linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Message-ID: <20201005175308.GI4225@quack2.suse.cz>
Date:   Mon, 5 Oct 2020 19:53:08 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     andrew Morton <akpm@...ux-foundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>, linux-mm@...ck.org,
        Hans Verkuil <hans.verkuil@...co.com>, Jan Kara <jack@...e.cz>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Mel Gorman <mgorman@...e.de>, stable@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

On Mon 05-10-20 14:38:54, Jason Gunthorpe wrote:
> When get_vaddr_frames() does its hacky follow_pfn() loop it should never
> be allowed to extract a struct page from a normal VMA. This could allow a
> serious use-after-free problem on any kernel memory.
> 
> Restrict this to only work on VMA's with one of VM_IO | VM_PFNMAP
> set. This limits the use-after-free problem to only IO memory, which while
> still serious, is an improvement.
> 
> Cc: stable@...r.kernel.org
> Fixes: 8025e5ddf9c1 ("[media] mm: Provide new get_vaddr_frames() helper")
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> ---
>  mm/frame_vector.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 10f82d5643b6de..26cb20544b6c37 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -99,6 +99,10 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  		if (ret >= nr_frames || start < vma->vm_end)
>  			break;
>  		vma = find_vma_intersection(mm, start, start + 1);
> +		if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
>  	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));

Hum, I fail to see how this helps. If vma has no VM_IO or VM_PFNMAP flag,
we'd exit the loop (to out: label) anyway due to the loop termination
condition and why not return the frames we already have? Furthermore
find_vma_intersection() can return NULL which would oops in your check
then. What am I missing?

								Honza

>  out:
>  	if (locked)
> -- 
> 2.28.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR