Message-ID: <20250225163253.22ddcaaf@eugeo>
Date: Tue, 25 Feb 2025 16:32:53 +0000
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>,
 Jann Horn <jannh@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, Alex
 Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, "
 Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, Trevor
 Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v14 8/8] task: rust: rework how current is accessed

On Thu, 13 Feb 2025 11:04:07 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> Introduce a new type called `CurrentTask` that lets you perform various
> operations that are only safe on the `current` task. Use the new type to
> provide a way to access the current mm without incrementing its
> refcount.
> 
> With this change, you can write stuff such as
> 
> 	let vma = current!().mm().lock_vma_under_rcu(addr);
> 
> without incrementing any refcounts.
> 
> This replaces the existing abstractions for accessing the current pid
> namespace. With the old approach, every field access to current involves
> both a macro and a unsafe helper function. The new approach simplifies
> that to a single safe function on the `CurrentTask` type. This makes it
> less heavy-weight to add additional current accessors in the future.
> 
> That said, creating a `CurrentTask` type like the one in this patch
> requires that we are careful to ensure that it cannot escape the current
> task or otherwise access things after they are freed. To do this, I
> declared that it cannot escape the current "task context" where I
> defined a "task context" as essentially the region in which `current`
> remains unchanged. So e.g., release_task() or begin_new_exec() would
> leave the task context.
> 
> If a userspace thread returns to userspace and later makes another
> syscall, then I consider the two syscalls to be different task contexts.
> This allows values stored in that task to be modified between syscalls,
> even if they're guaranteed to be immutable during a syscall.
> 
> Ensuring correctness of `CurrentTask` is slightly tricky if we also want
> the ability to have a safe `kthread_use_mm()` implementation in Rust. To
> support that safely, there are two patterns we need to ensure are safe:
> 
> 	// Case 1: current!() called inside the scope.
> 	let mm;
> 	kthread_use_mm(some_mm, || {
> 	    mm = current!().mm();
> 	});
> 	drop(some_mm);
> 	mm.do_something(); // UAF
> 
> and:
> 
> 	// Case 2: current!() called before the scope.
> 	let mm;
> 	let task = current!();
> 	kthread_use_mm(some_mm, || {
> 	    mm = task.mm();
> 	});
> 	drop(some_mm);
> 	mm.do_something(); // UAF
> 
> The existing `current!()` abstraction already natively prevents the
> first case: The `&CurrentTask` would be tied to the inner scope, so the
> borrow-checker ensures that no reference derived from it can escape the
> scope.
> 
> Fixing the second case is a bit more tricky. The solution is to
> essentially pretend that the contents of the scope execute on an
> different thread, which means that only thread-safe types can cross the
> boundary. Since `CurrentTask` is marked `NotThreadSafe`, attempts to
> move it to another thread will fail, and this includes our fake pretend
> thread boundary.
> 
> This has the disadvantage that other types that aren't thread-safe for
> reasons unrelated to `current` also cannot be moved across the
> `kthread_use_mm()` boundary. I consider this an acceptable tradeoff.

I like the approach used.

Reviewed-by: Gary Guo <gary@...yguo.net>

> 
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/task.rs | 247 +++++++++++++++++++++++++++-------------------------
>  1 file changed, 129 insertions(+), 118 deletions(-)

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