x86: stackprotector: Avoid random pool on rt

CPU bringup calls into the random pool to initialize the stack
canary. During boot that works nicely even on RT as the might sleep
checks are disabled. During CPU hotplug the might sleep checks
trigger. Making the locks in random raw is a major PITA, so avoid the
call on RT is the only sensible solution. This is basically the same
randomness which we get during boot where the random pool has no
entropy and we rely on the TSC randomnness.

Reported-by: Carsten Emde <carsten.emde@osadl.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 9 insertions, 1 deletions

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 157517763565..ac0703bcfe17 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -58,7 +58,7 @@
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	u64 canary;
+	u64 uninitialized_var(canary);
 	u64 tsc;
 
 #ifdef CONFIG_X86_64
@@ -69,8 +69,16 @@ static __always_inline void boot_init_stack_canary(void)
 	 * of randomness. The TSC only matters for very early init,
 	 * there it already has some randomness on most systems. Later
 	 * on during the bootup the random pool has true entropy too.
+	 *
+	 * For preempt-rt we need to weaken the randomness a bit, as
+	 * we can't call into the random generator from atomic context
+	 * due to locking constraints. We just leave canary
+	 * uninitialized and use the TSC based randomness on top of
+	 * it.
 	 */
+#ifndef CONFIG_PREEMPT_RT_FULL
 	get_random_bytes(&canary, sizeof(canary));
+#endif
 	tsc = __native_read_tsc();
 	canary += tsc + (tsc << 32UL);