s390: add support for BEAR enhancement facility

The Breaking-Event-Address-Register (BEAR) stores the address of the
last breaking event instruction. Breaking events are usually instructions
that change the program flow - for example branches, and instructions
that modify the address in the PSW like lpswe. This is useful for debugging
wild branches, because one could easily figure out where the wild branch
was originating from.

What is problematic is that lpswe is considered a breaking event, and
therefore overwrites BEAR on kernel exit. The BEAR enhancement facility
adds new instructions that allow to save/restore BEAR and also an lpswey
instruction that doesn't cause a breaking event. So we can save BEAR on
kernel entry and restore it on exit to user space.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c
index 2b1c6d916cf9..7d9705eeb02f 100644
--- a/arch/s390/mm/vmem.c
+++ b/arch/s390/mm/vmem.c
@@ -13,6 +13,7 @@
 #include <linux/hugetlb.h>
 #include <linux/slab.h>
 #include <asm/cacheflush.h>
+#include <asm/nospec-branch.h>
 #include <asm/pgalloc.h>
 #include <asm/setup.h>
 #include <asm/tlbflush.h>
@@ -584,8 +585,13 @@ void __init vmem_map_init(void)
 	__set_memory(__stext_amode31, (__etext_amode31 - __stext_amode31) >> PAGE_SHIFT,
 		     SET_MEMORY_RO | SET_MEMORY_X);
 
-	/* we need lowcore executable for our LPSWE instructions */
-	set_memory_x(0, 1);
+	if (nospec_uses_trampoline() || !static_key_enabled(&cpu_has_bear)) {
+		/*
+		 * Lowcore must be executable for LPSWE
+		 * and expoline trampoline branch instructions.
+		 */
+		set_memory_x(0, 1);
+	}
 
 	pr_info("Write protected kernel read-only data: %luk\n",
 		(unsigned long)(__end_rodata - _stext) >> 10);