arm64: mte: avoid clearing PSTATE.TCO on entry unless necessary

On some microarchitectures, clearing PSTATE.TCO is expensive. Clearing
TCO is only necessary if in-kernel MTE is enabled, or if MTE is
enabled in the userspace process in synchronous (or, soon, asymmetric)
mode, because we do not report uaccess faults to userspace in none
or asynchronous modes. Therefore, adjust the kernel entry code to
clear TCO only if necessary.

Because it is now possible to switch to a task in which TCO needs to
be clear from a task in which TCO is set, we also need to do the same
thing on task switch.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I52d82a580bd0500d420be501af2c35fa8c90729e
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220219012945.894950-2-pcc@google.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 075539f5f1c88f42298ae4c6b824cb34a9728a7e..adcb937342f14d2d431eab7bff46e90a19f9a026 100644 (file)
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -11,7 +11,9 @@
 #ifndef __ASSEMBLY__
 
 #include <linux/bitfield.h>
+#include <linux/kasan-enabled.h>
 #include <linux/page-flags.h>
+#include <linux/sched.h>
 #include <linux/types.h>
 
 #include <asm/pgtable-types.h>
@@ -86,6 +88,26 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
 
 #endif /* CONFIG_ARM64_MTE */
 
+static inline void mte_disable_tco_entry(struct task_struct *task)
+{
+       if (!system_supports_mte())
+               return;
+
+       /*
+        * Re-enable tag checking (TCO set on exception entry). This is only
+        * necessary if MTE is enabled in either the kernel or the userspace
+        * task in synchronous or asymmetric mode (SCTLR_EL1.TCF0 bit 0 is set
+        * for both). With MTE disabled in the kernel and disabled or
+        * asynchronous in userspace, tag check faults (including in uaccesses)
+        * are not reported, therefore there is no need to re-enable checking.
+        * This is beneficial on microarchitectures where re-enabling TCO is
+        * expensive.
+        */
+       if (kasan_hw_tags_enabled() ||
+           (task->thread.sctlr_user & (1UL << SCTLR_EL1_TCF0_SHIFT)))
+               asm volatile(SET_PSTATE_TCO(0));
+}
+
 #ifdef CONFIG_KASAN_HW_TAGS
 /* Whether the MTE asynchronous mode is enabled. */
 DECLARE_STATIC_KEY_FALSE(mte_async_or_asymm_mode);

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index ef7fcefb96bd1acfb178c72afee7d15727948c55..7093b578e32502acf38401d8d5c1e52a10e50e08 100644 (file)
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -6,6 +6,7 @@
  */
 
 #include <linux/context_tracking.h>
+#include <linux/kasan.h>
 #include <linux/linkage.h>
 #include <linux/lockdep.h>
 #include <linux/ptrace.h>
@@ -56,6 +57,7 @@ static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
 {
        __enter_from_kernel_mode(regs);
        mte_check_tfsr_entry();
+       mte_disable_tco_entry(current);
 }
 
 /*
@@ -103,6 +105,7 @@ static __always_inline void __enter_from_user_mode(void)
        CT_WARN_ON(ct_state() != CONTEXT_USER);
        user_exit_irqoff();
        trace_hardirqs_off_finish();
+       mte_disable_tco_entry(current);
 }
 
 static __always_inline void enter_from_user_mode(struct pt_regs *regs)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 772ec2ecf48884f24148772ab854497b5eada522..e1013a83d4f013e5a51d485163efff923ca8d39c 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -308,13 +308,6 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING
        msr_s   SYS_ICC_PMR_EL1, x20
 alternative_else_nop_endif
 
-       /* Re-enable tag checking (TCO set on exception entry) */
-#ifdef CONFIG_ARM64_MTE
-alternative_if ARM64_MTE
-       SET_PSTATE_TCO(0)
-alternative_else_nop_endif
-#endif
-
        /*
         * Registers that may be useful after this macro is invoked:
         *

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index f418ebc65f9509f079dff585c2ac714019fffbe9..f983795b5eda29d34c03ced50281a3e2014da02d 100644 (file)
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -253,6 +253,9 @@ void mte_thread_switch(struct task_struct *next)
        mte_update_sctlr_user(next);
        mte_update_gcr_excl(next);
 
+       /* TCO may not have been disabled on exception entry for the current task. */
+       mte_disable_tco_entry(next);
+
        /*
         * Check if an async tag exception occurred at EL1.
         *