fbmem: don't allow too huge resolutions

commit 8c28051cdcbe9dfcec6bd0a4709d67a09df6edae upstream.

syzbot is reporting page fault at vga16fb_fillrect() [1], for
vga16fb_check_var() is failing to detect multiplication overflow.

  if (vxres * vyres > maxmem) {
    vyres = maxmem / vxres;
    if (vyres < yres)
      return -ENOMEM;
  }

Since no module would accept too huge resolutions where multiplication
overflow happens, let's reject in the common path.

Link: https://syzkaller.appspot.com/bug?extid=04168c8063cfdde1db5e
Reported-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>
Debugged-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/185175d6-227a-7b55-433d-b070929b262c@i-love.sakura.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index de04c097d67c7f697581eb099e9c5f591130d519..2297dfb494d6a99516039ceb2cbe8076dd71ba22 100644 (file)
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -971,6 +971,7 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
        if ((var->activate & FB_ACTIVATE_FORCE) ||
            memcmp(&info->var, var, sizeof(struct fb_var_screeninfo))) {
                u32 activate = var->activate;
+               u32 unused;
 
                /* When using FOURCC mode, make sure the red, green, blue and
                 * transp fields are set to 0.
@@ -995,6 +996,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
                if (var->xres < 8 || var->yres < 8)
                        return -EINVAL;
 
+               /* Too huge resolution causes multiplication overflow. */
+               if (check_mul_overflow(var->xres, var->yres, &unused) ||
+                   check_mul_overflow(var->xres_virtual, var->yres_virtual, &unused))
+                       return -EINVAL;
+
                ret = info->fbops->fb_check_var(var, info);
 
                if (ret)