Re: [PATCH 1/2] uaccess: Simplify code pattern for masked user copies

[Linus Torvalds]

On Sun, 9 Feb 2025 at 11:48, David Laight <david.laight.linux@xxxxxxxxx> wrote:
>
> You almost need it to be 'void masked_user_access_begin(&uaddr)'.

Maybe we just need to make it a two-stage thing, with

        if (!user_access_ok(uaddr, size))
                return -EFAULT;
        user_read_access_begin(&uaddr);
        unsafe_get_user(val1, &uaddr->one, Efault);
        unsafe_get_user(val2, &uaddr->two, Efault);
        user_read_access_end();
        ... all done ..

Efault:
        user_read_access_end();
        return -EFAULT;

and that would actually simplify some things: right now we have
separate versions of the user address checking (for
read/write/either): user_read_access_begin() and friends.

We still need those three versions, but now we'd only need them for
the simpler non-conditional case that doesn't have to bother about the
size.

And then if you have user address masking, user_access_ok() just
unconditionally returns true and is a no-op, while
user_read_access_begin() does the masking and actually enables user
accesses.

And if you *don't* have user address masking, user_read_access_begin()
still enables user accesses and has the required speculation
synchronization, but doesn't do any address checking, because
user_access_ok() did that (and nothing else).

That seems like it might be a reasonable compromise and fairly hard to
get wrong (*)?

            Linus

(*) Obviously anybody can get anything wrong, but if you forget the
user_access_ok() entirely you're being wilful about it, and if you
forget the user_read_access_begin() the code won't work, so it seems
about as safe as it can be.