linux-kernel - Re: [PATCH v2 1/1] mm: introduce mmap_lock_speculation

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Message-ID: <ZvLzueEY9Sbyz1H4@casper.infradead.org>
Date: Tue, 24 Sep 2024 18:15:37 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jann Horn <jannh@...gle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
	linux-trace-kernel@...r.kernel.org, peterz@...radead.org,
	oleg@...hat.com, rostedt@...dmis.org, mhiramat@...nel.org,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org,
	paulmck@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
	mjguzik@...il.com, brauner@...nel.org, andrii@...nel.org
Subject: Re: [PATCH v2 1/1] mm: introduce mmap_lock_speculation_{start|end}

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).