fbcon: Fix global-out-of-bounds read in fbcon_get_font()

fbcon_get_font() is reading out-of-bounds. A malicious user may resize
`vc->vc_font.height` to a large value, causing fbcon_get_font() to
read out of `fontdata`.

fbcon_get_font() handles both built-in and user-provided fonts.
Fortunately, recently we have added FONT_EXTRA_WORDS support for built-in
fonts, so fix it by adding range checks using FNTSIZE().

This patch depends on patch "fbdev, newport_con: Move FONT_EXTRA_WORDS
macros into linux/font.h", and patch "Fonts: Support FONT_EXTRA_WORDS
macros for built-in fonts".

Cc: stable@vger.kernel.org
Reported-and-tested-by: syzbot+29d4ed7f3bdedf2aa2fd@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=08b8be45afea11888776f897895aef9ad1c3ecfd
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/b34544687a1a09d6de630659eb7a773f4953238b.1600953813.git.yepeilin.cs@gmail.com

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 8a31fc2..e432c9f 100644 (file)
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2471,6 +2471,9 @@ static int fbcon_get_font(struct vc_data *vc, struct console_font *font)
 
        if (font->width <= 8) {
                j = vc->vc_font.height;
+               if (font->charcount * j > FNTSIZE(fontdata))
+                       return -EINVAL;
+
                for (i = 0; i < font->charcount; i++) {
                        memcpy(data, fontdata, j);
                        memset(data + j, 0, 32 - j);
@@ -2479,6 +2482,9 @@ static int fbcon_get_font(struct vc_data *vc, struct console_font *font)
                }
        } else if (font->width <= 16) {
                j = vc->vc_font.height * 2;
+               if (font->charcount * j > FNTSIZE(fontdata))
+                       return -EINVAL;
+
                for (i = 0; i < font->charcount; i++) {
                        memcpy(data, fontdata, j);
                        memset(data + j, 0, 64 - j);
@@ -2486,6 +2492,9 @@ static int fbcon_get_font(struct vc_data *vc, struct console_font *font)
                        fontdata += j;
                }
        } else if (font->width <= 24) {
+               if (font->charcount * (vc->vc_font.height * sizeof(u32)) > FNTSIZE(fontdata))
+                       return -EINVAL;
+
                for (i = 0; i < font->charcount; i++) {
                        for (j = 0; j < vc->vc_font.height; j++) {
                                *data++ = fontdata[0];
@@ -2498,6 +2507,9 @@ static int fbcon_get_font(struct vc_data *vc, struct console_font *font)
                }
        } else {
                j = vc->vc_font.height * 4;
+               if (font->charcount * j > FNTSIZE(fontdata))
+                       return -EINVAL;
+
                for (i = 0; i < font->charcount; i++) {
                        memcpy(data, fontdata, j);
                        memset(data + j, 0, 128 - j);