package/cairo: fix CVE-2020-35492

Add an upstream patch to fix CVE-2020-35492:
A flaw was found in cairo's image-compositor.c in all versions prior to
1.17.4. This flaw allows an attacker who can provide a crafted input
file to cairo's image-compositor (for example, by convincing a user to
open a file in an application using cairo, or if an application uses
cairo on untrusted input) to cause a stack buffer overflow ->
out-of-bounds WRITE. The highest impact from this vulnerability is to
confidentiality, integrity, as well as system availability.

Important note: this is not the exact upstream patch. Indeed, the
upstream patch[1] contains a png file which appears as a binary diff
inside the patch. The `patch` tool which is used by Buildroot to apply
patches does not handle that kind of diff. Since it is just a test, it
shouldn't impact the quality of the CVE fix and all changes related to
the test are removed from the patch.

[1] https://gitlab.freedesktop.org/cairo/cairo/-/commit/03a820b173ed1fdef6ff14b4468f5dbc02ff59be
Cc: Quentin Schulz <foss+buildroot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/cairo/0004-Fix-mask-usage-in-image-compositor.patch

@@ -0,0 +1,56 @@
+From 03a820b173ed1fdef6ff14b4468f5dbc02ff59be Mon Sep 17 00:00:00 2001
+From: Heiko Lewin <heiko.lewin@worldiety.de>
+Date: Tue, 15 Dec 2020 16:48:19 +0100
+Subject: [PATCH] Fix mask usage in image-compositor
+
+[Retrieved from
+https://gitlab.freedesktop.org/cairo/cairo/-/commit/03a820b173ed1fdef6ff14b4468f5dbc02ff59be]
+[Removed changes in test/ directory to remove binary diff so that the
+patch can be applied by `patch` tool]
+Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
+---
+ src/cairo-image-compositor.c | 8 ++++----
+ 1 file changed, 4 insertions(+), 4 deletions(-)
+
+diff --git a/src/cairo-image-compositor.c b/src/cairo-image-compositor.c
+index bbf4cf228..2352c478e 100644
+--- a/src/cairo-image-compositor.c
++++ b/src/cairo-image-compositor.c
+@@ -2601,14 +2601,14 @@ _inplace_src_spans (void *abstract_renderer, int y, int h,
+ 		    unsigned num_spans)
+ {
+     cairo_image_span_renderer_t *r = abstract_renderer;
+-    uint8_t *m;
++    uint8_t *m, *base = (uint8_t*)pixman_image_get_data(r->mask);
+     int x0;
+ 
+     if (num_spans == 0)
+ 	return CAIRO_STATUS_SUCCESS;
+ 
+     x0 = spans[0].x;
+-    m = r->_buf;
++    m = base;
+     do {
+ 	int len = spans[1].x - spans[0].x;
+ 	if (len >= r->u.composite.run_length && spans[0].coverage == 0xff) {
+@@ -2646,7 +2646,7 @@ _inplace_src_spans (void *abstract_renderer, int y, int h,
+ 				      spans[0].x, y,
+ 				      spans[1].x - spans[0].x, h);
+ 
+-	    m = r->_buf;
++	    m = base;
+ 	    x0 = spans[1].x;
+ 	} else if (spans[0].coverage == 0x0) {
+ 	    if (spans[0].x != x0) {
+@@ -2675,7 +2675,7 @@ _inplace_src_spans (void *abstract_renderer, int y, int h,
+ #endif
+ 	    }
+ 
+-	    m = r->_buf;
++	    m = base;
+ 	    x0 = spans[1].x;
+ 	} else {
+ 	    *m++ = spans[0].coverage;
+-- 
+2.38.1
+

package/cairo/cairo.mk

@@ -16,6 +16,8 @@ CAIRO_INSTALL_STAGING = YES
 CAIRO_IGNORE_CVES += CVE-2018-19876
 # 0003-_arc_max_angle_for_tolerance_normalized-fix-infinite.patch
 CAIRO_IGNORE_CVES += CVE-2019-6462
+# 0004-Fix-mask-usage-in-image-compositor.patch
+CAIRO_IGNORE_CVES += CVE-2020-35492
 
 CAIRO_CONF_ENV = LIBS="$(CAIRO_LIBS)"