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From:   Jeff Moyer <jmoyer@redhat.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>
Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
References: <20181128183531.5139-1-willy@infradead.org>
Date:   Thu, 06 Dec 2018 17:21:31 -0500
In-Reply-To: <20181128183531.5139-1-willy@infradead.org> (Matthew Wilcox's
        message of "Wed, 28 Nov 2018 10:35:31 -0800")
Message-ID: <x49va46e1p0.fsf@segfault.boston.devel.redhat.com>
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Matthew Wilcox <willy@infradead.org> writes:

> This custom resizing array was vulnerable to a Spectre attack (speculating
> off the end of an array to a user-controlled offset).  The XArray is
> not vulnerable to Spectre as it always masks its lookups to be within
> the bounds of the array.

I'm not a big fan of completely re-writing the code to fix this.  Isn't
the below patch sufficient?

-Jeff

diff --git a/fs/aio.c b/fs/aio.c
index 97f983592925..9402ae0b63d5 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1038,6 +1038,7 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
 	if (!table || id >= table->nr)
 		goto out;
 
+	id = array_index_nospec(index, table->nr);
 	ctx = rcu_dereference(table->table[id]);
 	if (ctx && ctx->user_id == ctx_id) {
 		if (percpu_ref_tryget_live(&ctx->users))