From d2ffa389b8112282be1633bb4638f6f47e159299 Mon Sep 17 00:00:00 2001
From: Topi Miettinen <toiwoton@gmail.com>
Date: Wed, 26 Oct 2016 18:52:53 +0300
Subject: seccomp: also block shmat(..., SHM_EXEC) for MemoryDenyWriteExecute

shmat(..., SHM_EXEC) can be used to create writable and executable
memory, so let's block it when MemoryDenyWriteExecute is set.
---
 man/systemd.exec.xml | 11 +++++++----
 src/core/execute.c   | 11 +++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index dbe4594730..f9a15d8db0 100644
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -1516,12 +1516,15 @@
         <term><varname>MemoryDenyWriteExecute=</varname></term>
 
         <listitem><para>Takes a boolean argument. If set, attempts to create memory mappings that are writable and
-        executable at the same time, or to change existing memory mappings to become executable are prohibited.
+        executable at the same time, or to change existing memory mappings to become executable, or mapping shared memory
+        segments as executable are prohibited.
         Specifically, a system call filter is added that rejects
         <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
-        system calls with both <constant>PROT_EXEC</constant> and <constant>PROT_WRITE</constant> set
-        and <citerefentry><refentrytitle>mprotect</refentrytitle><manvolnum>2</manvolnum></citerefentry>
-        system calls with <constant>PROT_EXEC</constant> set. Note that this option is incompatible with programs
+        system calls with both <constant>PROT_EXEC</constant> and <constant>PROT_WRITE</constant> set,
+        <citerefentry><refentrytitle>mprotect</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        system calls with <constant>PROT_EXEC</constant> set and
+        <citerefentry><refentrytitle>shmat</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        system calls with <constant>SHM_EXEC</constant> set. Note that this option is incompatible with programs
         that generate program code dynamically at runtime, such as JIT execution engines, or programs compiled making
         use of the code "trampoline" feature of various C compilers. This option improves service security, as it makes
         harder for software exploits to change running code dynamically.
diff --git a/src/core/execute.c b/src/core/execute.c
index 5e7d7c25d7..7b42ac7bdc 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -29,8 +29,10 @@
 #include <sys/mman.h>
 #include <sys/personality.h>
 #include <sys/prctl.h>
+#include <sys/shm.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <sys/un.h>
 #include <unistd.h>
 #include <utmpx.h>
@@ -1394,6 +1396,15 @@ static int apply_memory_deny_write_execute(const Unit* u, const ExecContext *c)
         if (r < 0)
                 goto finish;
 
+        r = seccomp_rule_add(
+                        seccomp,
+                        SCMP_ACT_ERRNO(EPERM),
+                        SCMP_SYS(shmat),
+                        1,
+                        SCMP_A2(SCMP_CMP_MASKED_EQ, SHM_EXEC, SHM_EXEC));
+        if (r < 0)
+                goto finish;
+
         r = seccomp_load(seccomp);
 
 finish:
-- 
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