fbdev: bitblit: bound-check glyph index in bit_putcs*

bit_putcs_aligned()/unaligned() derived the glyph pointer from the
character value masked by 0xff/0x1ff, which may exceed the actual font's
glyph count and read past the end of the built-in font array.
Clamp the index to the actual glyph count before computing the address.

This fixes a global out-of-bounds read reported by syzbot.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
Tested-by: [e-mail]
Signed-off-by: Junjie Cao <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>
Cc: [e-mail]

drivers/video/fbdev/core/bitblit.c

···
 				     struct fb_image *image, u8 *buf, u8 *dst)
 {
 	u16 charmask = vc->vc_hi_font_mask ? 0x1ff : 0xff;
+	unsigned int charcnt = vc->vc_font.charcount;
 	u32 idx = vc->vc_font.width >> 3;
 	u8 *src;
 
 	while (cnt--) {
-		src = vc->vc_font.data + (scr_readw(s++)&
-					  charmask)*cellsize;
+		u16 ch = scr_readw(s++) & charmask;
+
+		if (ch >= charcnt)
+			ch = 0;
+		src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {
 			update_attr(buf, src, attr, vc);
···
 				       u8 *dst)
 {
 	u16 charmask = vc->vc_hi_font_mask ? 0x1ff : 0xff;
+	unsigned int charcnt = vc->vc_font.charcount;
 	u32 shift_low = 0, mod = vc->vc_font.width % 8;
 	u32 shift_high = 8;
 	u32 idx = vc->vc_font.width >> 3;
 	u8 *src;
 
 	while (cnt--) {
-		src = vc->vc_font.data + (scr_readw(s++)&
-					  charmask)*cellsize;
+		u16 ch = scr_readw(s++) & charmask;
+
+		if (ch >= charcnt)
+			ch = 0;
+		src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {
 			update_attr(buf, src, attr, vc);