[PATCH 4.14 60/91] fbcon: prevent user font height or width change from causing potential out-of-bounds access

Greg Kroah-Hartman
2020-09-01T17:04:23Z

From: George Kennedy <george.kennedy@oracle.com>

commit 39b3cffb8cf3111738ea993e2757ab382253d86a upstream.

Add a check to fbcon_resize() to ensure that a possible change to user font
height or user font width will not allow a font data out-of-bounds access.
NOTE: must use original charcount in calculation as font charcount can
change and cannot be used to determine the font data allocated size.

Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Cc: stable <stable@vger.kernel.org>
Reported-by: syzbot+38a3699c7eaf165b97a6@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/1596213192-6635-1-git-send-email-george.kennedy@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/video/fbdev/core/fbcon.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2130,6 +2130,9 @@ static void updatescrollmode(struct disp
}
}

+#define PITCH(w) (((w) + 7) >> 3)
+#define CALC_FONTSZ(h, p, c) ((h) * (p) * (c)) /* size = height * pitch * charcount */
+
static int fbcon_resize(struct vc_data *vc, unsigned int width,
unsigned int height, unsigned int user)
{
@@ -2139,6 +2142,24 @@ static int fbcon_resize(struct vc_data *
struct fb_var_screeninfo var = info->var;
int x_diff, y_diff, virt_w, virt_h, virt_fw, virt_fh;

+ if (ops->p && ops->p->userfont && FNTSIZE(vc->vc_font.data)) {
+ int size;
+ int pitch = PITCH(vc->vc_font.width);
+
+ /*
+ * If user font, ensure that a possible change to user font
+ * height or width will not allow a font data out-of-bounds access.
+ * NOTE: must use original charcount in calculation as font
+ * charcount can change and cannot be used to determine the
+ * font data allocated size.
+ */
+ if (pitch <= 0)
+ return -EINVAL;
+ size = CALC_FONTSZ(vc->vc_font.height, pitch, FNTCHARCNT(vc->vc_font.data));
+ if (size > FNTSIZE(vc->vc_font.data))
+ return -EINVAL;
+ }
+
virt_w = FBCON_SWAP(ops->rotate, width, height);
virt_h = FBCON_SWAP(ops->rotate, height, width);
virt_fw = FBCON_SWAP(ops->rotate, vc->vc_font.width,
@@ -2600,7 +2621,7 @@ static int fbcon_set_font(struct vc_data
int size;
int i, csum;
u8 *new_data, *data = font->data;
- int pitch = (font->width+7) >> 3;
+ int pitch = PITCH(font->width);

/* Is there a reason why fbconsole couldn't handle any charcount >256?
* If not this check should be changed to charcount < 256 */
@@ -2616,7 +2637,7 @@ static int fbcon_set_font(struct vc_data
if (fbcon_invalid_charcount(info, charcount))
return -EINVAL;

- size = h * pitch * charcount;
+ size = CALC_FONTSZ(h, pitch, charcount);

new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);