On 27.03.2023 10:36, Juergen Gross wrote:

Fix xenvif_get_requests() not to do grant copy operations across local page boundaries. This requires to double the maximum number of copy operations per queue, as each copy could now be split into 2.

Make sure that struct xenvif_tx_cb doesn't grow too large.

Cc: stable@vger.kernel.org Fixes: ad7f402ae4f4 ("xen/netback: Ensure protocol headers don't fall in the non-linear area") Signed-off-by: Juergen Gross jgross@suse.com

---

drivers/net/xen-netback/common.h | 2 +- drivers/net/xen-netback/netback.c | 25 +++++++++++++++++++++++-- 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h index 3dbfc8a6924e..1fcbd83f7ff2 100644 --- a/drivers/net/xen-netback/common.h +++ b/drivers/net/xen-netback/common.h @@ -166,7 +166,7 @@ struct xenvif_queue { /* Per-queue data for xenvif */ struct pending_tx_info pending_tx_info[MAX_PENDING_REQS]; grant_handle_t grant_tx_handle[MAX_PENDING_REQS];

• struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS];

• struct gnttab_copy tx_copy_ops[2 * MAX_PENDING_REQS]; struct gnttab_map_grant_ref tx_map_ops[MAX_PENDING_REQS]; struct gnttab_unmap_grant_ref tx_unmap_ops[MAX_PENDING_REQS]; /* passed to gnttab_[un]map_refs with pages under (un)mapping */

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c index 1b42676ca141..111c179f161b 100644 --- a/drivers/net/xen-netback/netback.c +++ b/drivers/net/xen-netback/netback.c @@ -334,6 +334,7 @@ static int xenvif_count_requests(struct xenvif_queue *queue, struct xenvif_tx_cb { u16 copy_pending_idx[XEN_NETBK_LEGACY_SLOTS_MAX + 1]; u8 copy_count;

• u32 split_mask;

}; #define XENVIF_TX_CB(skb) ((struct xenvif_tx_cb *)(skb)->cb) @@ -361,6 +362,8 @@ static inline struct sk_buff *xenvif_alloc_skb(unsigned int size) struct sk_buff *skb = alloc_skb(size + NET_SKB_PAD + NET_IP_ALIGN, GFP_ATOMIC | __GFP_NOWARN);

• BUILD_BUG_ON(sizeof(*XENVIF_TX_CB(skb)) > sizeof(skb->cb)); if (unlikely(skb == NULL)) return NULL;

@@ -396,11 +399,13 @@ static void xenvif_get_requests(struct xenvif_queue *queue, nr_slots = shinfo->nr_frags + 1; copy_count(skb) = 0;

• XENVIF_TX_CB(skb)->split_mask = 0;

/* Create copy ops for exactly data_len bytes into the skb head. */ __skb_put(skb, data_len); while (data_len > 0) { int amount = data_len > txp->size ? txp->size : data_len;

• bool split = false;
  

cop->source.u.ref = txp->gref; cop->source.domid = queue->vif->domid; @@ -413,6 +418,13 @@ static void xenvif_get_requests(struct xenvif_queue *queue, cop->dest.u.gmfn = virt_to_gfn(skb->data + skb_headlen(skb) - data_len);

• /* Don't cross local page boundary! */
  

• if (cop->dest.offset + amount > XEN_PAGE_SIZE) {
  

• 	amount = XEN_PAGE_SIZE - cop->dest.offset;
  

• 	XENVIF_TX_CB(skb)->split_mask |= 1U << copy_count(skb);
  

Maybe worthwhile to add a BUILD_BUG_ON() somewhere to make sure this shift won't grow too large a shift count. The number of slots accepted could conceivably be grown past XEN_NETBK_LEGACY_SLOTS_MAX (i.e. XEN_NETIF_NR_SLOTS_MIN) at some point.

