[glib/glib-2-70: 1/2] gutils: Avoid segfault in g_get_user_database_entry

[Simon McVittie <smcv src gnome org>]

commit 47114f52ddefe7b04b5c2c4e232bad287ad6af8d
Author: Jamie Bainbridge <jamie bainbridge gmail com>
Date:   Wed Sep 8 12:08:17 2021 +1000

    gutils: Avoid segfault in g_get_user_database_entry
    
    g_get_user_database_entry() capitalises the first letter of pw_name
    with g_ascii_toupper (pw->pw_name[0]).
    
    However, the manpage for getpwnam() and getpwuid() says the result of
    those calls "may point to a static area". GLib is then trying to edit
    static memory which belongs to a shared library, so segfaults.
    
    The reentrant variants of the above calls are supposed to fill the user
    buffer supplied to them, however Michael Catanzaro also found a bug in
    systemd where the data is not copied to the user buffer and still points
    to static memory, resulting in the same sort of segfault. See:
    https://github.com/systemd/systemd/issues/20679
    
    Solve both these cases in GLib by copying pw_name off to a temporary
    variable, set uppercase on that variable, and use the variable to join
    into the desired string. Free the variable after it is no longer needed.
    
    Signed-off-by: Jamie Bainbridge <jamie bainbridge gmail com>

 glib/gutils.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/glib/gutils.c b/glib/gutils.c
index b7a2113d4..4bccd7229 100644
--- a/glib/gutils.c
+++ b/glib/gutils.c
@@ -692,14 +692,17 @@ g_get_user_database_entry (void)
               {
                 gchar **gecos_fields;
                 gchar **name_parts;
+                gchar *uppercase_pw_name;
 
                 /* split the gecos field and substitute '&' */
                 gecos_fields = g_strsplit (pw->pw_gecos, ",", 0);
                 name_parts = g_strsplit (gecos_fields[0], "&", 0);
-                pw->pw_name[0] = g_ascii_toupper (pw->pw_name[0]);
-                e.real_name = g_strjoinv (pw->pw_name, name_parts);
+                uppercase_pw_name = g_strdup (pw->pw_name);
+                uppercase_pw_name[0] = g_ascii_toupper (uppercase_pw_name[0]);
+                e.real_name = g_strjoinv (uppercase_pw_name, name_parts);
                 g_strfreev (gecos_fields);
                 g_strfreev (name_parts);
+                g_free (uppercase_pw_name);
               }
 #endif