Re: [PATCH] powerpc/64s: support nospectre_v2 cmdline option

On May 5, 2019 at 9:32 PM Andrew Donnellan [off-list ref] wrote:


On 6/5/19 8:10 am, Christopher M. Riedl wrote:

quoted

Add support for disabling the kernel implemented spectre v2 mitigation
(count cache flush on context switch) via the nospectre_v2 cmdline
option.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Christopher M. Riedl <redacted>
---

  arch/powerpc/kernel/security.c | 14 ++++++++++++--
  1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
index b33bafb8fcea..f7c34745cd0f 100644
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c

@@ -28,7 +28,7 @@ static enum count_cache_flush_type count_cache_flush_type = COUNT_CACHE_FLUSH_NO
  bool barrier_nospec_enabled;
  static bool no_nospec;
  static bool btb_flush_enabled;
-#ifdef CONFIG_PPC_FSL_BOOK3E
+#if defined(CONFIG_PPC_FSL_BOOK3E) || defined(CONFIG_PPC_BOOK3S_64)
  static bool no_spectrev2;
  #endif
  

@@ -106,7 +106,7 @@ static __init int barrier_nospec_debugfs_init(void)
  device_initcall(barrier_nospec_debugfs_init);
  #endif /* CONFIG_DEBUG_FS */
  
-#ifdef CONFIG_PPC_FSL_BOOK3E
+#if defined(CONFIG_PPC_FSL_BOOK3E) || defined(CONFIG_PPC_BOOK3S_64)
  static int __init handle_nospectre_v2(char *p)
  {
  	no_spectrev2 = true;

@@ -114,6 +114,9 @@ static int __init handle_nospectre_v2(char *p)
  	return 0;
  }
  early_param("nospectre_v2", handle_nospectre_v2);
+#endif /* CONFIG_PPC_FSL_BOOK3E || CONFIG_PPC_BOOK3S_64 */
+
+#ifdef CONFIG_PPC_FSL_BOOK3E
  void setup_spectre_v2(void)
  {
  	if (no_spectrev2)

@@ -391,6 +394,13 @@ static void toggle_count_cache_flush(bool enable)
  
  void setup_count_cache_flush(void)
  {
+	if (no_spectrev2) {
+		if (security_ftr_enabled(SEC_FTR_BCCTRL_SERIALISED)
+		    || security_ftr_enabled(SEC_FTR_COUNT_CACHE_DISABLED))
+			pr_warn("Spectre v2 mitigations not under software control, can't disable\n");

If one of those ftrs is set, what's the impact of not calling 
toggle_count_cache_flush()?

The patchsite/callsite (kernel/entry_64.S:597) for flush_count_cache
inside _switch remains a nop.

Disassembly of vmlinux after build:
c00000000000e260:       00 00 23 f8     std     r1,0(r3)
c00000000000e264:       00 00 00 60     nop
c00000000000e268:       00 60 c0 3c     lis     r6,24576

Disassembly (xmon) after boot/during runtime in qemu:
c00000000000e260  f8230000	std     r1,0(r3)
c00000000000e264  4bffdb7d	bl      c00000000000bde0	# flush_count_cache+0x0/0x2420
c00000000000e268  3cc06000	lis     r6,24576

Disassembly (xmon) after boot/during runtime in qemu w/ nospectre_v2:
c00000000000e260  f8230000	std     r1,0(r3)
c00000000000e264  60000000	nop
c00000000000e268  3cc06000	lis     r6,24576

toggle_count_cache_flush() well uhh "toggles" the patchsite to either
contain a branch to the flush procedure or a nop.

quoted

+		return;
+	}
+
  	toggle_count_cache_flush(true);
  }
  

-- 
Andrew Donnellan              OzLabs, ADL Canberra
ajd@linux.ibm.com             IBM Australia Limited