[PATCH v5 05/40] arm64: mte: Add in-kernel tag fault handler

[Andrey Konovalov]

From: Vincenzo Frascino <vincenzo.frascino at arm.com>

Add the implementation of the in-kernel fault handler.

When a tag fault happens on a kernel address:
* MTE is disabled on the current CPU,
* the execution continues.

When a tag fault happens on a user address:
* the kernel executes do_bad_area() and panics.

The tag fault handler for kernel addresses is currently empty and will be
filled in by a future commit.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
Co-developed-by: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
---
Change-Id: I9b8aa79567f7c45f4d6a1290efcf34567e620717
---
 arch/arm64/include/asm/uaccess.h | 23 +++++++++++++++++++
 arch/arm64/mm/fault.c            | 38 +++++++++++++++++++++++++++++++-
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 991dd5f031e4..c7fff8daf2a7 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -200,13 +200,36 @@ do {									\
 				CONFIG_ARM64_PAN));			\
 } while (0)
 
+/*
+ * The Tag Check Flag (TCF) mode for MTE is per EL, hence TCF0
+ * affects EL0 and TCF affects EL1 irrespective of which TTBR is
+ * used.
+ * The kernel accesses TTBR0 usually with LDTR/STTR instructions
+ * when UAO is available, so these would act as EL0 accesses using
+ * TCF0.
+ * However futex.h code uses exclusives which would be executed as
+ * EL1, this can potentially cause a tag check fault even if the
+ * user disables TCF0.
+ *
+ * To address the problem we set the PSTATE.TCO bit in uaccess_enable()
+ * and reset it in uaccess_disable().
+ *
+ * The Tag check override (TCO) bit disables temporarily the tag checking
+ * preventing the issue.
+ */
 static inline void uaccess_disable(void)
 {
+	asm volatile(ALTERNATIVE("nop", SET_PSTATE_TCO(0),
+				 ARM64_MTE, CONFIG_KASAN_HW_TAGS));
+
 	__uaccess_disable(ARM64_HAS_PAN);
 }
 
 static inline void uaccess_enable(void)
 {
+	asm volatile(ALTERNATIVE("nop", SET_PSTATE_TCO(1),
+				 ARM64_MTE, CONFIG_KASAN_HW_TAGS));
+
 	__uaccess_enable(ARM64_HAS_PAN);
 }
 
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index a3bd189602df..d110f382dacf 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -33,6 +33,7 @@
 #include <asm/debug-monitors.h>
 #include <asm/esr.h>
 #include <asm/kprobes.h>
+#include <asm/mte.h>
 #include <asm/processor.h>
 #include <asm/sysreg.h>
 #include <asm/system_misc.h>
@@ -294,6 +295,11 @@ static void die_kernel_fault(const char *msg, unsigned long addr,
 	do_exit(SIGKILL);
 }
 
+static void report_tag_fault(unsigned long addr, unsigned int esr,
+			     struct pt_regs *regs)
+{
+}
+
 static void __do_kernel_fault(unsigned long addr, unsigned int esr,
 			      struct pt_regs *regs)
 {
@@ -641,10 +647,40 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
 	return 0;
 }
 
+static void do_tag_recovery(unsigned long addr, unsigned int esr,
+			   struct pt_regs *regs)
+{
+	static bool reported = false;
+
+	if (!READ_ONCE(reported)) {
+		report_tag_fault(addr, esr, regs);
+		WRITE_ONCE(reported, true);
+	}
+
+	/*
+	 * Disable MTE Tag Checking on the local CPU for the current EL.
+	 * It will be done lazily on the other CPUs when they will hit a
+	 * tag fault.
+	 */
+	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_NONE);
+	isb();
+}
+
+
 static int do_tag_check_fault(unsigned long addr, unsigned int esr,
 			      struct pt_regs *regs)
 {
-	do_bad_area(addr, esr, regs);
+	/*
+	 * The tag check fault (TCF) is per EL, hence TCF0 affects
+	 * EL0 and TCF affects EL1.
+	 * TTBR0 address belong by convention to EL0 hence to correctly
+	 * discriminate we use the is_ttbr0_addr() macro.
+	 */
+	if (is_ttbr0_addr(addr))
+		do_bad_area(addr, esr, regs);
+	else
+		do_tag_recovery(addr, esr, regs);
+
 	return 0;
 }
 
-- 
2.28.0.1011.ga647a8990f-goog