linux-kernel - Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

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Date:   Mon, 17 Oct 2016 14:19:47 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     viro@...iv.linux.org.uk, jack@...e.cz, dmonakhov@...nvz.org,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

Hi, Christoph,

Christoph Hellwig <hch@....de> writes:

> diff --git a/fs/aio.c b/fs/aio.c
> index 1157e13..bf315cd 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1078,6 +1078,17 @@ static void aio_complete(struct kiocb *kiocb, long res, long res2)
>  	unsigned tail, pos, head;
>  	unsigned long	flags;
>  
> +	if (kiocb->ki_flags & IOCB_WRITE) {
> +		struct file *file = kiocb->ki_filp;
> +
> +		/*
> +		 * Tell lockdep we inherited freeze protection from submission
> +		 * thread.
> +		 */
> +		__sb_writers_acquired(file_inode(file)->i_sb, SB_FREEZE_WRITE);
> +		file_end_write(file);

This ends up being a call to __sb_end_write:

void __sb_end_write(struct super_block *sb, int level)
{
        percpu_up_read(sb->s_writers.rw_sem + level-1);
}

Nothing guarantees that submission and completion happen on the same
CPU.  Is this safe?

-Jeff