arm64: access_ok() optimization

The TBI setup on arm64 is very strange: HW is set up to always do TBI,
but the kernel enforcement for system calls is purely a software
contract, and user space is supposed to mask off the top bits before the
system call.

Except all the actual brk/mmap/etc() system calls then mask it in kernel
space anyway, and accept any TBI address.

This basically unifies things and makes access_ok() also ignore it.

This is an ABI change, but the current situation is very odd, and this
change avoids the current mess and makes the kernel more permissive, and
as such is unlikely to break anything.

The way forward - for some possible future situation when people want to
use more bits - is probably to introduce a new "I actually want the full
64-bit address space" prctl.  But we should make sure that the software
and hardware rules actually match at that point.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm64/include/asm/uaccess.h

@@ -30,23 +30,20 @@ static inline int __access_ok(const void __user *ptr, unsigned long size);
 
 /*
  * Test whether a block of memory is a valid user space address.
- * Returns 1 if the range is valid, 0 otherwise.
  *
- * This is equivalent to the following test:
- * (u65)addr + (u65)size <= (u65)TASK_SIZE_MAX
+ * We only care that the address cannot reach the kernel mapping, and
+ * that an invalid address will fault.
  */
-static inline int access_ok(const void __user *addr, unsigned long size)
+static inline int access_ok(const void __user *p, unsigned long size)
 {
-	/*
-	 * 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) &&
-	    (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
-		addr = untagged_addr(addr);
+	unsigned long addr = (unsigned long)p;
 
-	return likely(__access_ok(addr, size));
+	/* Only bit 55 of the address matters */
+	addr |= addr+size;
+	addr = (addr >> 55) & 1;
+	size >>= 55;
+
+	return !(addr | size);
 }
 #define access_ok access_ok