| Message ID | 20240202-alice-file-v4-4-fc9c2080663b@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | File abstractions needed by Rust Binder | expand |
On 2/2/24 11:55, Alice Ryhl wrote: > From: Wedson Almeida Filho <wedsonaf@gmail.com> > > Add a wrapper around `struct cred` called `Credential`, and provide > functionality to get the `Credential` associated with a `File`. > > Rust Binder must check the credentials of processes when they attempt to > perform various operations, and these checks usually take a > `&Credential` as parameter. The security_binder_set_context_mgr function > would be one example. This patch is necessary to access these security_* > methods from Rust. > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > Co-developed-by: Alice Ryhl <aliceryhl@google.com> > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > --- > rust/bindings/bindings_helper.h | 1 + > rust/helpers.c | 13 ++++++ > rust/kernel/cred.rs | 72 +++++++++++++++++++++++++++++++++ > rust/kernel/file.rs | 13 ++++++ > rust/kernel/lib.rs | 1 + > 5 files changed, 100 insertions(+) > create mode 100644 rust/kernel/cred.rs Reviewed-by: Benno Lossin <benno.lossin@proton.me> -- Cheers, Benno
On Fri, Feb 2, 2024 at 5:57 AM Alice Ryhl <aliceryhl@google.com> wrote: > > From: Wedson Almeida Filho <wedsonaf@gmail.com> > > Add a wrapper around `struct cred` called `Credential`, and provide > functionality to get the `Credential` associated with a `File`. > > Rust Binder must check the credentials of processes when they attempt to > perform various operations, and these checks usually take a > `&Credential` as parameter. The security_binder_set_context_mgr function > would be one example. This patch is necessary to access these security_* > methods from Rust. > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > Co-developed-by: Alice Ryhl <aliceryhl@google.com> > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > --- > [...] > diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs > new file mode 100644 > index 000000000000..fabc50e48c9e > --- /dev/null > +++ b/rust/kernel/cred.rs > @@ -0,0 +1,72 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +//! Credentials management. > +//! > +//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h). > +//! > +//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html> > + > +use crate::{ > + bindings, > + types::{AlwaysRefCounted, Opaque}, > +}; > + > +/// Wraps the kernel's `struct cred`. Could you add a brief description to the summary line? > +/// Most fields of credentials are immutable. When things have their credentials changed, that > +/// happens by replacing the credential instad of changing an existing credential. See the [kernel s/instad/instead > [...] Reviewed-by: Trevor Gross <tmgross@umich.edu>
On 2/2/24 07:55, Alice Ryhl wrote: > From: Wedson Almeida Filho <wedsonaf@gmail.com> > > Add a wrapper around `struct cred` called `Credential`, and provide > functionality to get the `Credential` associated with a `File`. > > Rust Binder must check the credentials of processes when they attempt to > perform various operations, and these checks usually take a > `&Credential` as parameter. The security_binder_set_context_mgr function > would be one example. This patch is necessary to access these security_* > methods from Rust. > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > Co-developed-by: Alice Ryhl <aliceryhl@google.com> > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > --- > [...] > + /// Returns the credentials of the task that originally opened the file. > + pub fn cred(&self) -> &Credential { > + // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is > + // never changed after initialization of the file. > + let ptr = unsafe { (*self.as_ptr()).f_cred }; From what I can see `file.f_cred` is valid from the moment `file` was initialized, worth to notice too IMO. > + > + // SAFETY: The signature of this function ensures that the caller will only access the > + // returned credential while the file is still valid, and the C side ensures that the > + // credential stays valid at least as long as the file. > + unsafe { Credential::from_ptr(ptr) } > + } > [...] Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
On Thu, Feb 8, 2024 at 1:35 AM Martin Rodriguez Reboredo <yakoyoku@gmail.com> wrote: > > On 2/2/24 07:55, Alice Ryhl wrote: > > From: Wedson Almeida Filho <wedsonaf@gmail.com> > > > > Add a wrapper around `struct cred` called `Credential`, and provide > > functionality to get the `Credential` associated with a `File`. > > > > Rust Binder must check the credentials of processes when they attempt to > > perform various operations, and these checks usually take a > > `&Credential` as parameter. The security_binder_set_context_mgr function > > would be one example. This patch is necessary to access these security_* > > methods from Rust. > > > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > > Co-developed-by: Alice Ryhl <aliceryhl@google.com> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > > --- > > [...] > > + /// Returns the credentials of the task that originally opened the file. > > + pub fn cred(&self) -> &Credential { > > + // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is > > + // never changed after initialization of the file. > > + let ptr = unsafe { (*self.as_ptr()).f_cred }; > > From what I can see `file.f_cred` is valid from the moment `file` was > initialized, worth to notice too IMO. That's what it says? Alice
