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When declaring an immutable global variable in Rust, the compiler checks
that it looks thread safe, because it is generally safe to access said
global variable. When using C bindings types for these globals, we don't
really want this check, because it is conservative and assumes pointers
are not thread safe.
In the case of BINDER_VM_OPS, this is a challenge when combined with the
patch 'userfaultfd: introduce vm_uffd_ops' [1], which introduces a
pointer field to vm_operations_struct. It previously only held function
pointers, which are considered thread safe.
Rust Binder should not be assuming that vm_operations_struct contains no
pointer fields, so to fix this, use AssertSync (which Rust Binder has
already declared for another similar global of type struct
file_operations with the same problem). This ensures that even if
another commit adds a pointer field to vm_operations_struct, this does
not cause problems.
Fixes: 8ef2c15aeae0 ("rust_binder: check ownership before using vma")
Cc: stable <stable@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603121235.tpnRxFKO-lkp@intel.com/
Link: https://lore.kernel.org/r/20260306171815.3160826-8-rppt@kernel.org [1]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260314111951.4139029-1-aliceryhl@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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There's no reason to lock the whole mm when we are doing operations on
the vma if we can help it, so to reduce contention, use the
lock_vma_under_rcu() abstraction.
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Link: https://patch.msgid.link/20260218-binder-vma-rcu-v1-1-8bd45b2b1183@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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When installing missing pages (or zapping them), Rust Binder will look
up the vma in the mm by address, and then call vm_insert_page (or
zap_page_range_single). However, if the vma is closed and replaced with
a different vma at the same address, this can lead to Rust Binder
installing pages into the wrong vma.
By installing the page into a writable vma, it becomes possible to write
to your own binder pages, which are normally read-only. Although you're
not supposed to be able to write to those pages, the intent behind the
design of Rust Binder is that even if you get that ability, it should not
lead to anything bad. Unfortunately, due to another bug, that is not the
case.
To fix this, store a pointer in vm_private_data and check that the vma
returned by vma_lookup() has the right vm_ops and vm_private_data before
trying to use the vma. This should ensure that Rust Binder will refuse
to interact with any other VMA. The plan is to introduce more vma
abstractions to avoid this unsafe access to vm_ops and vm_private_data,
but for now let's start with the simplest possible fix.
C Binder performs the same check in a slightly different way: it
provides a vm_ops->close that sets a boolean to true, then checks that
boolean after calling vma_lookup(), but this is more fragile
than the solution in this patch. (We probably still want to do both, but
the vm_ops->close callback will be added later as part of the follow-up
vma API changes.)
It's still possible to remap the vma so that pages appear in the right
vma, but at the wrong offset, but this is a separate issue and will be
fixed when Rust Binder gets a vm_ops->close callback.
Cc: stable <stable@kernel.org>
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Reported-by: Jann Horn <jannh@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Link: https://patch.msgid.link/20260218-binder-vma-check-v2-1-60f9d695a990@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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When forward-porting Rust Binder to 6.18, I neglected to take commit
fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope") into
account, and apparently I did not end up running the shrinker callback
when I sanity tested the driver before submission. This leads to crashes
like the following:
============================================
WARNING: possible recursive locking detected
6.18.0-mainline-maybe-dirty #1 Tainted: G IO
--------------------------------------------
kswapd0/68 is trying to acquire lock:
ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: lock_list_lru_of_memcg+0x128/0x230
but task is already holding lock:
ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0
----
lock(&l->lock);
lock(&l->lock);
*** DEADLOCK ***
May be due to missing lock nesting notation
3 locks held by kswapd0/68:
#0: ffffffff90d2e260 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0x597/0x1160
#1: ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20
#2: ffffffff90cf3680 (rcu_read_lock){....}-{1:2}, at: lock_list_lru_of_memcg+0x2d/0x230
To fix this, remove the spin_lock() call from rust_shrink_free_page().
Cc: stable <stable@kernel.org>
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251202-binder-shrink-unspin-v1-1-263efb9ad625@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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We're generally not proponents of rewrites (nasty uncomfortable things
that make you late for dinner!). So why rewrite Binder?
