From 159042c71bbdd5909f792208dcdffffb1674ecfe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20P=C3=ADsa=C5=99?= <ppisar@redhat.com>
Date: Thu, 19 Aug 2021 16:07:06 +0200
Subject: [PATCH] Fix a crash in gss_release_oid() when destructing out_mech
 returned by gss_accept_sec_context()
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If Perl GSSAPI was built against MIT krb5, an example gss-server.pl
script crashed like this:

    Program terminated with signal SIGSEGV, Segmentation fault.
    #0  0x00007f27f3d48b23 in __GI___libc_free (mem=<optimized out>)
        at malloc.c:3131
    3131	  ar_ptr = arena_for_chunk (p);
    (gdb) bt
    #0  0x00007f27f3d48b23 in __GI___libc_free (mem=<optimized out>)
        at malloc.c:3131
    #1  0x00007f27f2fe17c6 in generic_gss_release_oid (
        minor_status=minor_status@entry=0x7fffc750333c,
        oid=oid@entry=0x7fffc7503340) at oid_ops.c:102
    #2  0x00007f27f2fee6df in gss_release_oid (
        minor_status=minor_status@entry=0x7fffc750333c,
        oid=oid@entry=0x7fffc7503340) at g_initialize.c:202
    #3  0x00007f27f322f5cf in XS_GSSAPI__OID_DESTROY (my_perl=<optimized out>,
        cv=0x564037c87130) at ./xs/OID.xs:24
    #4  0x00007f27f4f58149 in Perl_pp_entersub (my_perl=0x5640378d42a0)
        at pp_hot.c:4227

The cause is that gss_accept_sec_context() returns a pointer to
a static storage in out_mech argument. When GSSAPI passed out_mech to
a desctructor, the invoked gss_release_oid() crashed when freeing the
memory.

Accoding to RFC 2744, the static storage is correct. Hence the flaw is
on Perl GSSAPI side. This patch fixes it by copying the out_mech OID
object on a heap which is then correctly processed by
gss_release_oid().

CPAN RT#121873.

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 xs/Context.xs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/xs/Context.xs b/xs/Context.xs
index d176f08..4549595 100644
--- a/xs/Context.xs
+++ b/xs/Context.xs
@@ -80,6 +80,24 @@ accept(context, acc_cred, in_token, binding, out_name, out_mech, out_token, out_
 				       &in_token, binding, out_name, out_mech,
 				       &out_token, out_flags, out_time,
 				       delegated_cred);
+#if !defined(HEIMDAL)
+	if (out_mech && *out_mech) {
+		/* RFC 2744 documents that the returned *out_mech is a pointer
+		 * to static data. To prevent from freeing them when destructing
+		 * out_mech, we change *out_mech into a pointer to a heap-allocated
+		 * buffer with the same content. Otherwise, MITKRB5-provided
+		 * gss_release_oid() deallocator which cannot recognize this static
+		 * storage would crash. We use malloc() because gss_release_oid() used
+		 * free(). */
+		GSSAPI__OID copy = malloc(sizeof(*copy));
+		if (!copy) croak("Not enough memory for copying out_mech!");
+		copy->elements = malloc((*out_mech)->length);
+		if (!copy->elements) croak("Not enough memory for copying out_mech!");
+		memcpy(copy->elements, (*out_mech)->elements, (*out_mech)->length);
+		copy->length = (*out_mech)->length;
+		*out_mech = copy;
+    }
+#endif
     OUTPUT:
 	RETVAL
 	context
-- 
2.31.1