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 41fcd2905ed4..84a56e8b6b67 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -7,6 +7,7 @@ */ #include <kunit/test.h> +#include <linux/cred.h> #include <linux/errname.h> #include <linux/ethtool.h> #include <linux/file.h> diff --git a/rust/helpers.c b/rust/helpers.c index 03141a3608a4..10ed69f76424 100644 --- a/rust/helpers.c +++ b/rust/helpers.c @@ -23,6 +23,7 @@ #include <kunit/test-bug.h> #include <linux/bug.h> #include <linux/build_bug.h> +#include <linux/cred.h> #include <linux/err.h> #include <linux/errname.h> #include <linux/fs.h> @@ -164,6 +165,18 @@ struct file *rust_helper_get_file(struct file *f) } EXPORT_SYMBOL_GPL(rust_helper_get_file); +const struct cred *rust_helper_get_cred(const struct cred *cred) +{ + return get_cred(cred); +} +EXPORT_SYMBOL_GPL(rust_helper_get_cred); + +void rust_helper_put_cred(const struct cred *cred) +{ + put_cred(cred); +} +EXPORT_SYMBOL_GPL(rust_helper_put_cred); + /* * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can * use it in contexts where Rust expects a `usize` like slice (array) indices. diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs new file mode 100644 index 000000000000..fabc50e48c9e --- /dev/null +++ b/rust/kernel/cred.rs @@ -0,0 +1,72 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Credentials management. +//! +//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h). +//! +//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html> + +use crate::{ + bindings, + types::{AlwaysRefCounted, Opaque}, +}; + +/// Wraps the kernel's `struct cred`. +/// +/// Most fields of credentials are immutable. When things have their credentials changed, that +/// happens by replacing the credential instad of changing an existing credential. See the [kernel +/// documentation][ref] for more info on this. +/// +/// # Invariants +/// +/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the +/// allocation remains valid at least until the matching call to `put_cred`. +/// +/// [ref]: https://www.kernel.org/doc/html/latest/security/credentials.html +#[repr(transparent)] +pub struct Credential(Opaque<bindings::cred>); + +// SAFETY: +// - `Credential::dec_ref` can be called from any thread. +// - It is okay to send ownership of `Credential` across thread boundaries. +unsafe impl Send for Credential {} + +// SAFETY: It's OK to access `Credential` through shared references from other threads because +// we're either accessing properties that don't change or that are properly synchronised by C code. +unsafe impl Sync for Credential {} + +impl Credential { + /// Creates a reference to a [`Credential`] from a valid pointer. + /// + /// # Safety + /// + /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the + /// returned [`Credential`] reference. + pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential { + // SAFETY: The safety requirements guarantee the validity of the dereference, while the + // `Credential` type being transparent makes the cast ok. + unsafe { &*ptr.cast() } + } + + /// Returns the effective UID of the given credential. + pub fn euid(&self) -> bindings::kuid_t { + // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid` + // field of a credential is never changed after initialization, so there is no potential + // for data races. + unsafe { (*self.0.get()).euid } + } +} + +// SAFETY: The type invariants guarantee that `Credential` is always ref-counted. +unsafe impl AlwaysRefCounted for Credential { + fn inc_ref(&self) { + // SAFETY: The existence of a shared reference means that the refcount is nonzero. + unsafe { bindings::get_cred(self.0.get()) }; + } + + unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) { + // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay + // because `Credential` has the same representation as `struct cred`. + unsafe { bindings::put_cred(obj.cast().as_ptr()) }; + } +} diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs index 0d6ef32009c6..095775411979 100644 --- a/rust/kernel/file.rs +++ b/rust/kernel/file.rs @@ -7,6 +7,7 @@ use crate::{ bindings, + cred::Credential, error::{code::*, Error, Result}, types::{ARef, AlwaysRefCounted, Opaque}, }; @@ -202,6 +203,18 @@ pub fn as_ptr(&self) -> *mut bindings::file { self.0.get() } + /// Returns the credentials of the task that originally opened the file. + pub fn cred(&self) -> &Credential { + // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is + // never changed after initialization of the file. + let ptr = unsafe { (*self.as_ptr()).f_cred }; + + // SAFETY: The signature of this function ensures that the caller will only access the + // returned credential while the file is still valid, and the C side ensures that the + // credential stays valid at least as long as the file. + unsafe { Credential::from_ptr(ptr) } + } + /// Returns the flags associated with the file. /// /// The flags are a combination of the constants in [`flags`]. diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 9353dd713a20..f65e5978f807 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -33,6 +33,7 @@ #[cfg(not(testlib))] mod allocator; mod build_assert; +pub mod cred; pub mod error; pub mod file; pub mod init;