Messages in this thread | | | Date | Mon, 27 Apr 2020 13:45:46 -0700 | From | Sami Tolvanen <> | Subject | Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS) |
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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.
> 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?
> 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?
Sami
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