arm64: Validate tagged addresses in access_ok() called from kernel threads

__range_ok(), invoked from access_ok(), clears the tag of the user
address only if CONFIG_ARM64_TAGGED_ADDR_ABI is enabled and the thread
opted in to the relaxed ABI. The latter sets the TIF_TAGGED_ADDR thread
flag. In the case of asynchronous I/O (e.g. io_submit()), the
access_ok() may be called from a kernel thread. Since kernel threads
don't have TIF_TAGGED_ADDR set, access_ok() will fail for valid tagged
user addresses. Example from the ffs_user_copy_worker() thread:

	use_mm(io_data->mm);
	ret = ffs_copy_to_iter(io_data->buf, ret, &io_data->data);
	unuse_mm(io_data->mm);

Relax the __range_ok() check to always untag the user address if called
in the context of a kernel thread. The user pointers would have already
been checked via aio_setup_rw() -> import_{single_range,iovec}() at the
time of the asynchronous I/O request.

Fixes: 63f0c603 ("arm64: Introduce prctl() options to control the tagged user addresses ABI")
Cc: <stable@vger.kernel.org> # 5.4.x-
Cc: Will Deacon <will@kernel.org>
Reported-by: NEvgenii Stepanov <eugenis@google.com>
Tested-by: NEvgenii Stepanov <eugenis@google.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/uaccess.h

@@ -62,8 +62,13 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
 {
 	unsigned long ret, limit = current_thread_info()->addr_limit;
 
+	/*
+	 * Asynchronous I/O running in a kernel thread does not have the
+	 * TIF_TAGGED_ADDR flag of the process owning the mm, so always untag
+	 * the user address before checking.
+	 */
 	if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
-	    test_thread_flag(TIF_TAGGED_ADDR))
+	    (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
 		addr = untagged_addr(addr);
 
 	__chk_user_ptr(addr);