* 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. 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