• 	split = true;
  

• }
  

• cop->len = amount; cop->flags = GNTCOPY_source_gref;

@@ -420,7 +432,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue, pending_idx = queue->pending_ring[index]; callback_param(queue, pending_idx).ctx = NULL; copy_pending_idx(skb, copy_count(skb)) = pending_idx;

• copy_count(skb)++;
  

• if (!split)
  

• 	copy_count(skb)++;
  

cop++; data_len -= amount; @@ -441,7 +454,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue, nr_slots--; } else { /* The copy op partially covered the tx_request.

• 	 * The remainder will be mapped.
  

• 	 * The remainder will be mapped or copied in the next
  

• 	 * iteration.
   */
  txp->offset += amount;
  txp->size -= amount;
  

@@ -539,6 +553,13 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue, pending_idx = copy_pending_idx(skb, i); newerr = (*gopp_copy)->status;

• /* Split copies need to be handled together. */
  

• if (XENVIF_TX_CB(skb)->split_mask & (1U << i)) {
  

• 	(*gopp_copy)++;
  

• 	if (!newerr)
  

• 		newerr = (*gopp_copy)->status;
  

• }
  

It isn't guaranteed that a slot may be split only once, is it? Assuming a near-64k packet with all tiny non-primary slots, that'll cause those tiny slots to all be mapped, but due to

if (ret >= XEN_NETBK_LEGACY_SLOTS_MAX - 1 && data_len < txreq.size) data_len = txreq.size;

will, afaict, cause a lot of copying for the primary slot. Therefore I think you need a loop here, not just an if(). Plus tx_copy_ops[]'es dimension also looks to need further growing to accommodate this. Or maybe not - at least the extreme example given would still be fine; more generally packets being limited to below 64k means 2*16 slots would suffice at one end of the scale, while 2*MAX_PENDING_REQS would at the other end (all tiny, including the primary slot). What I haven't fully convinced myself of is whether there might be cases in the middle which are yet worse.

As I've been struggling with the code fragment quoted above already in the patch originally introducing it, I'd like to see that relaxed. Can't we avoid excessive copying by suitably growing tx_map_ops[] and then deleting that bumping of data_len? Then there also wouldn't be the risk multiple splits per copy anymore.

Alternatively to all of the above: Am I overlooking a check somewhere which would also constrain the primary slot (or more precisely its residual) to within a single page (along the lines of the check for non-primary slots in xenvif_count_requests())?

Jan

On 27.03.2023 12:07, Juergen Gross wrote:

On 27.03.23 11:49, Jan Beulich wrote:

...

On 27.03.2023 10:36, Juergen Gross wrote:

...

@@ -413,6 +418,13 @@ static void xenvif_get_requests(struct xenvif_queue *queue, cop->dest.u.gmfn = virt_to_gfn(skb->data + skb_headlen(skb) - data_len);

• /* Don't cross local page boundary! */
  

• if (cop->dest.offset + amount > XEN_PAGE_SIZE) {
  

• 	amount = XEN_PAGE_SIZE - cop->dest.offset;
  

• 	XENVIF_TX_CB(skb)->split_mask |= 1U << copy_count(skb);
  

Maybe worthwhile to add a BUILD_BUG_ON() somewhere to make sure this shift won't grow too large a shift count. The number of slots accepted could conceivably be grown past XEN_NETBK_LEGACY_SLOTS_MAX (i.e. XEN_NETIF_NR_SLOTS_MIN) at some point.

This is basically impossible due to the size restriction of struct xenvif_tx_cb.

If its size became a problem, it might simply take a level of indirection to overcome the limitation.

...

...

@@ -420,7 +432,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue, pending_idx = queue->pending_ring[index]; callback_param(queue, pending_idx).ctx = NULL; copy_pending_idx(skb, copy_count(skb)) = pending_idx;

• copy_count(skb)++;
  

• if (!split)
  

• 	copy_count(skb)++;
  

cop++; data_len -= amount; @@ -441,7 +454,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue, nr_slots--; } else { /* The copy op partially covered the tx_request.

• 	 * The remainder will be mapped.
  

