linux-kernel - Re: [PATCH v11 1/4] random: add vgetrandom

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Message-ID: <87cz8xr96o.fsf@oldenburg.str.redhat.com>
Date:   Mon, 05 Dec 2022 19:34:39 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        tglx@...utronix.de, linux-crypto@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v11 1/4] random: add vgetrandom_alloc() syscall

* 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