[PATCH net v4 0/2] gve: fix RX page refcount overflow with the pagecnt bias

[PATCH net v4 0/2] gve: fix RX page refcount overflow with the pagecnt bias

[Amaan Cheval]

gve's GQI and DQO-QPL RX paths bias every RX page's refcount to INT_MAX,
so a single get_page() from the stack, e.g. splice() of received data,
overflows it: splice/tee fail with EIO/EFAULT and GQI can reset the
NIC. Patch 2 lowers the bias to USHRT_MAX, as other drivers do.

With the smaller bias the driver gives up its last reference on a page
once every USHRT_MAX handoffs instead of once every INT_MAX, which is
only safe while every consumer holds its reference until the buffer is
recycled. That is not the case for an XDP_REDIRECT from DQO-QPL to an
AF_XDP socket in copy mode, a path the driver never meant to support:
commit d8a8ca14c937 dropped the queue_format check that used to refuse
it. Patch 1 restores that check for DQO-QPL, so patch 2 depends on it.
Trees before 6.17 still have the old check, so there patch 2 stands
alone.

Harshitha mentioned on v2 that Google would send a patch blocking XDP
on DQO-QPL; if one is already in flight I'm happy to drop patch 1 in
its favour.

v4:
 - Add patch 1/2 blocking XDP on DQO-QPL (Sashiko).
 - Say "overflow" instead of "headroom"; use USHRT_MAX directly instead
   of a macro (Maciej).
 - Say which trees need patch 1, for stable.
v3: https://lore.kernel.org/netdev/20261103090651.35399-1-amaan.cheval@gmail.com/
v2: https://lore.kernel.org/netdev/20261102112613.18334-1-amaan.cheval@gmail.com/
v1: https://lore.kernel.org/netdev/20261031173449.2439047-1-amaan.cheval@gmail.com/


Amaan Cheval (2):
  gve: block XDP on the DQO-QPL queue format
  gve: fix RX page refcount overflow from the pagecnt bias

 drivers/net/ethernet/google/gve/gve.h                |  2 +-
 .../net/ethernet/google/gve/gve_buffer_mgmt_dqo.c    |  4 ++--
 drivers/net/ethernet/google/gve/gve_main.c           |  6 ++++++
 drivers/net/ethernet/google/gve/gve_rx.c             |  8 ++++----
 drivers/net/ethernet/google/gve/gve_utils.c          | 12 ++++--------
 drivers/net/ethernet/google/gve/gve_utils.h          |  2 +-
 6 files changed, 18 insertions(+), 16 deletions(-)


base-commit: 21f57ec673a1917547c808cfa81890485fffa03b
-- 
2.47.3

[PATCH net v4 1/2] gve: block XDP on the DQO-QPL queue format

[Amaan Cheval]

Commit d8a8ca14c937 ("gve: add XDP_TX and XDP_REDIRECT support for DQ
RDA") enabled XDP on DQO-RDA by dropping the queue_format check from
verify_xdp_configuration(), which until then refused every format but
GQI-QPL. Nothing replaced it for DQO-QPL: gve_set_netdev_xdp_features()
advertises no XDP features there and gve_xdp_xmit() rejects it, but the
core only checks that ndo_bpf exists, so a native XDP program, and an
AF_XDP socket on top of it, can still be attached on DQO-QPL.

The DQO-QPL RX path was never written for that. Its page accounting
assumes the driver keeps a reference of its own across every handoff:
gve_xdp_done_dqo() passes the buffer to xdp_do_redirect() and then
recycles it with gve_reuse_buffer(), so a redirect target that releases
the buffer synchronously, such as an AF_XDP socket in copy mode, can
drop the page's last reference when pagecnt_bias runs out. The page is
then freed while it is still DMA-mapped and registered in the QPL, and
the driver posts it back to the device.

Refuse XDP program setup on DQO-QPL in gve_verify_xdp_configuration(),
matching the features the driver advertises. A zero-copy XSK pool bind
is already refused by the core for the same reason, and a copy-mode
bind never reaches the driver and is harmless without a program.

Fixes: d8a8ca14c937 ("gve: add XDP_TX and XDP_REDIRECT support for DQ RDA")
Cc: stable@vger.kernel.org
Signed-off-by: Amaan Cheval <amaan.cheval@gmail.com>
---
 drivers/net/ethernet/google/gve/gve_main.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ethernet/google/gve/gve_main.c b/drivers/net/ethernet/google/gve/gve_main.c
index 2437cd8ba633..4e9e692a0ed0 100644
--- a/drivers/net/ethernet/google/gve/gve_main.c
+++ b/drivers/net/ethernet/google/gve/gve_main.c
@@ -1723,6 +1723,12 @@ static int gve_verify_xdp_configuration(struct net_device *dev,
 	struct gve_priv *priv = netdev_priv(dev);
 	u16 max_xdp_mtu;
 
