Re: [PATCH v11 1/4] random: add vgetrandom_alloc() syscall

Florian Weimer
2022-12-05T18:34:51Z

* Jason A. Donenfeld:

> +/********************************************************************
> + *
> + * vDSO support helpers.
> + *
> + * The actual vDSO function is defined over in lib/vdso/getrandom.c,
> + * but this section contains the kernel-mode helpers to support that.
> + *
> + ********************************************************************/
> +
> +#ifdef CONFIG_VDSO_GETRANDOM
> +/**
> + * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
> + *
> + * @num: On input, a pointer to a suggested hint of how many states to
> + * allocate, and on return the number of states actually allocated.
> + *
> + * @size_per_each: On input, must be zero. On return, the size of each state allocated,
> + * so that the caller can split up the returned allocation into
> + * individual states.
> + *
> + * @addr: Reserved, must be zero.
> + *
> + * @flags: Reserved, must be zero.
> + *
> + * The getrandom() vDSO function in userspace requires an opaque state, which
> + * this function allocates by mapping a certain number of special pages into
> + * the calling process. It takes a hint as to the number of opaque states
> + * desired, and provides the caller with the number of opaque states actually
> + * allocated, the size of each one in bytes, and the address of the first
> + * state, which may be split up into @num states of @size_per_each bytes each,
> + * by adding @size_per_each to the returned first state @num times.
> + *
> + * Returns the address of the first state in the allocation on success, or a
> + * negative error value on failure.
> + *
> + * The returned address of the first state may be passed to munmap(2) with a
> + * length of `(size_t)num * (size_t)size_per_each`, in order to deallocate the
> + * memory, after which it is invalid to pass it to vDSO getrandom().
> + *
> + * States allocated by this function must not be dereferenced, written, read,
> + * or otherwise manipulated. The *only* supported operations are:
> + * - Splitting up the states in intervals of @size_per_each, no more than
> + * @num times from the first state.
> + * - Passing a state to the getrandom() vDSO function's @opaque_state
> + * parameter, but not passing the same state at the same time to two such
> + * calls.
> + * - Passing the first state to munmap(2), as described above.
> + * All other uses are undefined behavior, which is subject to change or removal

Suggest: “Passing the first state *and total length* to munmap(2)”

Rest of the documentation looks good to me. It addresses my concerns
about future evolution of this interface.

Thanks,
Florian