linux-kernel - [PATCH 3/3] xtensa: noMMU: allow handling protection faults

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Date:   Wed, 13 Apr 2022 16:49:30 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     linux-xtensa@...ux-xtensa.org
Cc:     Chris Zankel <chris@...kel.net>, linux-kernel@...r.kernel.org,
        Max Filippov <jcmvbkbc@...il.com>
Subject: [PATCH 3/3] xtensa: noMMU: allow handling protection faults

Many xtensa CPU cores without full MMU still have memory protection
features capable of raising exceptions for invalid instruction
fetches/data access. Allow handling such exceptions. This improves
behavior of processes that pass invalid memory pointers to syscalls in
noMMU configs: in case of exception the kernel instead of killing the
process is now able to return -EINVAL from a syscall.

Introduce CONFIG_PFAULT that controls whether protection fault code is
enabled and register handlers for common memory protection exceptions
when it is enabled.

Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
---
 arch/xtensa/Kconfig        | 11 +++++++++++
 arch/xtensa/kernel/traps.c | 20 ++++++++++----------
 arch/xtensa/mm/Makefile    |  3 ++-
 arch/xtensa/mm/fault.c     |  4 ++++
 4 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index bd113bc6e192..bca2763495a1 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -79,6 +79,7 @@ config STACKTRACE_SUPPORT
 
 config MMU
 	def_bool n
+	select PFAULT
 
 config HAVE_XTENSA_GPIO32
 	def_bool n
@@ -178,6 +179,16 @@ config XTENSA_FAKE_NMI
 
 	  If unsure, say N.
 
+config PFAULT
+	bool "Handle protection faults" if EXPERT && !MMU
+	default y
+	help
+	  Handle protection faults. MMU configurations must enable it.
+	  noMMU configurations may disable it if used memory map never
+	  generates protection faults or faults are always fatal.
+
+	  If unsure, say Y.
+
 config XTENSA_UNALIGNED_USER
 	bool "Unaligned memory access in user space"
 	help
diff --git a/arch/xtensa/kernel/traps.c b/arch/xtensa/kernel/traps.c
index 9345007d474d..82ced7b25b77 100644
--- a/arch/xtensa/kernel/traps.c
+++ b/arch/xtensa/kernel/traps.c
@@ -110,21 +110,21 @@ static dispatch_init_table_t __initdata dispatch_init_table[] = {
 { EXCCAUSE_UNALIGNED,		KRNL,	   fast_unaligned },
 #endif
 #ifdef CONFIG_MMU
-{ EXCCAUSE_ITLB_MISS,		0,	   do_page_fault },
-{ EXCCAUSE_ITLB_MISS,		USER|KRNL, fast_second_level_miss},
+{ EXCCAUSE_ITLB_MISS,			0,	   do_page_fault },
+{ EXCCAUSE_ITLB_MISS,			USER|KRNL, fast_second_level_miss},
+{ EXCCAUSE_DTLB_MISS,			USER|KRNL, fast_second_level_miss},
+{ EXCCAUSE_DTLB_MISS,			0,	   do_page_fault },
+{ EXCCAUSE_STORE_CACHE_ATTRIBUTE,	USER|KRNL, fast_store_prohibited },
+#endif /* CONFIG_MMU */
+#ifdef CONFIG_PFAULT
 { EXCCAUSE_ITLB_MULTIHIT,		0,	   do_multihit },
-{ EXCCAUSE_ITLB_PRIVILEGE,	0,	   do_page_fault },
-/* EXCCAUSE_SIZE_RESTRICTION unhandled */
+{ EXCCAUSE_ITLB_PRIVILEGE,		0,	   do_page_fault },
 { EXCCAUSE_FETCH_CACHE_ATTRIBUTE,	0,	   do_page_fault },
-{ EXCCAUSE_DTLB_MISS,		USER|KRNL, fast_second_level_miss},
-{ EXCCAUSE_DTLB_MISS,		0,	   do_page_fault },
 { EXCCAUSE_DTLB_MULTIHIT,		0,	   do_multihit },
-{ EXCCAUSE_DTLB_PRIVILEGE,	0,	   do_page_fault },
-/* EXCCAUSE_DTLB_SIZE_RESTRICTION unhandled */
-{ EXCCAUSE_STORE_CACHE_ATTRIBUTE,	USER|KRNL, fast_store_prohibited },
+{ EXCCAUSE_DTLB_PRIVILEGE,		0,	   do_page_fault },
 { EXCCAUSE_STORE_CACHE_ATTRIBUTE,	0,	   do_page_fault },
 { EXCCAUSE_LOAD_CACHE_ATTRIBUTE,	0,	   do_page_fault },
-#endif /* CONFIG_MMU */
+#endif
 /* XCCHAL_EXCCAUSE_FLOATING_POINT unhandled */
 #if XTENSA_HAVE_COPROCESSOR(0)
 COPROCESSOR(0),
diff --git a/arch/xtensa/mm/Makefile b/arch/xtensa/mm/Makefile
index f7fb08ae768f..44153a335951 100644
--- a/arch/xtensa/mm/Makefile
+++ b/arch/xtensa/mm/Makefile
@@ -4,7 +4,8 @@
 #
 
 obj-y			:= init.o misc.o
-obj-$(CONFIG_MMU)	+= cache.o fault.o ioremap.o mmu.o tlb.o
+obj-$(CONFIG_PFAULT)	+= fault.o
+obj-$(CONFIG_MMU)	+= cache.o ioremap.o mmu.o tlb.o
 obj-$(CONFIG_HIGHMEM)	+= highmem.o
 obj-$(CONFIG_KASAN)	+= kasan_init.o
 
diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index 01e66da4a6b0..16f0a5ff5799 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -25,6 +25,7 @@ void bad_page_fault(struct pt_regs*, unsigned long, int);
 
 static void vmalloc_fault(struct pt_regs *regs, unsigned int address)
 {
+#ifdef CONFIG_MMU
 	/* Synchronize this task's top level page-table
 	 * with the 'reference' page table.
 	 */
@@ -71,6 +72,9 @@ static void vmalloc_fault(struct pt_regs *regs, unsigned int address)
 
 bad_page_fault:
 	bad_page_fault(regs, address, SIGKILL);
+#else
+	WARN_ONCE(1, "%s in noMMU configuration\n", __func__);
+#endif
 }
 /*
  * This routine handles page faults.  It determines the address,
-- 
2.30.2