On Fri, 12 Sep 2025, Samasth Norway Ananda <[email protected]>
wrote:
> Fix integer overflow vulnerabilities in fbcon_do_set_font() where font
> size calculations could overflow when handling user-controlled font
> parameters.
>
> The vulnerabilities occur when:
> 1. CALC_FONTSZ(h, pitch, charcount) performs h * pith * charcount
> multiplication with user-controlled values that can overflow.
> 2. FONT_EXTRA_WORDS * sizeof(int) + size addition can also overflow
> 3. This results in smaller allocations than expected, leading to buffer
> overflows during font data copying.
>
> Add explicit overflow checking using check_mul_overflow() and
> check_add_overflow() kernel helpers to safety validate all size
> calculations before allocation.
>
> Signed-off-by: Samasth Norway Ananda <[email protected]>
> ---
> drivers/video/fbdev/core/fbcon.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c
> b/drivers/video/fbdev/core/fbcon.c
> index 55f5731e94c3..a507d05f8fea 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2531,9 +2531,16 @@ static int fbcon_set_font(struct vc_data *vc, const
> struct console_font *font,
> if (fbcon_invalid_charcount(info, charcount))
> return -EINVAL;
>
> - size = CALC_FONTSZ(h, pitch, charcount);
> + /* Check for integer overflow in font size calculation */
> + if (check_mul_overflow(h, pitch, &size) ||
> + check_mul_overflow(size, charcount, &size))
> + return -EINVAL;
> +
> + /* Check for overflow in allocation size calculation */
> + if (check_add_overflow(FONT_EXTRA_WORDS * sizeof(int), size, &size))
This change stores the intermediate value into size, but fails to take
into account that size is used just a bit later in the function,
expecting the original size:
new_data += FONT_EXTRA_WORDS * sizeof(int);
FNTSIZE(new_data) = size;
REFCOUNT(new_data) = 0; /* usage counter */
for (i=0; i< charcount; i++) {
memcpy(new_data + i*h*pitch, data + i*vpitch*pitch, h*pitch);
}
/* Since linux has a nice crc32 function use it for counting font
* checksums. */
csum = crc32(0, new_data, size);
What was supposed to address an unlikely integer overflow seems to have
caused a real buffer overflow [1].
BR,
Jani.
[1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15020
> + return -EINVAL;
>
> - new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
> + new_data = kmalloc(size, GFP_USER);
>
> if (!new_data)
> return -ENOMEM;
--
Jani Nikula, Intel
