Re: [PATCH 2/3] rust: add typed accessors for userspace pointers

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jan 25, 2024 at 5:00 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Thu, Jan 25, 2024, at 13:37, Alice Ryhl wrote:
> > On Thu, Jan 25, 2024 at 1:27 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >> On Wed, Jan 24, 2024, at 12:20, Alice Ryhl wrote:
>
> >> > +EXPORT_SYMBOL_GPL(rust_helper_copy_to_user_unsafe_skip_check_object_size);
> >>
> >> These functions are almost identical to the ones in
> >> lib/usercopy.c for !defined(INLINE_COPY_TO_USER).
> >>
> >> That version has an extra memset() after a partial
> >> copy_from_user(), and you probably want to have the
> >> same thing here for consistency.
> >>
> >> I think ideally we should only have one out-of-line copy
> >> of these two functions and have that one shared between
> >> rust and architectures that want the C version out of line
> >> as well.
> >
> > I had a bit of trouble figuring out all of the copy_[to/from]_user
> > methods that are available. I was hoping that a better solution would
> > be available, and it sounds like one is. Is _copy_from_user always
> > available as an exported symbol? If it's always available and skips
> > the check, then I can just use that. I don't think the memset matters
> > for my case.
>
> At the moment, it appears that it's available on the few architectures
> that don't #define INLINE_COPY_FROM_USER: alpha, csky, powerpc,
> riscv and x86. On the other architectures it is always an inline
> function.
>
> > Otherwise, I can add a helper in rust/helpers.c that wraps
> > _copy_from_user only when INLINE_COPY_FROM_USER is defined, and call
> > the helper in those cases, and otherwise call the exported symbol
> > directly. (I need an exported symbol to call into C from Rust.)
> >
> > Would that make sense?
>
> I don't think we can have a perfect abstraction here, but rather
> than putting knowledge of INLINE_COPY_FROM_USER into the rust
> wrapper, I would suggest putting a bit of information about
> rust into lib/usercopy.c.
>
> I've tried to come up with an idea below, see if that works
> for you.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 3064314f4832..835aa175d0ee 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -138,13 +138,18 @@ __copy_to_user(void __user *to, const void *from, unsigned long n)
>         return raw_copy_to_user(to, from, n);
>  }
>
> -#ifdef INLINE_COPY_FROM_USER
>  static inline __must_check unsigned long
> -_copy_from_user(void *to, const void __user *from, unsigned long n)
> +_inline_copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
>         unsigned long res = n;
>         might_fault();
>         if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> +               /*
> +                * Ensure that bad access_ok() speculation will not
> +                * lead to nasty side effects *after* the copy is
> +                * finished:
> +                */
> +               barrier_nospec();
>                 instrument_copy_from_user_before(to, from, n);
>                 res = raw_copy_from_user(to, from, n);
>                 instrument_copy_from_user_after(to, from, n, res);
> @@ -153,14 +158,11 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
>                 memset(to + (n - res), 0, res);
>         return res;
>  }
> -#else
>  extern __must_check unsigned long
>  _copy_from_user(void *, const void __user *, unsigned long);
> -#endif
>
> -#ifdef INLINE_COPY_TO_USER
>  static inline __must_check unsigned long
> -_copy_to_user(void __user *to, const void *from, unsigned long n)
> +_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
>         might_fault();
>         if (should_fail_usercopy())
> @@ -171,25 +173,32 @@ _copy_to_user(void __user *to, const void *from, unsigned long n)
>         }
>         return n;
>  }
> -#else
>  extern __must_check unsigned long
>  _copy_to_user(void __user *, const void *, unsigned long);
> -#endif
>
>  static __always_inline unsigned long __must_check
>  copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
> -       if (check_copy_size(to, n, false))
> -               n = _copy_from_user(to, from, n);
> -       return n;
> +       if (!check_copy_size(to, n, false))
> +               return n;
> +#ifdef INLINE_COPY_FROM_USER
> +       return _inline_copy_from_user(to, from, n);
> +#else
> +       return _copy_from_user(to, from, n);
> +#endif
>  }
>
>  static __always_inline unsigned long __must_check
>  copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
> -       if (check_copy_size(from, n, true))
> -               n = _copy_to_user(to, from, n);
> -       return n;
> +       if (!check_copy_size(from, n, true))
> +               return n;
> +
> +#ifdef INLINE_COPY_TO_USER
> +       return _inline_copy_to_user(to, from, n);
> +#else
> +       return _copy_to_user(to, from, n);
> +#endif
>  }
>
>  #ifndef copy_mc_to_kernel
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index d29fe29c6849..503a064d79e2 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -7,40 +7,18 @@
>
>  /* out-of-line parts */
>
> -#ifndef INLINE_COPY_FROM_USER
> +#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
>  unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
> -       unsigned long res = n;
> -       might_fault();
> -       if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> -               /*
> -                * Ensure that bad access_ok() speculation will not
> -                * lead to nasty side effects *after* the copy is
> -                * finished:
> -                */
> -               barrier_nospec();
> -               instrument_copy_from_user_before(to, from, n);
> -               res = raw_copy_from_user(to, from, n);
> -               instrument_copy_from_user_after(to, from, n, res);
> -       }
> -       if (unlikely(res))
> -               memset(to + (n - res), 0, res);
> -       return res;
> +       return _inline_copy_from_user(to, from, n);
>  }
>  EXPORT_SYMBOL(_copy_from_user);
>  #endif
>
> -#ifndef INLINE_COPY_TO_USER
> +#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
>  unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
> -       might_fault();
> -       if (should_fail_usercopy())
> -               return n;
> -       if (likely(access_ok(to, n))) {
> -               instrument_copy_to_user(to, from, n);
> -               n = raw_copy_to_user(to, from, n);
> -       }
> -       return n;
> +       return _inline_copy_to_user(to, from, n);
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif

Sure, if that's okay with you, then I'm happy to do it that way in v2.

Thank you!
Alice