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Validate guest-writable fields in try_pop_buffer_into before allocation#1262

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ludfjig merged 1 commit intohyperlight-dev:mainfrom
ludfjig:validate_guest_output
Feb 26, 2026
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Validate guest-writable fields in try_pop_buffer_into before allocation#1262
ludfjig merged 1 commit intohyperlight-dev:mainfrom
ludfjig:validate_guest_output

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@ludfjig ludfjig commented Feb 25, 2026 •
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try_pop_buffer_into reads two values from guest-writable shared memory

  • Size prefix: A malicious guest can set this to near u32::MAX, causing the host to attempt a ~4 GB allocation via vec![0; fb_buffer_size] (DoS).
  • Back-pointer: A malicious guest can point this outside the buffer, causing fill(0, ...) to zero unrelated host-managed data within shared memory (e.g., PEB fields, input buffer) and leaving the buffer stack in an inconsistent state.
  • Arithmetic overflow/underflow: A back-pointer just below the stack pointer causes underflow in stack_pointer_rel - last_element_offset_rel - 8. A size prefix of 0xFFFF_FFFD causes +4 to wrap around in debug (panic) or release (silent wrap to a small value).

Changes:

  • Validate last_element_offset_rel (back-pointer) is within [[8, stack_pointer_rel) before use
  • Validate fb_buffer_size does not exceed the actual element slot size before allocation
  • Use checked_add(4) on the size prefix instead of bare + 4, returning a descriptive error on u32 overflow
  • Add unit tests covering corrupt size prefix, corrupt back-pointer (too small / too large), off-by-one, back-pointer near stack pointer (underflow), and u32 size prefix overflow
  • Add integration tests with guest functions that write corrupt output buffers and halt

@ludfjig ludfjig added kind/bugfix For PRs that fix bugs area/security Involves security-related changes or fixes labels Feb 25, 2026
@ludfjig ludfjig requested a review from Copilot February 25, 2026 21:54

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@ludfjig ludfjig force-pushed the validate_guest_output branch from 24dccad to 7d51b11 Compare February 25, 2026 22:22
@ludfjig ludfjig requested a review from Copilot February 25, 2026 22:23
Copilot AI reviewed Feb 25, 2026
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Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

@ludfjig
Validate guest-writable fields in try_pop_buffer_into before allocation
56d40f6 
The back-pointer and flatbuffer size prefix in the shared output buffer
are written by the guest and were used without validation, allowing a
malicious guest to trigger a ~4 GB host-side allocation.

Add bounds checks on both fields before any heap allocation occurs and
return descriptive errors on violation. Add unit and integration tests
exercising corrupt size prefixes and back-pointers.

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>
@ludfjig ludfjig force-pushed the validate_guest_output branch from 7d51b11 to 56d40f6 Compare February 25, 2026 22:37
syntactically
syntactically reviewed Feb 26, 2026
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syntactically approved these changes Feb 26, 2026
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The arithmetic overflows causing unintentional panics issue I bet is something we have in other places. (I stand by some of the arguments I made earlier about panic-ing being reasonable behaviour in some situations---but it's important that we are directly aware of the panic opportunities and clear on whether they make sense---i.e. we should have a sensible reasoned argument why they ought to be impossible, even in the face of attacker-controlled data). It would be interesting to take a look at e.g. enumerating every cfg path from any exported function to panic (on an opt build, so that a bunch of the easy ones are gone) and see how much work it would be to make an argument either way on each path.

@ludfjig ludfjig merged commit 513b08e into hyperlight-dev:main Feb 26, 2026
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