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Message-ID: <0b28d134-eec4-4e81-1b26-e05e38fe5605@mellanox.com>
Date: Tue, 6 Mar 2018 18:13:24 +0200
From: Tariq Toukan <tariqt@...lanox.com>
To: Sarah Newman <srn@...mr.com>, Tariq Toukan <tariqt@...lanox.com>,
Yishai Hadas <yishaih@...lanox.com>
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH] net/mlx4_en: fix potential use-after-free with
dma_unmap_page
On 05/03/2018 11:10 PM, Sarah Newman wrote:
> On 03/05/2018 02:09 AM, Tariq Toukan wrote:
>>
>>
>> On 05/03/2018 6:20 AM, Sarah Newman wrote:
>>> Take an additional reference to a page whenever it is placed
>>> into the rx ring and put the page again after running
>>> dma_unmap_page.
>>>
>>> When swiotlb is in use, calling dma_unmap_page means that
>>> the original page mapped with dma_map_page must still be valid,
>>> as swiotlb will copy data from its internal cache back to the
>>> originally requested DMA location.
>>>
>>> When GRO is enabled, before this patch all references to the
>>> original frag may be put and the page freed before dma_unmap_page
>>> in mlx4_en_free_frag is called.
>>>
>>> It is possible there is a path where the use-after-free occurs
>>> even with GRO disabled, but this has not been observed so far.
>>>
>>> The bug can be trivially detected by doing the following:
>>>
>>> * Compile the kernel with DEBUG_PAGEALLOC
>>> * Run the kernel as a Xen Dom0
>>> * Leave GRO enabled on the interface
>>> * Run a 10 second or more test with iperf over the interface.
>>>
>>
>> Hi Sarah, thanks for your patch!
>>
>>> This bug was likely introduced in
>>> commit 4cce66cdd14a ("mlx4_en: map entire pages to increase throughput"),
>>> first part of u3.6.
>>>
>>> It was incidentally fixed in
>>> commit 34db548bfb95 ("mlx4: add page recycling in receive path"),
>>> first part of v4.12.
>>>
>>> This version applies to the v4.9 series.
>>>
>>> Signed-off-by: Sarah Newman <srn@...mr.com>
>>> Tested-by: Sarah Newman <srn@...mr.com>
>>> ---
>>> drivers/net/ethernet/mellanox/mlx4/en_rx.c | 39 +++++++++++++++++++++-------
>>> drivers/net/ethernet/mellanox/mlx4/en_tx.c | 3 ++-
>>> drivers/net/ethernet/mellanox/mlx4/mlx4_en.h | 1 +
>>> 3 files changed, 32 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
>>> index bcbb80f..d1fb087 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
>>> @@ -80,10 +80,14 @@ static int mlx4_alloc_pages(struct mlx4_en_priv *priv,
>>> page_alloc->page = page;
>>> page_alloc->dma = dma;
>>> page_alloc->page_offset = 0;
>>> + page_alloc->page_owner = true;
>>
>> Do we really need this boolean? I believe the issue can be fixed without it. We need to make sure we hold the correct refcnt at every stage, and
>> maintain symmetry between a flow and its inverse.
>
> The reason this was added is because the page address needs to stay around until after dma unmap_page is called, and right now setting page to NULL is
> used to indicate that put_page should not be called when frags are freed in mlx4_en_free_frag. So either the code needs to be rearranged so that
> dma_unmap_page while page is still set, or some variable needed to be used to indicate whether put_page should be called when the frags are freed.
>
rearranging sounds better.
> If dma_unmap_page was called before page was set to NULL, then this variable doesn't need to be added, yes. Then the call to dma_unmap_page in
> mlx4_en_free_frag would also be contingent on frags[i].page being set.
>
> There are two places where page is set to NULL without calling dma_unmap_page first, mlx4_en_complete_rx_desc and mlx4_en_xmit_frame.
>
In mlx4_en_xmit_frame, should not unmap, it should be done only upon
completion, this is done in mlx4_en_recycle_tx_desc.
In mlx4_en_complete_rx_desc I think it was just a bug.
>
> Is mlx4_en_complete_rx_desc the only place where a call to dma_unmap_page would need to be added? The other place page is set to NULL without a call
> to dma_unmap_page first is in mlx4_en_xmit_frame, and I believe there is no call to mlx4_en_free_frag if mlx4_en_xmit_frame executes.
