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

From: Max Filippov
Date: Wed Apr 13 2022 - 19:50:15 EST


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@xxxxxxxxx>
---
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