linux-kernel - RE: lockdep: holding locks across syscall boundaries

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Date:   Sun, 29 Oct 2023 22:01:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Jens Axboe <axboe@...nel.dk>
CC:     Ingo Molnar <mingo@...hat.com>, LKML <linux-kernel@...r.kernel.org>
Subject: RE: lockdep: holding locks across syscall boundaries

From: Peter Zijlstra
> Sent: 27 October 2023 17:00
> 
> On Fri, Oct 27, 2023 at 09:14:53AM -0600, Jens Axboe wrote:
> > Hi,
> >
> > Normally we'd expect locking state to be clean and consistent across
> > syscall entry and exit, as that is always the case for sync syscalls.
> 
> > We currently have a work-around for holding a lock from aio, see
> > kiocb_start_write(), which pretends to drop the lock from lockdeps
> > perspective, as it's held from submission to until kiocb_end_write() is
> > called at completion time.
> 
> I was not aware of this, the only such hack I knew about was the
> filesystem freezer thing.
> 
> The problem with holding locks past the end of a syscall is that you'll
> nest whatever random lock hierarchies possibly by every other syscall
> under that lock.
> 
...
> 
> Suppose syscall-a returns with your kiocb thing held, call it lock A
> Suppose syscall-b returns with your inode thing held, call it lock B
> 
> Then userspace does:
> 
> 	syscall-a
> 	syscall-b
> 
> while it also does:
> 
> 	syscall-b
> 	syscall-a
> 
> and we're up a creek, no?

Isn't it also open to a massive denial-of-service attack?
	syscall-a
	sleep(infinity)

assuming you actually catch:
	syscall-a
	_exit()

	David

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