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Subject: [PATCH v2] fbmem: don't allow too huge resolutions
To:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Colin King <colin.king@canonical.com>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Randy Dunlap <rdunlap@infradead.org>
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <750ed1ae-de80-b232-4aea-79d60c212fab@i-love.sakura.ne.jp>
Date:   Wed, 1 Sep 2021 10:14:43 +0900
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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 [1]
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>
---
Changes in v2:
  Use check_mul_overflow(), suggested by Geert Uytterhoeven <geert@linux-m68k.org>.

 drivers/video/fbdev/core/fbmem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1c855145711b..53d23b3d010c 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -962,6 +962,7 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	struct fb_var_screeninfo old_var;
 	struct fb_videomode mode;
 	struct fb_event event;
+	u32 unused;
 
 	if (var->activate & FB_ACTIVATE_INV_MODE) {
 		struct fb_videomode mode1, mode2;
@@ -1008,6 +1009,11 @@ 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)
-- 
2.25.1