3 ms·

> Why wouldn't the WASM JIT retpoline all code (probably only if the underlying processor needs it) rather than relying on the guest programs to do it themselve

> Why wouldn't the WASM JIT retpoline all code (probably only if the underlying processor needs it) rather than relying on the guest programs to do it themselves. Possible, but is that then a runtime option in the WASM header? How does it know if the program needs the cost of retpoline or not? And how can a program ensure that it's getting such mitigations when it needs them? > Once again, wouldn't the JIT be able to enforce this at runtime? The JIT has no insight into the app's malloc/free usage, so no, it can't. There is no system allocator in WASM, just a big chunk of linear memory that the app does whatever it wants with. For an actual case study in why this is risky all you have to do is look at OpenSSL's heartbleed. If it had been using the system's malloc/free (which WASM doesn't have) instead of using its internal free lists then heartbleed largely wouldn't have existed on platforms like OpenBSD.

monocasa 8y ago
> Possible, but is that then a runtime option in the WASM header? How does it know if the program needs the cost of retpoline or not? And how can a program ensure that it's getting such mitigations when it needs them? You're looking at it the wrong way. Untrusted code doesn't have a "I pinky promise that I'm safe" bit they can twiddle in their header. If the code running is untrusted, it doesn't get to make that decision, all code under that context needs to be retpolined. (V8 already does this https://chromium.googlesource.com/v8/v8/+/7d356ac4927e9da3e0c9757e214aa4b20f68acca https://chromium.googlesource.com/v8/v8/+/7d356ac4927e9da3e0... ) > The JIT has no insight into the app's malloc/free usage, so no, it can't. There is no system allocator in WASM, just a big chunk of linear memory that the app does whatever it wants with. It can retarget the code so that the virtual base address of the linear region isn't fixed. That's about all ASLR does anyway. > For an actual case study in why this is risky all you have to do is look at OpenSSL's heartbleed. If it had been using the system's malloc/free (which WASM doesn't have) instead of using its internal free lists then heartbleed largely wouldn't have existed on platforms like OpenBSD. You can implement all of the malloc protection strategies that OpenBSD uses in user space. It's completely legitimate to use your own malloc in those circumstances, libc's malloc isn't magically special, and it makes sense that they'd want to rely on memory allocation with additional guarantees than libc gives you. OpenSSL's sin wasn't re-implementing malloc; it was re-implementing it poorly (and not bounds checking input from attacker controlled packets).
- brynet 8y ago
  This is ignoring an important benefit of the libc implementations of malloc/free on OpenBSD, free pages are being opportunistically munmap'd at random. This catches actual bugs. Otto Moerbeek has a great recent Twitter thread on the topic: https://twitter.com/ottom6k/status/1065594523559096321 https://twitter.com/ottom6k/status/1065594523559096321 https://twitter.com/ottom6k/status/1063118597160091648 https://twitter.com/ottom6k/status/1063118597160091648 > The JIT has no insight into the app's malloc/free usage, so no, it can't. There is no system allocator in WASM, just a big chunk of linear memory that the app does whatever it wants with. This point by kllrnohj illustrates a pretty significant weakness.
  - monocasa 8y ago
    I figured kllrnohj was referencing the fact that OpenBSD clears data when it's allocated. That you can do from WASM pretty easily. Additionally WASM is moving towards multiple linear regions, which should allow mmap/munmap like functionality. And in the context of OpenSSL, this kind of fanciness in some libc's that'll make all sorts of syscalls randomly is exactly why they wrote their own allocator.
    - brynet 8y ago
      The Heartbleed bug would have been found then fixed by the munmap, as the process would have crashed. This has nothing to do with clearing data, it would not have found this. > And in the context of OpenSSL, this kind of fanciness in some libc's that'll make all sorts of syscalls randomly is exactly why they wrote their own allocator. "web asm -- if it was good enough for openssl!"
      - monocasa 8y ago
        Heartbleed fully allocated the larger buffer. It wouldn't have crashed. It was just uninitialized memory being read out. Yes, munmap and remap would have cleared that ram, but so would the malloc clearing. > "web asm -- if it was good enough for openssl!" More like, "OpenBSD, so slow that people feel the need to write their own malloc".

monocasa 8y ago

> Possible, but is that then a runtime option in the WASM header? How does it know if the program needs the cost of retpoline or not? And how can a program ensure that it's getting such mitigations when it needs them? You're looking at it the wrong way. Untrusted code doesn't have a "I pinky promise that I'm safe" bit they can twiddle in their header. If the code running is untrusted, it doesn't get to make that decision, all code under that context needs to be retpolined. (V8 already does this https://chromium.googlesource.com/v8/v8/+/7d356ac4927e9da3e0c9757e214aa4b20f68acca https://chromium.googlesource.com/v8/v8/+/7d356ac4927e9da3e0... ) > The JIT has no insight into the app's malloc/free usage, so no, it can't. There is no system allocator in WASM, just a big chunk of linear memory that the app does whatever it wants with. It can retarget the code so that the virtual base address of the linear region isn't fixed. That's about all ASLR does anyway. > For an actual case study in why this is risky all you have to do is look at OpenSSL's heartbleed. If it had been using the system's malloc/free (which WASM doesn't have) instead of using its internal free lists then heartbleed largely wouldn't have existed on platforms like OpenBSD. You can implement all of the malloc protection strategies that OpenBSD uses in user space. It's completely legitimate to use your own malloc in those circumstances, libc's malloc isn't magically special, and it makes sense that they'd want to rely on memory allocation with additional guarantees than libc gives you. OpenSSL's sin wasn't re-implementing malloc; it was re-implementing it poorly (and not bounds checking input from attacker controlled packets).

brynet 8y ago
This is ignoring an important benefit of the libc implementations of malloc/free on OpenBSD, free pages are being opportunistically munmap'd at random. This catches actual bugs. Otto Moerbeek has a great recent Twitter thread on the topic: https://twitter.com/ottom6k/status/1065594523559096321 https://twitter.com/ottom6k/status/1065594523559096321 https://twitter.com/ottom6k/status/1063118597160091648 https://twitter.com/ottom6k/status/1063118597160091648 > The JIT has no insight into the app's malloc/free usage, so no, it can't. There is no system allocator in WASM, just a big chunk of linear memory that the app does whatever it wants with. This point by kllrnohj illustrates a pretty significant weakness.
- monocasa 8y ago
  I figured kllrnohj was referencing the fact that OpenBSD clears data when it's allocated. That you can do from WASM pretty easily. Additionally WASM is moving towards multiple linear regions, which should allow mmap/munmap like functionality. And in the context of OpenSSL, this kind of fanciness in some libc's that'll make all sorts of syscalls randomly is exactly why they wrote their own allocator.
  - brynet 8y ago
    The Heartbleed bug would have been found then fixed by the munmap, as the process would have crashed. This has nothing to do with clearing data, it would not have found this. > And in the context of OpenSSL, this kind of fanciness in some libc's that'll make all sorts of syscalls randomly is exactly why they wrote their own allocator. "web asm -- if it was good enough for openssl!"
    - monocasa 8y ago
      Heartbleed fully allocated the larger buffer. It wouldn't have crashed. It was just uninitialized memory being read out. Yes, munmap and remap would have cleared that ram, but so would the malloc clearing. > "web asm -- if it was good enough for openssl!" More like, "OpenBSD, so slow that people feel the need to write their own malloc".