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Rewrite limit_to_pages to avoid storing 2^32 in u32 #1346

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merged 1 commit into from
Jul 11, 2022
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Rewrite limit_to_pages to avoid storing 2^32 in u32 #1346

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19 changes: 13 additions & 6 deletions packages/vm/src/wasm_backend/store.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ use super::limiting_tunables::LimitingTunables;
/// is 65536 (2^16) bytes. In WebAssembly version 1, a linear memory can have at
/// most 65536 pages, for a total of 2^32 bytes (4 gibibytes).
/// https://github.com/WebAssembly/memory64/blob/master/proposals/memory64/Overview.md
const MAX_WASM_MEMORY: usize = 4 * 1024 * 1024 * 1024;
const MAX_WASM_PAGES: u32 = 65536;

fn cost(_operator: &Operator) -> u64 {
// A flat fee for each operation
Expand Down Expand Up @@ -86,12 +86,17 @@ fn make_store_with_engine(engine: &dyn Engine, memory_limit: Option<Size>) -> St
}

fn limit_to_pages(limit: Size) -> Pages {
let capped = std::cmp::min(limit.0, MAX_WASM_MEMORY);
// round down to ensure the limit is less than or equal to the config
let pages: u32 = (capped / WASM_PAGE_SIZE)
.try_into()
.expect("Value must be <= 4 GiB/64KiB, i.e. fit in uint32. This is a bug.");
Pages(pages)
let limit_in_pages: usize = limit.0 / WASM_PAGE_SIZE;

let capped = match u32::try_from(limit_in_pages) {
Ok(x) => std::cmp::min(x, MAX_WASM_PAGES),
// The only case where TryFromIntError can happen is when
// limit_in_pages exceeds the u32 range. In this case it is way
// larger than MAX_WASM_PAGES and needs to be capped.
Err(_too_large) => MAX_WASM_PAGES,
};
Pages(capped)
}

#[cfg(test)]
Expand All @@ -113,10 +118,12 @@ mod tests {
assert_eq!(limit_to_pages(Size::kibi(63)), Pages(0));
assert_eq!(limit_to_pages(Size::kibi(64)), Pages(1));
assert_eq!(limit_to_pages(Size::kibi(65)), Pages(1));
assert_eq!(limit_to_pages(Size(u32::MAX as usize)), Pages(65535));
// caps at 4 GiB
assert_eq!(limit_to_pages(Size::gibi(3)), Pages(49152));
assert_eq!(limit_to_pages(Size::gibi(4)), Pages(65536));
assert_eq!(limit_to_pages(Size::gibi(5)), Pages(65536));
assert_eq!(limit_to_pages(Size(usize::MAX)), Pages(65536));
}

#[test]
Expand Down