[PATCH v4] arm64: errata: Fix exec handling in erratum 1418040 workaround

[D Scott Phillips]

The erratum 1418040 workaround enables vct access trapping when executing
compat threads. The workaround is applied when switching between tasks, but
the need for the workaround could also change at an exec(), when a
non-compat task execs a compat binary or vice versa. Apply the workaround
in arch_setup_new_exec().

The leaves a small window of time between SET_PERSONALITY and
arch_setup_new_exec where preemption could occur and confuse the old
workaround logic that compares TIF_32BIT between prev and next. Instead, we
can just read cntkctl to make sure it's in the state that the next task
needs. I measured cntkctl read time to be about the same as a mov from a
general-purpose register on N1. Update the workaround logic to examine the
current value of cntkctl instead of the previous task's compat state.

Fixes: d49f7d7376d0 ("arm64: Move handling of erratum 1418040 into C code")
Signed-off-by: D Scott Phillips <scott at os.amperecomputing.com>
Cc: <stable at vger.kernel.org> # 5.4.x
---

 v4: - Move exec() handling into arch_setup_new_exec(), drop prev32==next32
       comparison to fix possible confusion in the small window between
       SET_PERSONALITY() and arch_setup_new_exec(). (Catalin)

 v3: - Un-nest conditionals (Marc)

 v2: - Use sysreg_clear_set instead of open coding (Marc)
     - guard this_cpu_has_cap() check under IS_ENABLED() to avoid tons of
       WARN_ON(preemptible()) when built with !CONFIG_ARM64_ERRATUM_1418040

 arch/arm64/kernel/process.c | 34 ++++++++++++----------------------
 1 file changed, 12 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index aacf2f5559a8..b37ff23e625e 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -439,34 +439,23 @@ static void entry_task_switch(struct task_struct *next)
 
 /*
  * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT.
- * Assuming the virtual counter is enabled at the beginning of times:
- *
- * - disable access when switching from a 64bit task to a 32bit task
- * - enable access when switching from a 32bit task to a 64bit task
+ * Ensure access is disabled when switching to a 32bit task, ensure
+ * access is enabled when switching to a 64bit task.
  */
-static void erratum_1418040_thread_switch(struct task_struct *prev,
-					  struct task_struct *next)
+static void erratum_1418040_thread_switch(struct task_struct *next)
 {
-	bool prev32, next32;
-	u64 val;
-
-	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040))
-		return;
+	preempt_disable();
 
-	prev32 = is_compat_thread(task_thread_info(prev));
-	next32 = is_compat_thread(task_thread_info(next));
-
-	if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
+	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) ||
+	    !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
 		return;
 
-	val = read_sysreg(cntkctl_el1);
-
-	if (!next32)
-		val |= ARCH_TIMER_USR_VCT_ACCESS_EN;
+	if (is_compat_thread(task_thread_info(next)))
+		sysreg_clear_set(cntkctl_el1, ARCH_TIMER_USR_VCT_ACCESS_EN, 0);
 	else
-		val &= ~ARCH_TIMER_USR_VCT_ACCESS_EN;
+		sysreg_clear_set(cntkctl_el1, 0, ARCH_TIMER_USR_VCT_ACCESS_EN);
 
-	write_sysreg(val, cntkctl_el1);
+	preempt_enable();
 }
 
 /*
@@ -501,7 +490,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	contextidr_thread_switch(next);
 	entry_task_switch(next);
 	ssbs_thread_switch(next);
-	erratum_1418040_thread_switch(prev, next);
+	erratum_1418040_thread_switch(next);
 	ptrauth_thread_switch_user(next);
 
 	/*
@@ -611,6 +600,7 @@ void arch_setup_new_exec(void)
 	current->mm->context.flags = mmflags;
 	ptrauth_thread_init_user();
 	mte_thread_init_user();
+	erratum_1418040_thread_switch(current);
 
 	if (task_spec_ssb_noexec(current)) {
 		arch_prctl_spec_ctrl_set(current, PR_SPEC_STORE_BYPASS,
-- 
2.31.1