Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
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From: Jani Nikula <jani.nikula@linux.intel.com>
Date: 2025-09-22 12:43:35
Also in:
dri-devel, lkml
On Fri, 12 Sep 2025, Samasth Norway Ananda [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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
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+ 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