+CC Nicolas.

On Thu, Oct 21, 2021 at 03:39:35PM -0300, Marcelo Tosatti wrote:

Peter,

static __always_inline void arch_exit_to_user_mode(void)
{
        mds_user_clear_cpu_buffers();
}

/**
 * mds_user_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
 *
 * Clear CPU buffers if the corresponding static key is enabled
 */
static __always_inline void mds_user_clear_cpu_buffers(void)
{
        if (static_branch_likely(&mds_user_clear))
                mds_clear_cpu_buffers();
}

We were discussing how to perform objtool style validation 
that no code after the check for 

quoted

+             /* NMI happens here and must still do/finish CT_WORK_n */
+             sync_core();

But after the discussion with you, it seems doing the TLB checking 
and (also sync_core) checking very late/very early on exit/entry 
makes things easier to review.

Can then use a single atomic variable with USER/KERNEL state and cmpxchg
loops.

On Wed, Sep 29, 2021 at 05:17:34PM +0200, Peter Zijlstra wrote:

quoted

Use the new context_tracking infrastructure to avoid disturbing
userspace tasks when we rewrite kernel code.

XXX re-audit the entry code to make sure only the context_tracking
static_branch is before hitting this code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/sync_core.h |    2 ++
 arch/x86/kernel/alternative.c    |    8 +++++++-
 include/linux/context_tracking.h |    1 +
 kernel/context_tracking.c        |   12 ++++++++++++
 4 files changed, 22 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/sync_core.h
+++ b/arch/x86/include/asm/sync_core.h

@@ -87,6 +87,8 @@ static inline void sync_core(void)
 	 */
 	iret_to_self();
 }
+#define sync_core sync_core
+
 
 /*
  * Ensure that a core serializing instruction is issued before returning
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c

@@ -18,6 +18,7 @@
 #include <linux/mmu_context.h>
 #include <linux/bsearch.h>
 #include <linux/sync_core.h>
+#include <linux/context_tracking.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>

@@ -924,9 +925,14 @@ static void do_sync_core(void *info)
 	sync_core();
 }
 
+static bool do_sync_core_cond(int cpu, void *info)
+{
+	return !context_tracking_set_cpu_work(cpu, CT_WORK_SYNC);
+}
+
 void text_poke_sync(void)
 {
-	on_each_cpu(do_sync_core, NULL, 1);
+	on_each_cpu_cond(do_sync_core_cond, do_sync_core, NULL, 1);
 }
 
 struct text_poke_loc {
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h

@@ -11,6 +11,7 @@
 
 enum ct_work {
 	CT_WORK_KLP = 1,
+	CT_WORK_SYNC = 2,
 };
 
 /*
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c

@@ -51,6 +51,10 @@ static __always_inline void context_trac
 	__this_cpu_dec(context_tracking.recursion);
 }
 
+#ifndef sync_core
+static inline void sync_core(void) { }
+#endif
+
 /* CT_WORK_n, must be noinstr, non-blocking, NMI safe and deal with spurious calls */
 static noinstr void ct_exit_user_work(struct context_tracking *ct)
 {

@@ -64,6 +68,14 @@ static noinstr void ct_exit_user_work(struct
 		arch_atomic_andnot(CT_WORK_KLP, &ct->work);
 	}
 
+	if (work & CT_WORK_SYNC) {
+		/* NMI happens here and must still do/finish CT_WORK_n */
+		sync_core();
+
+		smp_mb__before_atomic();
+		arch_atomic_andnot(CT_WORK_SYNC, &ct->work);
+	}
+
 	smp_mb__before_atomic();
 	arch_atomic_andnot(CT_SEQ_WORK, &ct->seq);
 }