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

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Apr 17, 2023 at 09:29:07AM +0200, Thomas Zimmermann wrote:
> 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@loongson.cn>
> > > > 
> > > > 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@loongson.cn>
> > > > ---
> > > >   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.

So gem_bo are supposed to be at least PAGE_SIZE aligned. So maybe we just
put a comment in here why this assumption holds, and reinforce it with a
WARN_ON? I think that would address the concerns here?

But I concur with Sui that this is a separate issue which should be sorted
out in a separate patch.
-Daniel

> 
> 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




-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch