On Mon, Oct 10, 2022 at 01:13:05PM +0200, Florian Weimer wrote: > * Rick Edgecombe: > > > When operating with shadow stacks enabled, the kernel will automatically > > allocate shadow stacks for new threads, however in some cases userspace > > will need additional shadow stacks. The main example of this is the > > ucontext family of functions, which require userspace allocating and > > pivoting to userspace managed stacks. > > > > Unlike most other user memory permissions, shadow stacks need to be > > provisioned with special data in order to be useful. They need to be setup > > with a restore token so that userspace can pivot to them via the RSTORSSP > > instruction. But, the security design of shadow stack's is that they > > should not be written to except in limited circumstances. This presents a > > problem for userspace, as to how userspace can provision this special > > data, without allowing for the shadow stack to be generally writable. > > > > Previously, a new PROT_SHADOW_STACK was attempted, which could be > > mprotect()ed from RW permissions after the data was provisioned. This was > > found to not be secure enough, as other thread's could write to the > > shadow stack during the writable window. > > > > The kernel can use a special instruction, WRUSS, to write directly to > > userspace shadow stacks. So the solution can be that memory can be mapped > > as shadow stack permissions from the beginning (never generally writable > > in userspace), and the kernel itself can write the restore token. > > > > First, a new madvise() flag was explored, which could operate on the > > PROT_SHADOW_STACK memory. This had a couple downsides: > > 1. Extra checks were needed in mprotect() to prevent writable memory from > > ever becoming PROT_SHADOW_STACK. > > 2. Extra checks/vma state were needed in the new madvise() to prevent > > restore tokens being written into the middle of pre-used shadow stacks. > > It is ideal to prevent restore tokens being added at arbitrary > > locations, so the check was to make sure the shadow stack had never been > > written to. > > 3. It stood out from the rest of the madvise flags, as more of direct > > action than a hint at future desired behavior. > > > > So rather than repurpose two existing syscalls (mmap, madvise) that don't > > quite fit, just implement a new map_shadow_stack syscall to allow > > userspace to map and setup new shadow stacks in one step. While ucontext > > is the primary motivator, userspace may have other unforeseen reasons to > > setup it's own shadow stacks using the WRSS instruction. Towards this > > provide a flag so that stacks can be optionally setup securely for the > > common case of ucontext without enabling WRSS. Or potentially have the > > kernel set up the shadow stack in some new way. > > > > The following example demonstrates how to create a new shadow stack with > > map_shadow_stack: > > void *shstk = map_shadow_stack(adrr, stack_size, SHADOW_STACK_SET_TOKEN); > > Jason has recently been working on vDSO-based getrandom acceleration. > It needs a way for a userspace thread to allocate userspace memory in a > specific way. Jason proposed to use a vDSO call as the interface, not a > system call. Not quite so in the latest revision of that patch: https://lore.kernel.org/lkml/20220916125916.652546-1-Jason@xxxxxxxxx/ Jason > > Maybe this approach is applicable here as well? Or we can come up with > a more general interface for such per-thread allocations? > > Thanks, > Florian >
