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

[Dongliang Mu]

[ Upstream commit a49145acfb975d921464b84fe00279f99827d816 ]

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.

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 <george.kennedy@xxxxxxxxxx>
Reported-by: syzbot+e5fd3e65515b48c02a30@xxxxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: Dhaval Giani <dhaval.giani@xxxxxxxxxx>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/1594149963-13801-1-git-send-email-george.kennedy@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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;
+
 		ret = info->fbops->fb_check_var(var, info);
 
 		if (ret)
-- 
2.25.1