arm64: mte: Allow user control of the tag check mode via prctl()

By default, even if PROT_MTE is set on a memory range, there is no tag
check fault reporting (SIGSEGV). Introduce a set of option to the
exiting prctl(PR_SET_TAGGED_ADDR_CTRL) to allow user control of the tag
check fault mode:

  PR_MTE_TCF_NONE  - no reporting (default)
  PR_MTE_TCF_SYNC  - synchronous tag check fault reporting
  PR_MTE_TCF_ASYNC - asynchronous tag check fault reporting

These options translate into the corresponding SCTLR_EL1.TCF0 bitfield,
context-switched by the kernel. Note that the kernel accesses to the
user address space (e.g. read() system call) are not checked if the user
thread tag checking mode is PR_MTE_TCF_NONE or PR_MTE_TCF_ASYNC. If the
tag checking mode is PR_MTE_TCF_SYNC, the kernel makes a best effort to
check its user address accesses, however it cannot always guarantee it.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>

arch/arm64/include/asm/processor.h

@@ -151,6 +151,9 @@ struct thread_struct {
 	struct ptrauth_keys_user	keys_user;
 	struct ptrauth_keys_kernel	keys_kernel;
 #endif
+#ifdef CONFIG_ARM64_MTE
+	u64			sctlr_tcf0;
+#endif
 };
 
 static inline void arch_thread_struct_whitelist(unsigned long *offset,