• 	 * The remainder will be mapped or copied in the next
  

• 	 * iteration.
   */
  txp->offset += amount;
  txp->size -= amount;
  

@@ -539,6 +553,13 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue, pending_idx = copy_pending_idx(skb, i); newerr = (*gopp_copy)->status;

• /* Split copies need to be handled together. */
  

• if (XENVIF_TX_CB(skb)->split_mask & (1U << i)) {
  

• 	(*gopp_copy)++;
  

• 	if (!newerr)
  

• 		newerr = (*gopp_copy)->status;
  

• }
  

It isn't guaranteed that a slot may be split only once, is it? Assuming a

I think it is guaranteed.

No slot can cover more than XEN_PAGE_SIZE bytes due to the grants being restricted to that size. There is no way how such a data packet could cross 2 page boundaries.

In the end the problem isn't the copies for the linear area not crossing multiple page boundaries, but the copies for a single request slot not doing so. And this can't happen IMO.

You're thinking of only well-formed requests. What about said request providing a large size with only tiny fragments? xenvif_get_requests() will happily process such, creating bogus grant-copy ops. But them failing once submitted to Xen will be only after damage may already have occurred (from bogus updates of internal state; the logic altogether is too involved for me to be convinced that nothing bad can happen).

Interestingly (as I realize now) the shifts you add are not be at risk of turning UB in this case, as the shift count won't go beyond 16.

...

near-64k packet with all tiny non-primary slots, that'll cause those tiny slots to all be mapped, but due to

if (ret >= XEN_NETBK_LEGACY_SLOTS_MAX - 1 && data_len < txreq.size)
	data_len = txreq.size;

will, afaict, cause a lot of copying for the primary slot. Therefore I think you need a loop here, not just an if(). Plus tx_copy_ops[]'es dimension also looks to need further growing to accommodate this. Or maybe not - at least the extreme example given would still be fine; more generally packets being limited to below 64k means 2*16 slots would suffice at one end of the scale, while 2*MAX_PENDING_REQS would at the other end (all tiny, including the primary slot). What I haven't fully convinced myself of is whether there might be cases in the middle which are yet worse.

See above reasoning. I think it is okay, but maybe I'm missing something.

Well, the main thing I'm missing is a "primary request fits in a page" check, even more so with the new copying logic that the commit referenced by Fixes: introduced into xenvif_get_requests().

Jan

On 27.03.2023 18:22, Juergen Gross wrote:

On 27.03.23 17:38, Jan Beulich wrote:

...

On 27.03.2023 12:07, Juergen Gross wrote:

...

On 27.03.23 11:49, Jan Beulich wrote:

...

On 27.03.2023 10:36, Juergen Gross wrote:

...

@@ -539,6 +553,13 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue, pending_idx = copy_pending_idx(skb, i); newerr = (*gopp_copy)->status;

• /* Split copies need to be handled together. */
  

• if (XENVIF_TX_CB(skb)->split_mask & (1U << i)) {
  

• 	(*gopp_copy)++;
  

• 	if (!newerr)
  

• 		newerr = (*gopp_copy)->status;
  

• }
  

It isn't guaranteed that a slot may be split only once, is it? Assuming a

I think it is guaranteed.

No slot can cover more than XEN_PAGE_SIZE bytes due to the grants being restricted to that size. There is no way how such a data packet could cross 2 page boundaries.

In the end the problem isn't the copies for the linear area not crossing multiple page boundaries, but the copies for a single request slot not doing so. And this can't happen IMO.

You're thinking of only well-formed requests. What about said request providing a large size with only tiny fragments? xenvif_get_requests() will happily process such, creating bogus grant-copy ops. But them failing once submitted to Xen will be only after damage may already have occurred (from bogus updates of internal state; the logic altogether is too involved for me to be convinced that nothing bad can happen).

There are sanity checks after each relevant RING_COPY_REQUEST() call, which will bail out if "(txp->offset + txp->size) > XEN_PAGE_SIZE" (the first one is after the call of xenvif_count_requests(), as this call will decrease the size of the request, the other check is in xenvif_count_requests()).

Oh, indeed - that's the check I've been overlooking. (The messages logged there could do with also mentioning "Cross page boundary", like the one in xenvif_count_requests() does.)

Jan