block_buffer: panic corrupts inline buffer position

Summary

A caught panic may leave the cursor position of EagerBuffer or ReadBuffer in a corrupted state; this in turn allows out-of-bounds reads/writes.

Details & PoC

The following two tests fail miri:

#[cfg(miri)]
#[test]
fn eager_digest_blocks_panic_corrupts_inline_position() {
    // `EagerBuffer` stores its cursor in the last byte of the internal block.
    // When `digest_blocks` completes a previously partial block, it overwrites
    // that byte with input data before invoking the caller-provided `compress`
    // callback. If the callback panics, safe code can catch the panic and keep
    // using the buffer while its cursor byte no longer satisfies the internal
    // `pos < block_size` invariant. Under Miri this `get_pos` call reaches the
    // `unreachable_unchecked` used for the assumed-valid cursor.
    let mut buf = EagerBuffer::<U4>::new(&[1, 2]);

    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        buf.digest_blocks(&[3, 0xff], |_| panic!("simulated compression failure"));
    }));

    let _ = buf.get_pos();
}

#[cfg(miri)]
#[test]
fn read_buffer_generator_panic_corrupts_inline_position() {
    // `ReadBuffer` stores its cursor in `buffer[0]`, but `write_block` gives
    // `gen_block` mutable access to the whole internal block before restoring
    // `buffer[0]` to a valid cursor. If `gen_block` writes an arbitrary first
    // byte and panics, safe code can catch the panic and later observe an
    // invalid cursor. Under Miri this `get_pos` call reaches the
    // `unreachable_unchecked` used for the assumed-valid cursor.
    let mut buf = ReadBuffer::<U4>::default();

    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        buf.write_block(
            1,
            |block| {
                block[0] = 0xff;
                panic!("simulated block generation failure");
            },
            |_| {},
        );
    }));

    let _ = buf.get_pos();
}

They fail on an unreachable_unchecked!() under the invariant for the pos to always be within bounds of the block.

Impact

While the byte that overwrites pos may come from untrusted input and is therefore attacker-controlled, this still relies on the surrounding code catching the panic and carrying on, which should be uncommon in practice.

For this to be exploitable, the attacker also needs a way to trigger a panic here; I have not investigated how feasible that is.

Credits

The issue was discovered by GPT-5.5