linux-kernel - Re: [PATCH v2] uaccess: rust: use newtype for user pointers

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <DA7Y9ELFKI6U.32MUDCD0VQEE2@kernel.org>
Date: Wed, 28 May 2025 18:55:08 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Alexander Viro" <viro@...iv.linux.org.uk>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Arnd Bergmann" <arnd@...db.de>
Cc: "Andrew Morton" <akpm@...ux-foundation.org>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <benno.lossin@...ton.me>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] uaccess: rust: use newtype for user pointers

On Tue May 27, 2025 at 3:53 PM CEST, Alice Ryhl wrote:
> In C code we use sparse with the __user annotation to detect cases where
> a user pointer is mixed up with other things. To replicate that, we
> introduce a new struct UserPtr that serves the same purpose using the
> newtype pattern.
>
> The UserPtr type is not marked with #[derive(Debug)], which means that

Should we still implement it, not printing the address of course? So
something like `UserPtr(/* .. */)` or whatever people print instead of
showing the address?

> it's not possible to print values of this type. This avoids ASLR
> leakage.
>
> The type is added to the prelude as it is a fairly fundamental type
> similar to c_int. The wrapping_add() method is renamed to
> wrapping_byte_add() for consistency with the method name found on raw
> pointers.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

I left one question below, but it already looks good, so:

Reviewed-by: Benno Lossin <lossin@...nel.org>

> @@ -444,7 +484,11 @@ fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<us
>  
>      // SAFETY: `dst` is valid for writing `dst.len()` bytes.
>      let res = unsafe {
> -        bindings::strncpy_from_user(dst.as_mut_ptr().cast::<c_char>(), src as *const c_char, len)
> +        bindings::strncpy_from_user(
> +            dst.as_mut_ptr().cast::<c_char>(),
> +            src.as_const_ptr().cast::<c_char>(),

Should we also have `as_{const,mut}_char_ptr` functions? After a quick
search, I found that `char __user *` is a common type for user pointers.

---
Cheers,
Benno

> +            len,
> +        )
>      };