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Date:   Sat, 24 Jul 2021 01:31:54 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] io_uring: refactor io_sq_offload_create()
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On Sat, Jul 24, 2021 at 12:45:12AM +0100, Matthew Wilcox wrote:
> On Fri, Jul 23, 2021 at 08:19:49PM +0000, Al Viro wrote:
> > To elaborate: ->release() instance may not assume anything about current->mm,
> > or assume anything about current, for that matter.  It is entirely possible
> > to arrange its execution in context of a process that is not yours and had not
> > consent to doing that.  In particular, it's a hard bug to have _any_ visible
> > effects depending upon the memory mappings, memory contents or the contents of
> > descriptor table of the process in question.
> 
> Hmm.  Could we add a poison_current() function?  Something like ...
> 
> static inline void call_release(struct file *file, struct inode *inode)
> {
> 	void *tmp = poison_current();
> 	if (file->f_op->release)
> 		file->f_op->release(inode, file);
> 	restore_current(tmp);
> }
> 
> Should be straightforward for asm-generic/current.h and for x86 too.
> Probably have to disable preemption?  Maybe interrupts too?  Not sure
> what's kept in current these days that an interrupt handler might
> rely on being able to access temporarily.

->release() might grab a mutex, for example.  Scheduler is going to be unhappy
if it runs into somebody playing silly buggers with current...