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 <20240418-alice-mm-v6-3-cb8f3e5d688f@google.com> <20240425171358.54dc96e4@eugeo>
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From: Alice Ryhl <aliceryhl@google.com>
Date: Fri, 26 Apr 2024 09:13:18 +0200
Message-ID: <CAH5fLgjHALMTsWNj_rRBZTgKPTPzWBEo=5VVahS_Vxag-nizPA@mail.gmail.com>
Subject: Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers
To: Gary Guo <gary@garyguo.net>
Cc: Miguel Ojeda <ojeda@kernel.org>, Matthew Wilcox <willy@infradead.org>, 
	Al Viro <viro@zeniv.linux.org.uk>, Andrew Morton <akpm@linux-foundation.org>, 
	Kees Cook <keescook@chromium.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, Trevor Gross <tmgross@umich.edu>, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	rust-for-linux@vger.kernel.org, Christian Brauner <brauner@kernel.org>
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On Thu, Apr 25, 2024 at 6:14=E2=80=AFPM Gary Guo <gary@garyguo.net> wrote:
>
> On Thu, 18 Apr 2024 08:59:19 +0000
> Alice Ryhl <aliceryhl@google.com> wrote:
>
> > Add safe methods for reading and writing Rust values to and from
> > userspace pointers.
> >
> > The C methods for copying to/from userspace use a function called
> > `check_object_size` to verify that the kernel pointer is not dangling.
> > However, this check is skipped when the length is a compile-time
> > constant, with the assumption that such cases trivially have a correct
> > kernel pointer.
> >
> > In this patch, we apply the same optimization to the typed accessors.
> > For both methods, the size of the operation is known at compile time to
> > be size_of of the type being read or written. Since the C side doesn't
> > provide a variant that skips only this check, we create custom helpers
> > for this purpose.
> >
> > The majority of reads and writes to userspace pointers in the Rust
> > Binder driver uses these accessor methods. Benchmarking has found that
> > skipping the `check_object_size` check makes a big difference for the
> > cases being skipped here. (And that the check doesn't make a difference
> > for the cases that use the raw read/write methods.)
> >
> > This code is based on something that was originally written by Wedson o=
n
> > the old rust branch. It was modified by Alice to skip the
> > `check_object_size` check, and to update various comments, including th=
e
> > notes about kernel pointers in `WritableToBytes`.
> >
> > Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> > Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> > Reviewed-by: Trevor Gross <tmgross@umich.edu>
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
>
> Reviewed-by: Gary Guo <gary@garyguo.net>
>
> > ---
> >  rust/kernel/types.rs   | 64 ++++++++++++++++++++++++++++++++++++++++
> >  rust/kernel/uaccess.rs | 79 ++++++++++++++++++++++++++++++++++++++++++=
++++++--
> >  2 files changed, 141 insertions(+), 2 deletions(-)
> >
> > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> > index 8fad61268465..9c57c6c75553 100644
> > --- a/rust/kernel/types.rs
> > +++ b/rust/kernel/types.rs
> >
> > +/// Types that can be viewed as an immutable slice of initialized byte=
s.
> > +///
> > +/// If a struct implements this trait, then it is okay to copy it byte=
-for-byte to userspace. This
> > +/// means that it should not have any padding, as padding bytes are un=
initialized. Reading
> > +/// uninitialized memory is not just undefined behavior, it may even l=
ead to leaking sensitive
> > +/// information on the stack to userspace.
> > +///
> > +/// The struct should also not hold kernel pointers, as kernel pointer=
 addresses are also considered
> > +/// sensitive. However, leaking kernel pointers is not considered unde=
fined behavior by Rust, so
> > +/// this is a correctness requirement, but not a safety requirement.
> > +///
> > +/// # Safety
> > +///
> > +/// Values of this type may not contain any uninitialized bytes. This =
type must not have interior
> > +/// mutability.
> > +pub unsafe trait AsBytes {}
> > +
> > +// SAFETY: Instances of the following types have no uninitialized port=
ions.
> > +unsafe impl AsBytes for u8 {}
> > +unsafe impl AsBytes for u16 {}
> > +unsafe impl AsBytes for u32 {}
> > +unsafe impl AsBytes for u64 {}
> > +unsafe impl AsBytes for usize {}
> > +unsafe impl AsBytes for i8 {}
> > +unsafe impl AsBytes for i16 {}
> > +unsafe impl AsBytes for i32 {}
> > +unsafe impl AsBytes for i64 {}
> > +unsafe impl AsBytes for isize {}
> > +unsafe impl AsBytes for bool {}
> > +unsafe impl AsBytes for char {}
> > +unsafe impl AsBytes for str {}
> > +// SAFETY: If individual values in an array have no uninitialized port=
ions, then the array itself
> > +// does not have any uninitialized portions either.
> > +unsafe impl<T: AsBytes> AsBytes for [T] {}
>
> nit: I would move `str` to here, since `str` is essentially `[u8]` with
> UTF-8 guarantee.
>
> > +unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {}

Yes ... but the safety comment here talks about arrays and their
individual values. I don't think it transfers cleanly to str, and that
the other safety comment fits str better.

Alice