[PATCH] arm64: mte: Ensure TIF_MTE_ASYNC_FAULT is set atomically

[Catalin Marinas]

The entry from EL0 code checks the TFSRE0_EL1 register for any
asynchronous tag check faults in user space and sets the
TIF_MTE_ASYNC_FAULT flag. This is not done atomically, potentially
racing with another CPU calling set_tsk_thread_flag().

Replace the non-atomic ORR+STR with an STSET instruction. While STSET
requires ARMv8.1 and an assembler that understands LSE atomics, the MTE
feature is part of ARMv8.5 and already requires an updated assembler.

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Fixes: 637ec831ea4f ("arm64: mte: Handle synchronous and asynchronous tag check faults")
Cc: <stable at vger.kernel.org> # 5.10.x
Reported-by: Will Deacon <will at kernel.org>
Cc: Will Deacon <will at kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
---

Slightly updated from the inline diff I posted. The MTE dependency is only on
AS_HAS_LSE_ATOMICS rather than the whole LSE feature being enabled.

 arch/arm64/Kconfig        |  6 +++++-
 arch/arm64/kernel/entry.S | 10 ++++++----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 9b4d629f7628..1573cbdf372e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1401,10 +1401,13 @@ config ARM64_PAN
 config AS_HAS_LDAPR
 	def_bool $(as-instr,.arch_extension rcpc)
 
+config AS_HAS_LSE_ATOMICS
+	def_bool $(as-instr,.arch_extension lse)
+
 config ARM64_LSE_ATOMICS
 	bool
 	default ARM64_USE_LSE_ATOMICS
-	depends on $(as-instr,.arch_extension lse)
+	depends on AS_HAS_LSE_ATOMICS
 
 config ARM64_USE_LSE_ATOMICS
 	bool "Atomic instructions"
@@ -1648,6 +1651,7 @@ config ARM64_MTE
 	default y
 	depends on ARM64_AS_HAS_MTE && ARM64_TAGGED_ADDR_ABI
 	depends on AS_HAS_ARMV8_5
+	depends on AS_HAS_LSE_ATOMICS
 	# Required for tag checking in the uaccess routines
 	depends on ARM64_PAN
 	select ARCH_USES_HIGH_VMA_FLAGS
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a45b4ebbfe7d..ad29892f2974 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -148,16 +148,18 @@ alternative_cb_end
 	.endm
 
 	/* Check for MTE asynchronous tag check faults */
-	.macro check_mte_async_tcf, flgs, tmp
+	.macro check_mte_async_tcf, tmp, ti_flags
 #ifdef CONFIG_ARM64_MTE
+	.arch_extension lse
 alternative_if_not ARM64_MTE
 	b	1f
 alternative_else_nop_endif
 	mrs_s	\tmp, SYS_TFSRE0_EL1
 	tbz	\tmp, #SYS_TFSR_EL1_TF0_SHIFT, 1f
 	/* Asynchronous TCF occurred for TTBR0 access, set the TI flag */
-	orr	\flgs, \flgs, #_TIF_MTE_ASYNC_FAULT
-	str	\flgs, [tsk, #TSK_TI_FLAGS]
+	mov	\tmp, #_TIF_MTE_ASYNC_FAULT
+	add	\ti_flags, tsk, #TSK_TI_FLAGS
+	stset	\tmp, [\ti_flags]
 	msr_s	SYS_TFSRE0_EL1, xzr
 1:
 #endif
@@ -244,7 +246,7 @@ alternative_else_nop_endif
 	disable_step_tsk x19, x20
 
 	/* Check for asynchronous tag check faults in user space */
-	check_mte_async_tcf x19, x22
+	check_mte_async_tcf x22, x23
 	apply_ssbd 1, x22, x23
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23