glib2/CVE-2026-58013.patch

From b440f8dc6d2764e95655fd2d384cbfecfab9f199 Mon Sep 17 00:00:00 2001
From: Philip Withnall <pwithnall@gnome.org>
Date: Tue, 28 Apr 2026 16:45:14 +0100
Subject: [PATCH] giochannel: Fix memcmp() off the end of the buffer with long
 terminators
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If the line terminator is longer than a single byte, and the current
line extends to the end of the buffer, and the buffer (which is a
`GString`) is near a power of two in length (as that’s how `GString`s
are allocated) it’s possible for the `memcmp()` which checks the
terminator to read off the end of the string buffer.

Fix that by checking the terminator length against the last character
before calling `memcmp()`. Add a unit test.

Spotted by linhlhq as #YWH-PGM9867-199. The fix is theirs (validated by
me), and the unit test is adapted from their proof of concept.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: #3925
---
 glib/giochannel.c       |  3 +-
 glib/tests/io-channel.c | 61 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/glib/giochannel.c b/glib/giochannel.c
index b44fff35b..4e682e2a9 100644
--- a/glib/giochannel.c
+++ b/glib/giochannel.c
@@ -1822,7 +1822,8 @@ read_again:
         {
           if (channel->line_term)
             {
-              if (memcmp (channel->line_term, nextchar, line_term_len) == 0)
+              if ((size_t) (lastchar - nextchar) >= line_term_len &&
+                  memcmp (channel->line_term, nextchar, line_term_len) == 0)
                 {
                   line_length = nextchar - use_buf->str;
                   got_term_len = line_term_len;
diff --git a/glib/tests/io-channel.c b/glib/tests/io-channel.c
index c5dd01d04..4b4189b68 100644
--- a/glib/tests/io-channel.c
+++ b/glib/tests/io-channel.c
@@ -30,6 +30,7 @@
 
 #include <glib.h>
 #include <glib/gstdio.h>
+#include <stdint.h>
 
 static void
 test_small_writes (void)
@@ -216,6 +217,65 @@ test_read_line_embedded_nuls (void)
   g_free (filename);
 }
 
+static void
+test_read_line_long_terminator (void)
+{
+  uint8_t *test_data = NULL;
+  size_t test_data_len = 0;
+  int fd;
+  char *filename = NULL;
+  GIOChannel *channel = NULL;
+  GError *local_error = NULL;
+  char *line = NULL;
+  size_t line_length, terminator_pos;
+  const char *line_term;
+  int line_term_length;
+  GIOStatus status;
+
+  g_test_summary ("Test that reading a line when using a long terminator doesn’t over-read the buffer.");
+  g_test_bug ("https://gitlab.gnome.org/GNOME/glib/-/work_items/3925");
+
+  /* Write out a temporary file containing 2047 bytes. This is enough to make it
+   * near the length of the GString buffer when read back in. */
+  fd = g_file_open_tmp ("glib-test-io-channel-XXXXXX", &filename, &local_error);
+  g_assert_no_error (local_error);
+  g_close (g_steal_fd (&fd), NULL);
+
+  test_data_len = 2047;
+  test_data = g_malloc (test_data_len);
+  memset (test_data, 'M', test_data_len);
+  g_file_set_contents (filename, (const gchar *) test_data, test_data_len, &local_error);
+  g_assert_no_error (local_error);
+
+  /* Create the channel. */
+  channel = g_io_channel_new_file (filename, "r", &local_error);
+  g_assert_no_error (local_error);
+
+  /* Use a long line terminator so it could potentially over-read the end of the buffer. */
+  g_io_channel_set_line_term (channel, "DEADBEEF", 8);
+
+  line_term = g_io_channel_get_line_term (channel, &line_term_length);
+  g_assert_cmpstr (line_term, ==, "DEADBEEF");
+  g_assert_cmpint (line_term_length, ==, 8);
+
+  g_io_channel_set_encoding (channel, "UTF-8", &local_error);
+  g_assert_no_error (local_error);
+
+  status = g_io_channel_read_line (channel, &line, &line_length,
+                                   &terminator_pos, &local_error);
+  g_assert_no_error (local_error);
+  g_assert_cmpint (status, ==, G_IO_STATUS_NORMAL);
+  g_assert_cmpuint (line_length, ==, 2047);
+  g_assert_cmpuint (terminator_pos, ==, 2047);
+  g_assert_cmpmem (line, line_length, test_data, test_data_len);
+
+  g_free (line);
+  g_io_channel_unref (channel);
+  g_free (test_data);
+  g_unlink (filename);
+  g_free (filename);
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -224,6 +284,7 @@ main (int   argc,
 
   g_test_add_func ("/io-channel/read-write", test_read_write);
   g_test_add_func ("/io-channel/read-line/embedded-nuls", test_read_line_embedded_nuls);
+  g_test_add_func ("/io-channel/read-line/long-terminator", test_read_line_long_terminator);
 
   return g_test_run ();
 }