>
Yes, only in mlx4_en_complete_rx_desc, see above.
>>
>> Upon alloc, refcnt is 1. This alloc refcnt should be inverted by a call to put_page. We might want to introduce a page free API (symmetric to
>> mlx4_alloc_pages), that does: dma unmap the page, call put_page, nullify pointer.
>
> That seems reasonable.
>
Yes, let's use it in mlx4_en_free_frag.
>> Once alloced, page refcnt is bumped up by the amount of possible frags populating it, which is (page_size / frag_stride), as you do here.
>>
>>> /* Not doing get_page() for each frag is a big win
>>> * on asymetric workloads. Note we can not use atomic_set().
>>> */
>>> - page_ref_add(page, page_alloc->page_size / frag_info->frag_stride - 1);
>>> + /* Since the page must be valid until after dma_unmap_page is called,
>>> + * take an additional reference we would not have otherwise.
>>> + */
>>> + page_ref_add(page, page_alloc->page_size / frag_info->frag_stride);
>>> return 0;
>>> }
>>> @@ -105,9 +109,13 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
>>> page_alloc[i].page_offset += frag_info->frag_stride;
>>> if (page_alloc[i].page_offset + frag_info->frag_stride <=
>>> - ring_alloc[i].page_size)
>>> - continue;
>>> -
>>> + ring_alloc[i].page_size) {
>>> + WARN_ON(!page_alloc[i].page);
>>> + WARN_ON(!page_alloc[i].page_owner);
>>
>> Why WARN before the likely() check?
>> Move after the check, for a better performance.
>
> No particular reason.
>
please move them.
>>
>>> + if (likely(page_alloc[i].page &&
>>> + page_alloc[i].page_owner))
>>> + continue;
>>> + }
>>> if (unlikely(mlx4_alloc_pages(priv, &page_alloc[i],
>>> frag_info, gfp)))
>>> goto out;
>>> @@ -131,7 +139,7 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
>>> page = page_alloc[i].page;
>>> /* Revert changes done by mlx4_alloc_pages */
>>> page_ref_sub(page, page_alloc[i].page_size /
>>> - priv->frag_info[i].frag_stride - 1);
>>> + priv->frag_info[i].frag_stride);
>>> put_page(page);
>>> }
>>> }
>>> @@ -146,11 +154,13 @@ static void mlx4_en_free_frag(struct mlx4_en_priv *priv,
>>> u32 next_frag_end = frags[i].page_offset + 2 * frag_info->frag_stride;
>>> - if (next_frag_end > frags[i].page_size)
>>> + if (next_frag_end > frags[i].page_size) {
>>> dma_unmap_page(priv->ddev, frags[i].dma, frags[i].page_size,
>>> frag_info->dma_dir);
>>> + put_page(frags[i].page);
>>> + }
>>> - if (frags[i].page)
>>> + if (frags[i].page_owner)
>>> put_page(frags[i].page);
>>> }
>>> @@ -184,9 +194,10 @@ static int mlx4_en_init_allocator(struct mlx4_en_priv *priv,
>>> page = page_alloc->page;
>>> /* Revert changes done by mlx4_alloc_pages */
>>> page_ref_sub(page, page_alloc->page_size /
>>> - priv->frag_info[i].frag_stride - 1);
>>> + priv->frag_info[i].frag_stride);
>>> put_page(page);
>>> page_alloc->page = NULL;
>>> + page_alloc->page_owner = false;
>>> }
>>> return -ENOMEM;
>>> }
>>> @@ -206,12 +217,14 @@ static void mlx4_en_destroy_allocator(struct mlx4_en_priv *priv,
>>> dma_unmap_page(priv->ddev, page_alloc->dma,
>>> page_alloc->page_size, frag_info->dma_dir);
>>> + put_page(page_alloc->page);
>>
>> for symmetry, i'd move this after the while loop.
>
> Or use the wrapper function you suggested for dma_unmap_page?
>
yes.
