arm64/mte: Remove asymmetric mode from the prctl() interface

As pointed out by Evgenii Stepanov one potential issue with the new ABI for
enabling asymmetric is that if there are multiple places where MTE is
configured in a process, some of which were compiled with the old prctl.h
and some of which were compiled with the new prctl.h, there may be problems
keeping track of which MTE modes are requested. For example some code may
disable only sync and async modes leaving asymmetric mode enabled when it
intended to fully disable MTE.

In order to avoid such mishaps remove asymmetric mode from the prctl(),
instead implicitly allowing it if both sync and async modes are requested.
This should not disrupt userspace since a process requesting both may
already see a mix of sync and async modes due to differing defaults between
CPUs or changes in default while the process is running but it does mean
that userspace is unable to explicitly request asymmetric mode without
changing the system default for CPUs.

Reported-by: Evgenii Stepanov <eugenis@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Evgenii Stepanov <eugenis@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Branislav Rankov <branislav.rankov@arm.com>
Link: https://lore.kernel.org/r/20220309131200.112637-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 10 insertions, 3 deletions

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index cbbd8d93fc50..78b3e0f8e997 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -311,7 +311,16 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 		mte_ctrl |= MTE_CTRL_TCF_ASYNC;
 	if (arg & PR_MTE_TCF_SYNC)
 		mte_ctrl |= MTE_CTRL_TCF_SYNC;
-	if (arg & PR_MTE_TCF_ASYMM)
+
+	/*
+	 * If the system supports it and both sync and async modes are
+	 * specified then implicitly enable asymmetric mode.
+	 * Userspace could see a mix of both sync and async anyway due
+	 * to differing or changing defaults on CPUs.
+	 */
+	if (cpus_have_cap(ARM64_MTE_ASYMM) &&
+	    (arg & PR_MTE_TCF_ASYNC) &&
+	    (arg & PR_MTE_TCF_SYNC))
 		mte_ctrl |= MTE_CTRL_TCF_ASYMM;
 
 	task->thread.mte_ctrl = mte_ctrl;
@@ -341,8 +350,6 @@ long get_mte_ctrl(struct task_struct *task)
 		ret |= PR_MTE_TCF_ASYNC;
 	if (mte_ctrl & MTE_CTRL_TCF_SYNC)
 		ret |= PR_MTE_TCF_SYNC;
-	if (mte_ctrl & MTE_CTRL_TCF_ASYMM)
-		ret |= PR_MTE_TCF_ASYMM;
 
 	return ret;
 }