Binder has been evolving over the past 15+ years to meet the evolving
needs of Android. Its responsibilities, expectations, and complexity
have grown considerably during that time. While we expect Binder to
continue to evolve along with Android, there are a number of factors
that currently constrain our ability to develop/maintain it. Briefly
those are:
1. Complexity: Binder is at the intersection of everything in Android and
fulfills many responsibilities beyond IPC. It has become many things
to many people, and due to its many features and their interactions
with each other, its complexity is quite high. In just 6kLOC it must
deliver transactions to the right threads. It must correctly parse
and translate the contents of transactions, which can contain several
objects of different types (e.g., pointers, fds) that can interact
with each other. It controls the size of thread pools in userspace,
and ensures that transactions are assigned to threads in ways that
avoid deadlocks where the threadpool has run out of threads. It must
track refcounts of objects that are shared by several processes by
forwarding refcount changes between the processes correctly. It must
handle numerous error scenarios and it combines/nests 13 different
locks, 7 reference counters, and atomic variables. Finally, It must
do all of this as fast and efficiently as possible. Minor performance
regressions can cause a noticeably degraded user experience.
2. Things to improve: Thousand-line functions [1], error-prone error
handling [2], and confusing structure can occur as a code base grows
organically. After more than a decade of development, this codebase
could use an overhaul.
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/android/binder.c?h=v6.5#n2896
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/android/binder.c?h=v6.5#n3658
3. Security critical: Binder is a critical part of Android's sandboxing
strategy. Even Android's most de-privileged sandboxes (e.g. the
Chrome renderer, or SW Codec) have direct access to Binder. More than
just about any other component, it's important that Binder provide
robust security, and itself be robust against security
vulnerabilities.
It's #1 (high complexity) that has made continuing to evolve Binder and
resolving #2 (tech debt) exceptionally difficult without causing #3
(security issues). For Binder to continue to meet Android's needs, we
need better ways to manage (and reduce!) complexity without increasing
the risk.
The biggest change is obviously the choice of programming language. We
decided to use Rust because it directly addresses a number of the
challenges within Binder that we have faced during the last years. It
prevents mistakes with ref counting, locking, bounds checking, and also
does a lot to reduce the complexity of error handling. Additionally,
we've been able to use the more expressive type system to encode the
ownership semantics of the various structs and pointers, which takes the
complexity of managing object lifetimes out of the hands of the
programmer, reducing the risk of use-after-frees and similar problems.
Rust has many different pointer types that it uses to encode ownership
semantics into the type system, and this is probably one of the most
important aspects of how it helps in Binder. The Binder driver has a lot
of different objects that have complex ownership semantics; some
pointers own a refcount, some pointers have exclusive ownership, and
some pointers just reference the object and it is kept alive in some
other manner. With Rust, we can use a different pointer type for each
kind of pointer, which enables the compiler to enforce that the
ownership semantics are implemented correctly.
Another useful feature is Rust's error handling. Rust allows for more
simplified error handling with features such as destructors, and you get
compilation failures if errors are not properly handled. This means that
even though Rust requires you to spend more lines of code than C on
things such as writing down invariants that are left implicit in C, the
Rust driver is still slightly smaller than C binder: Rust is 5.5kLOC and
C is 5.8kLOC. (These numbers are excluding blank lines, comments,
binderfs, and any debugging facilities in C that are not yet implemented
in the Rust driver. The numbers include abstractions in rust/kernel/
that are unlikely to be used by other drivers than Binder.)
Although this rewrite completely rethinks how the code is structured and
how assumptions are enforced, we do not fundamentally change *how* the
driver does the things it does. A lot of careful thought has gone into
the existing design. The rewrite is aimed rather at improving code
health, structure, readability, robustness, security, maintainability
and extensibility. We also include more inline documentation, and
improve how assumptions in the code are enforced. Furthermore, all
unsafe code is annotated with a SAFETY comment that explains why it is
correct.
We have left the binderfs filesystem component in C. Rewriting it in
Rust would be a large amount of work and requires a lot of bindings to
the file system interfaces. Binderfs has not historically had the same
challenges with security and complexity, so rewriting binderfs seems to
have lower value than the rest of Binder.
Correctness and feature parity
------------------------------
Rust binder passes all tests that validate the correctness of Binder in
the Android Open Source Project. We can boot a device, and run a variety
of apps and functionality without issues. We have performed this both on
the Cuttlefish Android emulator device, and on a Pixel 6 Pro.
As for feature parity, Rust binder currently implements all features
that C binder supports, with the exception of some debugging facilities.
The missing debugging facilities will be added before we submit the Rust
implementation upstream.
Tracepoints
-----------
I did not include all of the tracepoints as I felt that the mechansim
for making C access fields of Rust structs should be discussed on list
separately. I also did not include the support for building Rust Binder
as a module since that requires exporting a bunch of additional symbols
on the C side.
Original RFC Link with old benchmark numbers:
https://lore.kernel.org/r/20231101-rust-binder-v1-0-08ba9197f637@google.com
Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Matt Gilbride <mattgilbride@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250919-rust-binder-v2-1-a384b09f28dd@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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