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