linux-kernel - Re: [PATCH] aio: fix mremap after fork null-deref

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Date:   Fri, 13 Jan 2023 11:18:33 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Seth Jenkins <sethjenkins@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Benjamin LaHaise <bcrl@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Pavel Emelyanov <xemul@...allels.com>, stable@...r.kernel.org
Subject: Re: [PATCH] aio: fix mremap after fork null-deref

Seth Jenkins <sethjenkins@...gle.com> writes:

> Commit e4a0d3e720e7 ("aio: Make it possible to remap aio ring") introduced
> a null-deref if mremap is called on an old aio mapping after fork as
> mm->ioctx_table will be set to NULL.
>
> Fixes: e4a0d3e720e7 ("aio: Make it possible to remap aio ring")
> Cc: stable@...r.kernel.org
> Signed-off-by: Seth Jenkins <sethjenkins@...gle.com>
> ---
>  fs/aio.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 5b2ff20ad322..74eae7de7323 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -361,16 +361,18 @@ static int aio_ring_mremap(struct vm_area_struct *vma)
>  	spin_lock(&mm->ioctx_lock);
>  	rcu_read_lock();
>  	table = rcu_dereference(mm->ioctx_table);
> -	for (i = 0; i < table->nr; i++) {
> -		struct kioctx *ctx;
> -
> -		ctx = rcu_dereference(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;
> +	if (table) {
> +		for (i = 0; i < table->nr; i++) {
> +			struct kioctx *ctx;
> +
> +			ctx = rcu_dereference(table->table[i]);
> +			if (ctx && ctx->aio_ring_file == file) {
> +				if (!atomic_read(&ctx->dead)) {
> +					ctx->user_id = ctx->mmap_base = vma->vm_start;

This is now over 80 columns.  I'd prefer it if you would just invert the
NULL pointer check and add a label:

if (!table)
	goto out_unlock;

Other than that, it looks good to me.

Cheers,
Jeff