On Tue, 08/07/2025 at 20.02 -0500, Jacob Bachmeyer wrote:
> On 7/8/25 14:38, Guido Trentalancia via Gnupg-devel wrote:
> > The following new v10 patch has been created to fix a missing
> > #ifdef
> > and header file check for the case of L1D cache flushing.
> > [...]
>
> Two major issues, further explained inline below:
> - You are installing the signal handler incorrectly; this will
> interfere with other possible uses of SIGBUS.
> - Your message from configure when testing the option to request
> L1 cache flushes is misleading.
> Further, none of this actually *fixes* anything; these are
> *workarounds* for widespread hardware bugs.
> Also, have you actually tested this on a machine where the request
> for L1 cache flushes raises SIGBUS?
I have tested it with SIGBUS (just boot with "l1d_flush=on", but
without adding "nosmt"): it needs a 1-second delay in order to properly
set up the signal handlers before actually logging the error, such
delay is introduced in v14 patch that 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 <gu...@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 +136,68 @@ writestring_via_estream (int mode, const
}
+#ifdef ENABLE_L1D_CACHE_FLUSH
+void sigbus_handler (int signo)
+{
+ if (signo == SIGBUS)
+ {
+ printf ("Fatal: Level 1 Data Cache flushing requires the \"nosmt\" boot
parameter.\n");
+ 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.
#
> > 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 2025-07-08 21:29:23.071406450 +0200
> > [...]
> > /* 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)
> > + if (signal(SIGBUS, sigbus_handler) == SIG_ERR)
> > + {
> > + log_info ("Warning: cannot catch the SIGBUS signal.\n");
> > + }
>
> You cannot use signal() here in a library because you must restore
> any previous signal handler immediately after the prctl() call.
> Other code in the program might have its own reasons to catch SIGBUS,
> and this handler will interfere with that. You must use sigaction()
> here to obtain the old handler in a form that you can use to restore
> it after calling prctl().
> > + if (prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH,
> > PR_SPEC_ENABLE, 0, 0) < 0)
> > + {
> > + log_info ("Warning: Level 1 Data Cache flushing requires the
> > \"l1d_flush=on\" boot parameter.\n");
> > + }
>
> This is where you need to restore the previous state of SIGBUS
> handling.
> > +#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])
>
> This message does not accurately describe what is going on. This
> should say "whether L1 data cache should be flushed on context
> switch" because it does *not* test whether the cache is *actually*
> flushed, but only if the option to *request* flushing the cache is
> set.
> > +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],
> > [...]
>
> -- Jacob
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