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Comment by Joker_vD

3 days ago

I mean, if the stacks grew upwards, that alone would nip 90% of buffer overflow attacks in the bud. Moving the return address from the activation frame into a separate stack would help as well, but I understand that having an activation frame to be a single piece of data (a current continuation's closure, essentially) can be quite convenient.

16 comments

Joker_vD

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musicale  3 days ago

The PL/I stack growing up rather than down reduced potential impact of stack overflows in Multics (and PL/I already had better memory safety, with bounded strings, etc.) TFA's author would probably have appreciated the segmented memory architecture as well.

There is no reason why the C/C++ stack can't grow up rather than down. On paged hardware, both the stack and heap could (and probably should) grow up. "C's stack should grow up", one might say.

  • Joker_vD  3 days ago

    > There is no reason why the C/C++ stack can't grow up rather than down.

    Historical accident. Imagine if PDP-7/PDP-11 easily allowed for the following memory layout:

        FFFF +---------------+
         |     text      |  X
         +---------------+
         |    rodata     |  R
         +---------------+
         |  data + bss   |  RW
         +---------------+
         |     heap      |
         |      ||       |  RW
         |      \/       |
         +---------------+
         |  empty space  |  unmapped
         +---------------+
         |      /\       |
         |      ||       |  RW
         |     stack     |
    0000 +---------------+

    Things could have turned out very differently than they have. Oh well.

    • musicale  2 days ago

      Nice diagram. I might put read-only pages on both sides of 0 though to mitigate null pointer effects.

  • josephg  3 days ago

    Is there anything stopping us from doing this today on modern hardware? Why do we grow the stack down?

    • Veserv  3 days ago

      x86-64 call instruction decrements the stack pointer to push the return address. x86-64 push instructions decrement the stack pointer. The push instructions are easy to work around because most compilers already just push the entire stack frame at once and then do offset accesses, but the call instruction would be kind of annoying.

      ARM does not suffer from that problem due to the usage of link registers and generic pre/post-modify. RISC-V is probably also safe, but I have not looked specifically.

      2 replies →

    • sph  3 days ago

      Nothing stops you from having upward growing stacks in RISC-V, for example, as there are no dedicated stack instructions.

      Instead of

        addi sp, sp, -16
  sd a0, 0(sp)
  sd a1, 8(sp)

      Do:

        addi sp, sp, 16
  sd a0, -8(sp)
  sd a1, -16(sp)

ch_123  3 days ago

HP-UX on PA-RISC had an upward-growing stack. In practice, various exploits were developed which adapted to the changed direction of the stack.

One source from a few mins of searching: https://phrack.org/issues/58/11

  • LukeShu  3 days ago

    Linux on PA-RISC also has an upward-growing stack (AFAIK, it's the only architecture Linux has ever had an upward-growing stack on; it's certainly the only currently-supported one).

    • musicale  3 days ago

      Both this and parent comment about PA-RISC are very interesting.

      As noted, stack growing up doesn't prevent all stack overflows, but it makes it less trivially easy to overwrite a return address. Bounded strings also made it less trivially easy to create string buffer overflows.

    • ch_123  3 days ago

      Yeah, my assumption is that all the PA-RISC operating systems did, but I only know about HP-UX for certain.

dmitrygr  3 days ago

In ARMv4/v5 (non-thumb-mode) stack is purely a convention that hardware does not enforce. Nobody forces you to use r13 as the stack pointer or to make the stack descending. You can prototype your approach trivially with small changes to gcc and linux kernel. As this is a standard architectural feature, qemu and the like will support emulating this. And it would run fine on real hardware too. I'd read the paper you publish based on this.

axoltl  3 days ago

For modern systems, stack buffer overflow bugs haven't been great to exploit for a while. You need at least a stack cookie leak and on Apple Silicon the return addresses are MACed so overwriting them is a fools errand (2^-16 chance of success).

Most exploitable memory corruption bugs are heap buffer overflows.