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

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 <s...@prgmr.com>
Tested-by: Sarah Newman <s...@prgmr.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 
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 

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 
           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);
@@ -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,
+        put_page(frags[i].page);
+    }
   -    if (frags[i].page)
+    if (frags[i].page_owner)
   @@ -184,9 +194,10 @@ static int mlx4_en_init_allocator(struct mlx4_en_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);
           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 
             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?


           while (page_alloc->page_offset + frag_info->frag_stride <
                  page_alloc->page_size) {
               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 
       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 
             dma_unmap_page(priv->ddev, frame->dma, frame->page_size,
+        WARN_ON(frame->page_owner);
       ring->page_cache.index = 0;
@@ -595,7 +614,7 @@ static int mlx4_en_complete_rx_desc(struct mlx4_en_priv 
           frag_info = &priv->frag_info[nr];
           if (length <= frag_info->frag_prefix_size)
-        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 
           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 
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 
       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 
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;


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.


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