linux-kernel - Re: [PATCH v4 1/3] aio_ring_remap: kill the bogus ctx->dead check

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Date:	Thu, 18 Jun 2015 16:01:26 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin LaHaise <bcrl@...ck.org>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] aio_ring_remap: kill the bogus ctx->dead check

Oleg Nesterov <oleg@...hat.com> writes:

> kill_ioctx() sets ctx->dead and removes ctx from ->ioctx_table
> "atomically" under mm->ioctx_lock, so aio_ring_remap() can never
> see a dead ctx.
>
> And even -EINVAL doesn't look necessary. Yes, if mremap() races
> with kill_ioctx() vm_munmap(ctx->mmap_base, ctx->mmap_size) can
> unmap the wrong region. In this case the buggy application should
> blame itself. And there are other reasons why that vm_munmap() can
> be wrong. Say, an application can mremap() the part of aio region
> and then do io_destroy(). We could change aio_ring_remap() to
> verify vma->that vma_end - vma->vma_start == ctx->mmap_size but
> this won't help if the application does munmap() instead.

I don't think this paragraph really fits with the patch.  It's
interesting commentary, but if you feel strongly enough about it, send a
patch that undoes b2edffdd912b.  ;-)

So, drop that paragraph from the commit message, and you can add my
reviewed by.  The patch itself looks good to me.

Cheers,
Jeff

>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  fs/aio.c |    9 +++------
>  1 files changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 480440f..893d300 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -325,14 +325,11 @@ static int aio_ring_remap(struct file *file, struct vm_area_struct *vma)
>  	rcu_read_lock();
>  	table = rcu_dereference(mm->ioctx_table);
>  	for (i = 0; i < table->nr; i++) {
> -		struct kioctx *ctx;
> +		struct kioctx *ctx = table->table[i];
>  
> -		ctx = table->table[i];
>  		if (ctx && ctx->aio_ring_file == file) {
> -			if (!atomic_read(&ctx->dead)) {
> -				ctx->user_id = ctx->mmap_base = vma->vm_start;
> -				res = 0;
> -			}
> +			ctx->user_id = ctx->mmap_base = vma->vm_start;
> +			res = 0;
>  			break;
>  		}
>  	}
--
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