[glib/wip/oholy/x-gvfs-notrash: 7/10] guri: Fix buffer overrun when decoding %-encoded URI components

[Philip Withnall <pwithnall src gnome org>]

commit 63d326cb53e05bae574bf6d9d357c5c2b6c30655
Author: Philip Withnall <withnall endlessm com>
Date:   Wed Jul 1 11:54:05 2020 +0100

    guri: Fix buffer overrun when decoding %-encoded URI components
    
    There is a limited (1 or 2 byte) read off the end of the buffer if its
    final or penultimate byte is `%` and it’s not nul-terminated after that.
    If the buffer *is* nul-terminated then the first `g_ascii_isxdigit()`
    call safely returns `FALSE` and the code moves on.
    
    Fix it by adding an additional check, and some unit tests to catch the
    behaviour.
    
    This bug is present in libsoup, which `GUri` is based on, but not
    exploitable due to how the external API only exposes nul-terminated
    strings. See https://gitlab.gnome.org/GNOME/libsoup/-/merge_requests/126
    for the fix there.
    
    oss-fuzz#23815
    oss-fuzz#23818
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 glib/guri.c      | 5 +++--
 glib/tests/uri.c | 2 ++
 2 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/glib/guri.c b/glib/guri.c
index 486501fcb..18fed304a 100644
--- a/glib/guri.c
+++ b/glib/guri.c
@@ -253,10 +253,11 @@ uri_decoder (gchar       **out,
     {
       if (*s == '%')
         {
-          if (!g_ascii_isxdigit (s[1]) ||
+          if (s + 2 >= end ||
+              !g_ascii_isxdigit (s[1]) ||
               !g_ascii_isxdigit (s[2]))
             {
-              /* % followed by non-hex; this is an error */
+              /* % followed by non-hex or the end of the string; this is an error */
               if (flags & G_URI_FLAGS_PARSE_STRICT)
                 {
                   g_set_error_literal (error, G_URI_ERROR, parse_error,
diff --git a/glib/tests/uri.c b/glib/tests/uri.c
index 71bfad289..4e0f07366 100644
--- a/glib/tests/uri.c
+++ b/glib/tests/uri.c
@@ -378,6 +378,7 @@ test_uri_unescape_bytes (gconstpointer test_data)
     {
       { "%00%00", 2, (const guint8 *) "\x00\x00" },
       { "%%", -1, NULL },
+      { "%", -1, NULL },
     };
   gsize i;
 
@@ -1284,6 +1285,7 @@ test_uri_parse_params (gconstpointer test_data)
       { "%00=foo", '&', FALSE, -1, { NULL, }},
       { "p1=%00", '&', FALSE, -1, { NULL, }},
       { "p1=foo&P1=bar", '&', TRUE, 1, { "p1", "bar", NULL, }},
+      { "=%", '&', FALSE, 1, { "", "%", NULL, }},
     };
   gsize i;