ARM: 9350/1: fault: Implement copy_from_kernel_nofault_allowed()

Under PAN emulation when dumping backtraces from things like the
LKDTM EXEC_USERSPACE test[1], a double fault (which would hang a CPU)
would happen because of dump_instr() attempting to read a userspace
address. Make sure copy_from_kernel_nofault() does not attempt this
any more.

Closes: https://lava.sirena.org.uk/scheduler/job/497571
Link: https://lore.kernel.org/all/202401181125.D48DCB4C@keescook/ [1]

Reported-by: Mark Brown <[email protected]>
Suggested-by: Russell King (Oracle) <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Tested-by: Mark Brown <[email protected]>
Cc: Wang Kefeng <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Russell King (Oracle) <[email protected]>

Author: kees

arch/arm/mm/fault.c

@@ -25,6 +25,13 @@
 
 #include "fault.h"
 
+bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
+{
+	unsigned long addr = (unsigned long)unsafe_src;
+
+	return addr >= TASK_SIZE && ULONG_MAX - addr >= size;
+}
+
 #ifdef CONFIG_MMU
 
 /*