Hi Andrey, On Thu, Aug 11, 2022 at 12:23 PM Andrey Strachuk <strochuk@xxxxxxxxx> wrote: > Add virtual screen size check to drm_fb_helper_check_var() in > order to validate userspace input. > > Found by Linux Verification Center (linuxtesting.org) with syzkaller. > > Signed-off-by: Andrey Strachuk <strochuk@xxxxxxxxx> Thanks for your patch! > Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()") This Fixes tag is misleading: the bug has existed since basically forever. > --- a/drivers/gpu/drm/drm_fb_helper.c > +++ b/drivers/gpu/drm/drm_fb_helper.c > @@ -1355,6 +1355,16 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var, > (drm_format_info_block_height(fb->format, 0) > 1)) > return -EINVAL; > > + /* verify that virtual resolution >= physical resolution */ > + if (var->xres_virtual < var->xres || > + var->yres_virtual < var->yres) { > + drm_dbg_kms(dev, "requested virtual screen size that is " > + "below the physical size(%ux%u vs. %ux%u)\n", > + var->xres_virtual, var->yres_virtual, > + var->xres, var->yres); > + return -EINVAL; According to the fbdev rounding rules, invalid values should be rounded up, if possible. Hence it is better to not return an error, but round up: if (var->xres_virtual < var->xres) var->xres_virtual = var->xres; if (var->yres_virtual < var->yres) var->yres_virtual = var->yres; > + } > + > /* > * Changes struct fb_var_screeninfo are currently not pushed back > * to KMS, hence fail if different settings are requested. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx 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