On 8/27/20 10:54 AM, Catalin Marinas wrote:

On Fri, Aug 14, 2020 at 07:27:03PM +0200, Andrey Konovalov wrote:

quoted

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 5e832b3387f1..c62c8ba85c0e 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>

@@ -222,6 +223,20 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 	return 1;
 }
 
+static bool is_el1_mte_sync_tag_check_fault(unsigned int esr)
+{
+	unsigned int ec = ESR_ELx_EC(esr);
+	unsigned int fsc = esr & ESR_ELx_FSC;
+
+	if (ec != ESR_ELx_EC_DABT_CUR)
+		return false;
+
+	if (fsc == ESR_ELx_FSC_MTE)
+		return true;
+
+	return false;
+}
+
 static bool is_el1_instruction_abort(unsigned int esr)
 {
 	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_CUR;

@@ -294,6 +309,18 @@ 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)
+{
+	bool is_write = ((esr & ESR_ELx_WNR) >> ESR_ELx_WNR_SHIFT) != 0;
+
+	pr_alert("Memory Tagging Extension Fault in %pS\n", (void *)regs->pc);
+	pr_alert("  %s at address %lx\n", is_write ? "Write" : "Read", addr);
+	pr_alert("  Pointer tag: [%02x], memory tag: [%02x]\n",
+			mte_get_ptr_tag(addr),
+			mte_get_mem_tag((void *)addr));
+}
+
 static void __do_kernel_fault(unsigned long addr, unsigned int esr,
 			      struct pt_regs *regs)
 {

@@ -317,12 +344,16 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
 			msg = "execute from non-executable memory";
 		else
 			msg = "read from unreadable memory";
+	} else if (is_el1_mte_sync_tag_check_fault(esr)) {
+		report_tag_fault(addr, esr, regs);
+		msg = "memory tagging extension fault";

IIUC, that's dead code. See my comment below on do_tag_check_fault().

That's correct. This was useful with "panic_on_mte_fault" kernel command line
parameter. Since it has now been replaced by a similar kasan feature, this code
can be safely removed.

quoted

 	} else if (addr < PAGE_SIZE) {
 		msg = "NULL pointer dereference";
 	} else {
 		msg = "paging request";
 	}
 
+

Unnecessary empty line.

Agree.

quoted

 	die_kernel_fault(msg, addr, esr, regs);
 }
 

@@ -658,10 +689,27 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
 	return 0;
 }
 
+static int do_tag_recovery(unsigned long addr, unsigned int esr,
+			   struct pt_regs *regs)
+{
+	report_tag_fault(addr, esr, regs);
+
+	/* Skip over the faulting instruction and continue: */
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);

Ooooh, do we expect the kernel to still behave correctly after this? I
thought the recovery means disabling tag checking altogether and
restarting the instruction rather than skipping over it. We only skip if
we emulated it.

I tried to dig it out but I am not sure why we need this as well.

quoted

+
+	return 0;
+}
+
+
 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 TTBR */
+	if (is_ttbr0_addr(addr))
+		do_bad_area(addr, esr, regs);
+	else
+		do_tag_recovery(addr, esr, regs);

So we never invoke __do_kernel_fault() for a synchronous tag check in
the kernel. What's with all the is_el1_mte_sync_tag_check_fault() check
above?

That's correct. This had a meaning with "panic_on_mte_fault" but since the
feature has been replaced is_el1_mte_sync_tag_check_fault() is not useful anymore.

-- 
Regards,
Vincenzo

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