[PATCH libgcrypt] Disable CPU speculation-related misfeatures

[Guido Trentalancia]

Disable CPU speculation-related misfeatures which are in
fact vulnerabilities causing data leaks:

 - Speculative Store Bypass
 - Indirect Branch Speculation
 - Flush L1D Cache on context switch out of the task

For further information see the kernel documentation:
Documentation/userspace-api/spec_ctrl.rst

Signed-off-by: Guido Trentalancia <guido at trentalancia.com>
---
 src/global.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

--- a/src/global.c	2024-04-26 14:08:12.000000000 +0200
+++ b/src/global.c	2025-05-25 16:03:24.718598884 +0200
@@ -33,6 +33,7 @@
 #ifdef HAVE_SYSLOG
 # include <syslog.h>
 #endif /*HAVE_SYSLOG*/
+#include <sys/prctl.h>
 
 #include "g10lib.h"
 #include "gcrypt-testapi.h"
@@ -95,6 +96,26 @@ global_init (void)
     return;
   _gcry_global_any_init_done = 1;
 
+/* Disable CPU speculation-related misfeatures which are in
+ * fact vulnerabilities causing data leaks: see the kernel
+ * documentation: Documentation/userspace-api/spec_ctrl.rst
+ *
+ * - Speculative Store Bypass
+ * - Indirect Branch Speculation
+ * - Flush L1D Cache on context switch out of the task
+ */
+#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
+
+#ifdef PR_SPEC_L1D_FLUSH
+  prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_FORCE_DISABLE, 0, 0);
+#endif
+
   /* Tell the random module that we have seen an init call.  */
   _gcry_set_preferred_rng_type (0);