Re: [PATCH 4.19] fbmem: add margin check to fb_check_caps()

On Thu, Sep 2, 2021 at 9:15 PM Geert Uytterhoeven [off-list ref] wrote:

Hi Dongliang,

On Thu, Sep 2, 2021 at 8:04 AM Dongliang Mu [off-list ref] wrote:

quoted

[ Upstream commit a49145acfb975d921464b84fe00279f99827d816 ]

Oops, looks like I missed when that one was submitted for review...

This patch cannot directly apply to old stable trees. Maybe that's the reason.

quoted

A fb_ioctl() FBIOPUT_VSCREENINFO call with invalid xres setting
or yres setting in struct fb_var_screeninfo will result in a
KASAN: vmalloc-out-of-bounds failure in bitfill_aligned() as
the margins are being cleared. The margins are cleared in
chunks and if the xres setting or yres setting is a value of
zero upto the chunk size, the failure will occur.

Add a margin check to validate xres and yres settings.

Shouldn't (the caller of) bitfill_aligned() be fixed instead?
Can this be triggered by e.g. using the mini_4x6 font?

I am sorry. I don't know much detail about this subsystem. I just
found syzkaller can still trigger this bug in linux-4.19.

Combined with the bug information, I found this patch is not merged
into the old kernel trees. So I send this patch rebased on linux-4.19.
Also I have tested it on linux-4.14 and below.

quoted

Note that, this patch needs special handling to backport it to linux
kernel 4.19, 4.14, 4.9, 4.4.

Signed-off-by: George Kennedy <redacted>
Reported-by: syzbot+e5fd3e65515b48c02a30@syzkaller.appspotmail.com
Reviewed-by: Dan Carpenter <redacted>
Cc: Dhaval Giani <redacted>
Signed-off-by: Bartlomiej Zolnierkiewicz <redacted>
Link: https://patchwork.freedesktop.org/patch/msgid/1594149963-13801-1-git-send-email-george.kennedy@oracle.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/video/fbdev/core/fbmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 84845275dbef..de04c097d67c 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c

@@ -991,6 +991,10 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
                        goto done;
                }

+               /* bitfill_aligned() assumes that it's at least 8x8 */
+               if (var->xres < 8 || var->yres < 8)
+                       return -EINVAL;

Are you sure there don't exist such small displays (e.g. OLED)?

quoted

+
                ret = info->fbops->fb_check_var(var, info);

                if (ret)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds