On Tue, Apr 04, 2023 at 03:53:09PM +0200, Geert Uytterhoeven wrote:
Hi Daniel,
CC vkmsdrm maintainer
Thanks for your patch!
On Tue, Apr 4, 2023 at 2:36 PM Daniel Vetter daniel.vetter@ffwll.ch wrote:
There's a few reasons the kernel should not spam dmesg on bad userspace ioctl input:
- at warning level it results in CI false positives
- it allows userspace to drown dmesg output, potentially hiding real issues.
None of the other generic EINVAL checks report in the FBIOPUT_VSCREENINFO ioctl do this, so it's also inconsistent.
I guess the intent of the patch which introduced this warning was that the drivers ->fb_check_var routine should fail in that case. Reality is that there's too many fbdev drivers and not enough people maintaining them by far, and so over the past few years we've simply handled all these validation gaps by tighning the checks in the core, because that's realistically really all that will ever happen.
Reported-by: syzbot+20dcf81733d43ddff661@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cef...
WARNING: fbcon: Driver 'vkmsdrmfb' missed to adjust virtual screensize (0x0 vs. 64x768)
This is a bug in the vkmsdrmfb driver and/or DRM helpers.
The message was added to make sure the individual drivers are fixed. Perhaps it should be changed to BUG() instead, so dmesg output cannot be drown?
So you're solution is to essentially force us to replicate this check over all the drivers which cannot change the virtual size?
Are you volunteering to field that audit and type all the patches?
Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()") Cc: Helge Deller deller@gmx.de Cc: Geert Uytterhoeven geert@linux-m68k.org Cc: stable@vger.kernel.org # v5.4+ Cc: Daniel Vetter daniel@ffwll.ch Cc: Javier Martinez Canillas javierm@redhat.com Cc: Thomas Zimmermann tzimmermann@suse.de Signed-off-by: Daniel Vetter daniel.vetter@intel.com
NAKed-by: Geert Uytterhoeven geert@linux-m68k.org
Yes I know it's not pretty, but realistically unless someone starts typing a _lot_ of patches this is the solution. It's exactly the same solution we've implemented for all other gaps syzcaller has find in fbdev input validation. Unless you can show that this is papering over a more severe bug somewhere, but then I guess it really should be a BUG to prevent worse things from happening. -Daniel
--- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -1021,10 +1021,6 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var) /* verify that virtual resolution >= physical resolution */ if (var->xres_virtual < var->xres || var->yres_virtual < var->yres) {
pr_warn("WARNING: fbcon: Driver '%s' missed to adjust virtual screen size (%ux%u vs. %ux%u)\n",info->fix.id,var->xres_virtual, var->yres_virtual,var->xres, var->yres); return -EINVAL; }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