Re: [RFC PATCH v2 2/2] rust: add Random Number Generator algorithms support

[Benno Lossin <benno.lossin@proton.me>]

> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/helpers.c                  | 12 ++++++
>  rust/kernel/crypto.rs           |  1 +
>  rust/kernel/crypto/rng.rs       | 72 +++++++++++++++++++++++++++++++++
>  4 files changed, 86 insertions(+)
>  create mode 100644 rust/kernel/crypto/rng.rs
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 2f198c6d5de5..089ac38c6461 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -7,6 +7,7 @@
>   */
> 
>  #include <crypto/hash.h>
> +#include <crypto/rng.h>
>  #include <linux/errname.h>
>  #include <linux/slab.h>
>  #include <linux/refcount.h>
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 7966902ed8eb..e4dcd611738f 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -19,6 +19,7 @@
>   */
> 
>  #include <crypto/hash.h>
> +#include <crypto/rng.h>
>  #include <linux/bug.h>
>  #include <linux/build_bug.h>
>  #include <linux/err.h>
> @@ -52,6 +53,17 @@ int rust_helper_crypto_shash_init(struct shash_desc *desc) {
>  	return crypto_shash_init(desc);
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_crypto_shash_init);
> +
> +void rust_helper_crypto_free_rng(struct crypto_rng *tfm) {
> +	crypto_free_rng(tfm);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_crypto_free_rng);
> +
> +int rust_helper_crypto_rng_generate(struct crypto_rng *tfm, const u8 *src,
> +	unsigned int slen, u8 *dst, unsigned int dlen) {
> +	return crypto_rng_generate(tfm, src, slen, dst, dlen);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_crypto_rng_generate);
>  #endif
> 
>  __noreturn void rust_helper_BUG(void)
> diff --git a/rust/kernel/crypto.rs b/rust/kernel/crypto.rs
> index f80dd7bd3381..a1995e6c85d4 100644
> --- a/rust/kernel/crypto.rs
> +++ b/rust/kernel/crypto.rs
> @@ -3,3 +3,4 @@
>  //! Cryptography.
> 
>  pub mod hash;
> +pub mod rng;
> diff --git a/rust/kernel/crypto/rng.rs b/rust/kernel/crypto/rng.rs
> new file mode 100644
> index 000000000000..2215a8344669
> --- /dev/null
> +++ b/rust/kernel/crypto/rng.rs
> @@ -0,0 +1,72 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Random number generator.
> +//!
> +//! C headers: [`include/crypto/rng.h`](../../../../include/crypto/rng.h)
> +
> +use crate::{
> +    error::{from_err_ptr, to_result, Result},
> +    str::CStr,
> +};
> +
> +/// Corresponds to the kernel's `struct crypto_rng`.
> +///
> +/// # Invariants
> +///
> +/// The pointer is valid.
> +pub struct Rng(*mut bindings::crypto_rng);
> +
> +impl Drop for Rng {
> +    fn drop(&mut self) {
> +        // SAFETY: The type invariant guarantees that `self.0` is valid.

This safety comment should explain why it is safe to access this mutable
static variable.

> +        if unsafe { bindings::crypto_default_rng } == self.0 {
> +            // SAFETY: FFI call.
> +            unsafe {
> +                bindings::crypto_put_default_rng();
> +            }
> +        } else {
> +            // SAFETY: The type invariant guarantees that `self.0` is valid.
> +            unsafe { bindings::crypto_free_rng(self.0) };
> +        }
> +    }
> +}
> +
> +impl Rng {
> +    /// Creates a [`Rng`] instance.
> +    pub fn new(name: &CStr, t: u32, mask: u32) -> Result<Self> {
> +        // SAFETY: FFI call.

If there are no requirements then say so.

> +        let ptr = unsafe { from_err_ptr(bindings::crypto_alloc_rng(name.as_char_ptr(), t, mask)) }?;
> +        // INVARIANT: `ptr` is valid and non-null since `crypto_alloc_rng`
> +        // returned a valid pointer which was null-checked.
> +        Ok(Self(ptr))
> +    }
> +
> +    /// Creates a [`Rng`] instance with a default algorithm.
> +    pub fn new_with_default() -> Result<Self> {
> +        // SAFETY: FFI call.
> +        to_result(unsafe { bindings::crypto_get_default_rng() })?;
> +        // SAFETY: The C API guarantees that `crypto_default_rng` is valid until
> +        // `crypto_put_default_rng` is called.
> +        Ok(Self(unsafe { bindings::crypto_default_rng }))
> +    }
> +
> +    /// Get a random number.
> +    pub fn generate(&mut self, src: &[u8], dst: &mut [u8]) -> Result {
> +        // SAFETY: The type invariant guarantees that the pointer is valid.
> +        to_result(unsafe {
> +            bindings::crypto_rng_generate(
> +                self.0,
> +                src.as_ptr(),
> +                src.len() as u32,

What happens when `src.len() > u32::MAX`? Then it will overflow here and
write less than `len` random bytes. Not sure what we want to do in this 
case though, maybe call it in a loop? Return an error? Also below.

> +                dst.as_mut_ptr(),
> +                dst.len() as u32,
> +            )
> +        })
> +    }
> +
> +    /// Re-initializes the [`Rng`] instance.
> +    pub fn reset(&mut self, seed: &[u8]) -> Result {
> +        // SAFETY: The type invariant guarantees that the pointer is valid.
> +        to_result(unsafe { bindings::crypto_rng_reset(self.0, seed.as_ptr(), seed.len() as u32) })

Same problem with the as u32 cast here.

--
Cheers,
Benno

> +    }
> +}
> --
> 2.34.1
>