>>
>>> while (page_alloc->page_offset + frag_info->frag_stride <
>>> page_alloc->page_size) {
>>> put_page(page_alloc->page);
>>> page_alloc->page_offset += frag_info->frag_stride;
>>> }
>>> page_alloc->page = NULL;
>>> + page_alloc->page_owner = false;
>>> }
>>> }
>>> @@ -251,6 +264,11 @@ static int mlx4_en_prepare_rx_desc(struct mlx4_en_priv *priv,
>>> if (ring->page_cache.index > 0) {
>>> frags[0] = ring->page_cache.buf[--ring->page_cache.index];
>>> rx_desc->data[0].addr = cpu_to_be64(frags[0].dma);
>>> + WARN_ON(frags[0].page_owner);
>>> + if (likely(!frags[0].page_owner)) {
>>> + page_ref_inc(frags[0].page);
>>> + frags[0].page_owner = true;
>>> + }
>>
>> Why? If I'm not mistaken, the page is cached with refcnt == 2. No?
>
> In mlx4_en_deactivate_rx_ring, pages assigned to frames in the page_cache are only put once. If refcnt == 2 when it's inserted, isn't that a memory
> leak? I can confirm one way or another if you haven't already.
>
I think you're right. But I didn't check this yet.
>
>>
>>> return 0;
>>> }
>>> @@ -569,6 +587,7 @@ void mlx4_en_deactivate_rx_ring(struct mlx4_en_priv *priv,
>>> dma_unmap_page(priv->ddev, frame->dma, frame->page_size,
>>> priv->frag_info[0].dma_dir);
>>> + WARN_ON(frame->page_owner);
>>> put_page(frame->page);
>>> }
>>> ring->page_cache.index = 0;
>>> @@ -595,7 +614,7 @@ static int mlx4_en_complete_rx_desc(struct mlx4_en_priv *priv,
>>> frag_info = &priv->frag_info[nr];
>>> if (length <= frag_info->frag_prefix_size)
>>> break;
>>> - if (unlikely(!frags[nr].page))
>>> + if (unlikely(!frags[nr].page_owner))
>>> goto fail;
>>> dma = be64_to_cpu(rx_desc->data[nr].addr);
>>> @@ -607,7 +626,7 @@ static int mlx4_en_complete_rx_desc(struct mlx4_en_priv *priv,
>>> skb_frag_size_set(&skb_frags_rx[nr], frag_info->frag_size);
>>> skb_frags_rx[nr].page_offset = frags[nr].page_offset;
>>> skb->truesize += frag_info->frag_stride;
>>> - frags[nr].page = NULL;
>>> + frags[nr].page_owner = false;
>>> }
>>> /* Adjust size of last fragment to match actual length */
>>> if (nr > 0)
>>> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>>> index e2509bb..25f7f9e 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>>> @@ -356,6 +356,7 @@ u32 mlx4_en_recycle_tx_desc(struct mlx4_en_priv *priv,
>>> .dma = tx_info->map0_dma,
>>> .page_offset = 0,
>>> .page_size = PAGE_SIZE,
>>> + .page_owner = false,
>>
>> I don't understand why this is needed.
>
> Not strictly needed but there for clarity.
>
Let's obsolete it.
>>
>>> };
>>> if (!mlx4_en_rx_recycle(ring->recycle_ring, &frame)) {
>>> @@ -1128,7 +1129,7 @@ netdev_tx_t mlx4_en_xmit_frame(struct mlx4_en_rx_alloc *frame,
>>> dma = frame->dma;
>>> tx_info->page = frame->page;
>>> - frame->page = NULL;
>>> + frame->page_owner = false;
>>> tx_info->map0_dma = dma;
>>> tx_info->map0_byte_count = length;
>>> tx_info->nr_txbb = nr_txbb;
>>> diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
>>> index df0f396..2c9d9a6 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
>>> +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
>>> @@ -261,6 +261,7 @@ struct mlx4_en_rx_alloc {
>>> dma_addr_t dma;
>>> u32 page_offset;
>>> u32 page_size;
>>> + bool page_owner;
>>> };
>>> #define MLX4_EN_CACHE_SIZE (2 * NAPI_POLL_WEIGHT)
>>>
>>
>> Thanks,
>> Tariq
>
> Thanks, Sarah
>
I have a general question about the process.
I don't totally get what branch this patch is targeted to.
It touches critical areas in datapath and should go through regression
tests before it is accepted to any branch.
Thanks,
Tariq
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