> But this changes are helps to prevent the damage box computed go out of
> bound,
>
> the bound of the displayable shadow buffer on multiple display case.
>
> It is the minimum width x height which could be fit in for all
> display/minotor.
>
>
> For example, one is 1920x1080 monitor, another is 1280x800 monitor.
>
> connected to the motherboard simultaneously.
>
>
> Then, 1920x1080x4 (suppose we are using the XRGB) scanout buffer will be
>
> allocate by the GEM backend. But the the actual display area is 1280x800.
>
> This is true at least for my driver on my platform, In this case,
>
> ```
>
> info->var.xres ==1280;
>
> info->var.yres == 800;
>
> ```
>
> If don't restrict this, the damage box computed out of the bound of (0, 0)
> ~ (1280, 800) rectangle.
>
> a 1920x1080 damage box will came out.
>
>
> When running fbdev test of IGT, the smaller screen display will be OK.
>
> but the larger screen, the area outsize of 1280x800 will also be written.
>
> The background color became completely white from completely black before
> carry out the test,
>
> luckily, linux kernel do not hung, this time.
>
>
> On multi-screen case, we still need to restrict the damage box computed,
>
> Do not go out of 1280x800, right?
>
>
> >
> > > x1 = bit_off / info->var.bits_per_pixel;
> > > x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);
> > > 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];
> >
> > I guess we simply go back to this line. I'd R-b a patch that does
> > exactly this.
> >
> > But some explanation is in order. Maybe you can add this as a comment to
> > the computation, as it's not obvious:
> >
> > The value of screen_size should actually be the size of the gem buffer.
> > In a physical framebuffer (i.e., video memory), the size would be a
> > multiple of the page size, but not necessarily a multiple of the screen
> > resolution. There are also pan fbdev's operations, and we could possibly
> > use DRM buffers that are not multiples of the screen width. But the
> > update code requires the use of drm_framebuffer_funcs.dirty, which takes
> > a clipping rectangle and therefore doesn't work well with these odd
> > values for screen_size.
> >
> > Best regards
> > Thomas
> >
> > > screen_buffer = vzalloc(screen_size);
> > > if (!screen_buffer) {
> > > ret = -ENOMEM;
> >
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch