[PATCH gnupg v15] Disable CPU speculation-related misfeatures

[Guido Trentalancia]

Thanks for the feedback Jeff, I have now created a v15 patch which
follows...

common: Disable CPU speculative execution security
vulnerabilities (CVE-2018-3639 aka Spectre variant 4,
CVE-2017-5715 and optionally CVE-2020-0550)

* configure.ac: add a new L1D Cache flushing option
(--enable-l1d-cache-flushing) to fix CVE-2020-0550
and check for sys/prctl.h on Linux systems

* common/init.c (early_system_init): Disable CPU
speculative execution security vulnerabilities
potentially causing data leaks:

 - Speculative Store Bypass (always disabled)
 - Indirect Branch Speculation (always disabled)
 - Flush L1D Cache on context switch out of the
   task (use the --enable-l1d-cache-flushing
   configure option and "nosmt l1d_flush=on" on the
   boot command line to mitigate the vulnerability)

For further information see the kernel documentation:
Documentation/userspace-api/spec_ctrl.rst
Documentation/admin-guide/hw-vuln/l1d_flush.rst

Signed-off-by: Guido Trentalancia <guido at trentalancia.com>

diff -pru a/common/init.c b/common/init.c
--- a/common/init.c	2025-05-25 15:43:45.871984100 +0200
+++ b/common/init.c	2025-07-09 13:39:44.036998821 +0200
@@ -29,6 +29,16 @@
 
 #include <config.h>
 
+#if defined(__linux__) && defined(HAVE_SYS_PRCTL_H)
+# include <sys/prctl.h>
+#endif
+
+#if defined(ENABLE_L1D_CACHE_FLUSH)
+# include "util.h"
+# include "sysutils.h"
+# include <signal.h>
+#endif
+
 #ifdef HAVE_W32_SYSTEM
 # if _WIN32_WINNT < 0x0600
 #   define _WIN32_WINNT 0x0600  /* Required for SetProcessDEPPolicy.  */
@@ -128,10 +136,69 @@ writestring_via_estream (int mode, const
 }
 
 
+#ifdef ENABLE_L1D_CACHE_FLUSH
+#define SIGBUS_ERROR "Fatal: Level 1 Data Cache flushing requires the \"nosmt\" boot parameter.\n"
+void sigbus_handler (int signo)
+{
+  if (signo == SIGBUS)
+    {
+      write (STDOUT_FILENO, SIGBUS_ERROR, strlen(SIGBUS_ERROR));
+      exit (SIGBUS);
+    }
+}
+#endif
+
+
 /* This function should be the first called after main.  */
 void
 early_system_init (void)
 {
+#if defined(__linux__) && defined(HAVE_SYS_PRCTL_H)
+
+/* Disable CPU speculative execution security vulnerabilities
+ * causing data leaks: see the Linux kernel documentation
+ * Documentation/userspace-api/spec_ctrl.rst
+ *
+ * - Speculative Store Bypass (CVE-2018-3639, always
+ *   disabled)
+ * - Indirect Branch Speculation (CVE-2017-5715, always
+ *   disabled)
+ * - Flush L1D Cache on context switch out of the task (it
+ *   requires the "nosmt l1d_flush=on" kernel boot parameter)
+ */
+#ifdef PR_SPEC_STORE_BYPASS
+  prctl (PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
+#endif
+
+#ifdef PR_SPEC_INDIRECT_BRANCH
+  prctl (PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
+#endif
+
+#if defined(ENABLE_L1D_CACHE_FLUSH) && defined(PR_SPEC_L1D_FLUSH)
+  struct sigaction old_action, new_action;
+
+  new_action.sa_handler = sigbus_handler;
+  sigemptyset (&new_action.sa_mask);
+  new_action.sa_flags = 0;
+
+  if (sigaction (SIGBUS, &new_action, &old_action) == -1)
+    {
+      printf ("Warning: cannot catch the SIGBUS signal.\n");
+    }
+
+  if (prctl (PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0) < 0)
+    {
+      printf ("Warning: Level 1 Data Cache flushing requires the \"l1d_flush=on\" boot parameter.\n");
+    }
+
+  gnupg_sleep (1);
+  if (sigaction (SIGBUS, &old_action, NULL) == -1)
+    {
+      printf ("Warning: cannot restore previous action on SIGBUS.\n");
+    }
+#endif
+
+#endif /* __linux__ && HAVE_SYS_PRCTL_H */
 }
 
 
diff -pru a/configure.ac b/configure.ac
--- a/configure.ac	2025-07-06 18:01:54.128546282 +0200
+++ b/configure.ac	2025-07-08 21:32:32.674405293 +0200
@@ -244,6 +244,16 @@ AC_ARG_ENABLE(selinux-support,
 AC_MSG_RESULT($selinux_support)
 
 
+# Fix security vulnerability CVE-2020-0550 by enabling
+# Level 1 Data Cache flushing on context switch.
+AC_MSG_CHECKING([whether Level 1 Data Cache is flushed on context switch])
+AC_ARG_ENABLE(l1d-cache-flushing,
+              AS_HELP_STRING([--enable-l1d-cache-flushing],
+                             [enable L1D cache flushing]),
+              l1d_cache_flushing=$enableval, l1d_cache_flushing=no)
+AC_MSG_RESULT($l1d_cache_flushing)
+
+
 AC_MSG_CHECKING([whether to allocate extra secure memory])
 AC_ARG_ENABLE(large-secmem,
               AS_HELP_STRING([--enable-large-secmem],
@@ -1313,6 +1323,16 @@ fi
 
 
 #
+# Level 1 Data Cache flushing on context switch (CVE-2020-0550)
+#
+if test "$l1d_cache_flushing" = yes ; then
+  AC_DEFINE(ENABLE_L1D_CACHE_FLUSH,1,
+          [Define to enable Layer 1 Data Cache flushing])
+  AC_CHECK_HEADERS([signal.h])
+fi
+
+
+#
 # Checks for header files.
 #
 AC_MSG_NOTICE([checking for header files])
@@ -1322,6 +1342,13 @@ AC_CHECK_HEADERS([unistd.h langinfo.h te
                   ucred.h sys/ucred.h sys/sysmacros.h sys/mkdev.h])
 
 
+# See whether libc supports the prctl()
+case "${host}" in
+    *-*-linux*)
+        AC_CHECK_HEADERS([sys/prctl.h])
+        ;;
+esac
+
 #
 # Checks for typedefs, structures, and compiler characteristics.
 #

On Thu, 10/07/2025 at 13.51 -0400, Jeffrey Walton wrote:
> On Thu, Jul 10, 2025 at 11:15 AM Guido Trentalancia via Gnupg-devel
> <gnupg-devel at gnupg.org> wrote:
> > 
> > [...]
> > I have switched to simple printf() calls in a new v13 patch, which
> > is also my last attempt to fix this.
> 
> You cannot use printf in a signal handler. It is _not_ AS-Safe. See
> the signal-safety(7) man page, like
> <https://man7.org/linux/man-pages/man7/signal-safety.7.html>.
> 
> Jeff