On Mon, Apr 27, 2020 at 01:45:46PM -0700, Sami Tolvanen wrote: > On Fri, Apr 24, 2020 at 12:21:14PM +0100, Will Deacon wrote: > > Also, since you mentioned the lack of redzoning, isn't it a bit dodgy > > allocating blindly out of the kmem_cache? It means we don't have a redzone > > or a guard page, so if you can trigger something like a recursion bug then > > could you scribble past the SCS before the main stack overflows? Would this > > clobber somebody else's SCS? > > I agree that allocating from a kmem_cache isn't ideal for safety. It's a > compromise to reduce memory overhead. Do you think it would be a problem if we always allocated a page for the SCS? > > The vmap version that I asked Sami to drop > > is at least better in this regard, although the guard page is at the wrong > > end of the stack and we just hope that the allocation below us didn't pass > > VM_NO_GUARD. Looks like the same story for vmap stack :/ > > SCS grows up and the guard page is after the allocation, so how is it at > the wrong end? Am I missing something here? Sorry, I'd got the SCS upside-down in my head (hey, that second 'S' stands for 'Stack'!). But I think I'm right about vmap stack, which feels a little fragile even though it seems to work out today with the very limited uses of VM_NO_GUARD. > > If we split the pointer in two (base, offset) then we could leave the > > base live in the thread_info, not require alignment of the stacks (which > > may allow for unconditional redzoning?) and then just update the offset > > value on context switch, which could be trivially checked as part of the > > existing stack overflow checking on kernel entry. > > I sent out v13 with split pointers, but I'm not sure it's convenient to > add an overflow check to kernel_ventry where the VMAP_STACK check is > done. I suppose I could add a check to kernel_entry after we load x18 > from tsk. Thoughts? I'll take a look at v13, since at this stage I'm keen to get something queued up so that we can use it as a base for further improvements without you having to repost the whole stack every time. Cheers, Will
