Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr

On Mon, Jun 10, 2019 at 07:53:30PM +0100, Catalin Marinas wrote:

On Mon, Jun 10, 2019 at 11:07:03AM -0700, Kees Cook wrote:

quoted

On Mon, Jun 10, 2019 at 06:53:27PM +0100, Catalin Marinas wrote:

quoted

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 3767fb21a5b8..fd191c5b92aa 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c

@@ -552,3 +552,18 @@ void arch_setup_new_exec(void)
 
 	ptrauth_thread_init_user(current);
 }
+
+/*
+ * Enable the relaxed ABI allowing tagged user addresses into the kernel.
+ */
+int untagged_uaddr_set_mode(unsigned long arg)
+{
+	if (is_compat_task())
+		return -ENOTSUPP;
+	if (arg)
+		return -EINVAL;
+
+	set_thread_flag(TIF_UNTAGGED_UADDR);
+
+	return 0;
+}

I think this should be paired with a flag clearing in copy_thread(),
yes? (i.e. each binary needs to opt in)

It indeed needs clearing though not in copy_thread() as that's used on
clone/fork but rather in flush_thread(), called on the execve() path.

Ah! Yes, thanks.

And a note to myself: I think PR_UNTAGGED_ADDR (not UADDR) looks better
in a uapi header, the user doesn't differentiate between uaddr and
kaddr.

Good point. I would agree. :)

-- 
Kees Cook

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