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Date: Sat, 1 Jun 2024 12:56:40 +0200
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-kernel@vger.kernel.org, patches@lists.linux.dev,
	tglx@linutronix.de, linux-crypto@vger.kernel.org,
	linux-api@vger.kernel.org, x86@kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
	Carlos O'Donell <carlos@redhat.com>,
	Florian Weimer <fweimer@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Jann Horn <jannh@google.com>,
	Christian Brauner <brauner@kernel.org>,
	David Hildenbrand <dhildenb@redhat.com>
Subject: Re: [PATCH v16 2/5] random: add vgetrandom_alloc() syscall
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References: <20240528122352.2485958-1-Jason@zx2c4.com>
 <20240528122352.2485958-3-Jason@zx2c4.com>
 <20240531035917.GD6505@sol.localdomain>
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In-Reply-To: <20240531035917.GD6505@sol.localdomain>

On Thu, May 30, 2024 at 08:59:17PM -0700, Eric Biggers wrote:
> On Tue, May 28, 2024 at 02:19:51PM +0200, Jason A. Donenfeld wrote:
> > +/**
> > + * 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, while
> > + * ensuring that no single state straddles a page boundary.
> > + *
> > + * 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().
> 
> Wouldn't a munmap with '(size_t)num * (size_t)size_per_each' be potentially too
> short, due to how the allocation is sized such that states don't cross page
> boundaries?

You're right, I think. The calculation should instead be something like:

    DIV_ROUND_UP(num, PAGE_SIZE / size_per_each) * PAGE_SIZE

Does that seem correct to you?

Jason