arm64: compat: Implement misalignment fixups for multiword loads

The 32-bit ARM kernel implements fixups on behalf of user space when
using LDM/STM or LDRD/STRD instructions on addresses that are not 32-bit
aligned. This is not something that is supported by the architecture,
but was done anyway to increase compatibility with user space software,
which mostly targeted x86 at the time and did not care about aligned
accesses.

This feature is one of the remaining impediments to being able to switch
to 64-bit kernels on 64-bit capable hardware running 32-bit user space,
so let's implement it for the arm64 compat layer as well.

Note that the intent is to implement the exact same handling of
misaligned multi-word loads and stores as the 32-bit kernel does,
including what appears to be missing support for user space programs
that rely on SETEND to switch to a different byte order and back. Also,
like the 32-bit ARM version, we rely on the faulting address reported by
the CPU to infer the memory address, instead of decoding the instruction
fully to obtain this information.

This implementation is taken from the 32-bit ARM tree, with all pieces
removed that deal with instructions other than LDRD/STRD and LDM/STM, or
that deal with alignment exceptions taken in kernel mode.

Cc: debian-arm@lists.debian.org
Cc: Vagrant Cascadian <vagrant@debian.org>
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Steve McIntyre <steve@einval.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20220701135322.3025321-1-ardb@kernel.org
[catalin.marinas@arm.com: change the option to 'default n']
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -1562,6 +1562,9 @@ config THUMB2_COMPAT_VDSO
 	  Compile the compat vDSO with '-mthumb -fomit-frame-pointer' if y,
 	  otherwise with '-marm'.
 
+config COMPAT_ALIGNMENT_FIXUPS
+	bool "Fix up misaligned multi-word loads and stores in user space"
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on SYSCTL

arch/arm64/include/asm/exception.h

@@ -70,6 +70,7 @@ void do_sysinstr(unsigned long esr, struct pt_regs *regs);
 void do_sp_pc_abort(unsigned long addr, unsigned long esr, struct pt_regs *regs);
 void bad_el0_sync(struct pt_regs *regs, int reason, unsigned long esr);
 void do_cp15instr(unsigned long esr, struct pt_regs *regs);
+int do_compat_alignment_fixup(unsigned long addr, struct pt_regs *regs);
 void do_el0_svc(struct pt_regs *regs);
 void do_el0_svc_compat(struct pt_regs *regs);
 void do_ptrauth_fault(struct pt_regs *regs, unsigned long esr);

arch/arm64/kernel/Makefile

@@ -45,6 +45,7 @@ $(obj)/%.stub.o: $(obj)/%.o FORCE
 obj-$(CONFIG_COMPAT)			+= sys32.o signal32.o			\
 					   sys_compat.o
 obj-$(CONFIG_COMPAT)			+= sigreturn32.o
+obj-$(CONFIG_COMPAT_ALIGNMENT_FIXUPS)	+= compat_alignment.o
 obj-$(CONFIG_KUSER_HELPERS)		+= kuser32.o
 obj-$(CONFIG_FUNCTION_TRACER)		+= ftrace.o entry-ftrace.o
 obj-$(CONFIG_MODULES)			+= module.o

arch/arm64/mm/fault.c

@@ -691,6 +691,9 @@ static int __kprobes do_translation_fault(unsigned long far,
 static int do_alignment_fault(unsigned long far, unsigned long esr,
 			      struct pt_regs *regs)
 {
+	if (IS_ENABLED(CONFIG_COMPAT_ALIGNMENT_FIXUPS) &&
+	    compat_user_mode(regs))
+		return do_compat_alignment_fixup(far, regs);
 	do_bad_area(far, esr, regs);
 	return 0;
 }