On Fri, Sep 13, 2024 at 12:52:39AM +0200, Jann Horn wrote: > FWIW, I would still feel happier if this was a 64-bit number, though I > guess at least with uprobes the attack surface is not that large even > if you can wrap that counter... 2^31 counter increments are not all > that much, especially if someone introduces a kernel path in the > future that lets you repeatedly take the mmap_lock for writing within > a single syscall without doing much work, or maybe on some machine > where syscalls are really fast. I really don't like hinging memory > safety on how fast or slow some piece of code can run, unless we can > make strong arguments about it based on how many memory writes a CPU > core is capable of doing per second or stuff like that.
You could repeatedly call munmap(1, 0) which will take the mmap_write_lock, do no work and call mmap_write_unlock(). We could fix that by moving the start/len validation outside the mmap_write_lock(), but it won't increase the path length by much. How many syscalls can we do per second? https://blogs.oracle.com/linux/post/syscall-latency suggests 217ns per syscall, so we'll be close to 4.6m syscalls/second or 466 seconds (7 minutes, 46 seconds).
