On Fri, Feb 2, 2024 at 9:05 AM Theo de Raadt <deraadt@xxxxxxxxxxx> wrote: > > Another interaction to consider is sigaltstack(). > > In OpenBSD, sigaltstack() forces MAP_STACK onto the specified > (pre-allocated) region, because on kernel-entry we require the "sp" > register to point to a MAP_STACK region (this severely damages ROP pivot > methods). Linux does not have MAP_STACK enforcement (yet), but one day > someone may try to do that work. > > This interacted poorly with mimmutable() because some applications > allocate the memory being provided poorly. I won't get into the details > unless pushed, because what we found makes me upset. Over the years, > we've upstreamed diffs to applications to resolve all the nasty > allocation patterns. I think the software ecosystem is now mostly > clean. > > I suggest someone in Linux look into whether sigaltstack() is a mseal() > bypass, perhaps somewhat similar to madvise MADV_FREE, and consider the > correct strategy. > Thanks for bringing this up. I will follow up on sigaltstack() in Linux. > This is our documented strategy: > > On OpenBSD some additional restrictions prevent dangerous address space > modifications. The proposed space at ss_sp is verified to be > contiguously mapped for read-write permissions (no execute) and incapable > of syscall entry (see msyscall(2)). If those conditions are met, a page- > aligned inner region will be freshly mapped (all zero) with MAP_STACK > (see mmap(2)), destroying the pre-existing data in the region. Once the > sigaltstack is disabled, the MAP_STACK attribute remains on the memory, > so it is best to deallocate the memory via a method that results in > munmap(2). > > OK, I better provide the details of what people were doing. > sigaltstacks() in .data, in .bss, using malloc(), on a buffer on the > stack, we even found one creating a sigaltstack inside a buffer on a > pthread stack. We told everyone to use mmap() and munmap(), with MAP_STACK > if #ifdef MAP_STACK finds a definition. >
