On Sat, Jun 01, 2024 at 12:56:40PM +0200, Jason A. Donenfeld wrote:

On Thu, May 30, 2024 at 08:59:17PM -0700, Eric Biggers wrote:

quoted

On Tue, May 28, 2024 at 02:19:51PM +0200, Jason A. Donenfeld wrote:

quoted

+/**
+ * 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?

Yes, though I wonder if it would be better to give userspace the number of pages
instead of the number of states.

- Eric