+	if (priv->queue_format == GVE_DQO_QPL_FORMAT) {
+		NL_SET_ERR_MSG_MOD(extack,
+				   "XDP is not supported in DQO QPL queue format.");
+		return -EOPNOTSUPP;
+	}
+
 	if (dev->features & NETIF_F_GRO_HW) {
 		NL_SET_ERR_MSG_MOD(extack,
 				   "XDP is not supported when HW-GRO is on.");
-- 
2.47.3

[PATCH net v4 2/2] gve: fix RX page refcount overflow from the pagecnt bias

[Amaan Cheval]

gve's GQI and DQO-QPL RX paths set every RX page's refcount to INT_MAX and
hand references to the stack by decrementing page_info->pagecnt_bias, to
avoid an atomic op per packet. This leaves no room for anyone else: while
a fresh page's buffer sits in an skb its refcount is already INT_MAX, so a
single get_page() from the stack overflows it to negative.

splice() of received TCP data into a pipe takes such a reference, and the
onward splice into a socket then fails: skb_splice_from_iter() sees
page_count() < 1, sendpage_ok() fails and the send returns -EIO (since
commit 2e910b95329c ("net: Add a function to splice pages into an
skbuff for MSG_SPLICE_PAGES")). tee() of the pipe fails with -EFAULT via
try_get_page(). On GQI the reuse check gve_rx_can_recycle_buffer() also
sees page_count() < pagecnt_bias, WARNs and calls gve_schedule_reset(),
so a task that merely pins a fresh RX page (e.g. holds spliced data in
an undrained tee() pipe) can reset the NIC.

Use USHRT_MAX in a u16, as igb, igc, ixgbe, i40e and hns3 do: the bias
only has to make the per-packet atomic rare, so a small value keeps
page_count() far below INT_MAX however many references the stack takes.
Refill by adding a fixed batch and setting the bias to it, which keeps
the rule `references = page_count() - bias` intact.

Refilling at 0 hands the driver's last reference to the consumer once
every USHRT_MAX handoffs rather than once every INT_MAX, which is only
safe because every consumer on these paths holds its reference until
the driver has recycled the buffer. The previous patch keeps it that
way by blocking XDP on DQO-QPL, so this one depends on it. Trees before
6.17, without commit d8a8ca14c937 ("gve: add XDP_TX and XDP_REDIRECT
support for DQ RDA"), still refuse XDP on every format but GQI-QPL in
verify_xdp_configuration(), so there this patch stands alone.

Reproduced on gVNIC in GQI-QPL mode (GCE n2-standard-8, 6.12) and in
DQO-RDA mode (GCE c3-standard-8, 6.12, where DQO-RDA still uses the
bias) with the help of relay.py -- a small splice(2) TCP proxy --
relaying a download while the RX ring is refilled with fresh pages:

  ethtool -G eth0 rx 512 && ethtool -G eth0 rx 1024   # fresh RX pages
  ./relay.py $HOST 8080 &                             # splice()s both ways
  for i in $(seq 200); do curl -so /dev/null \
      --connect-to $HOST:80:127.0.0.1:8080 http://$HOST/file; done

Most downloads failed with EIO before the fix (tee() with EFAULT) and
all completed after it. relay.py is at the Link below.

This bug was found and fixed with the help of an AI coding assistant
(Claude Code) over an interactive session; it read the driver and
splice/pipe code and ran the tests above.

Link: https://github.com/amaanc/kernel-bias
Fixes: 9b8dd5e5ea48 ("gve: DQO: Add RX path")
Fixes: 58401b2a46e7 ("gve: Add rx buffer pagecnt bias")
Fixes: 82fd151d38d9 ("gve: Reduce alloc and copy costs in the GQ rx path")
Cc: stable@vger.kernel.org
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Tested-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Amaan Cheval <amaan.cheval@gmail.com>
---
v4:
 - Add patch 1/2, blocking XDP on DQO-QPL, which this fix depends on
   (Sashiko). Harshitha said Google would send such a patch; I'm happy
   to drop 1/2 in favour of theirs.
 - Say "overflow" instead of "headroom" in the subject and the commit
   message (Maciej).
 - Use USHRT_MAX directly instead of a GVE_PAGECNT_BIAS macro, as the
   other drivers do, and move its rationale next to the refill in
   gve_dec_pagecnt_bias() (Maciej). No functional change.
 - Added Reviewed-by from Maciej Fijalkowski.
 - Say which trees need patch 1/2, for stable.
v3: https://lore.kernel.org/netdev/20261103090651.35399-1-amaan.cheval@gmail.com/
 - Refill when the bias reaches 0 again, as in v1. DQO-QPL is not
   supposed to support XDP at all, and it will be blocked explicitly in
   a separate patch, so the v2 refill-at-1 change is not needed.
 - Added Reviewed-by and Tested-by from Harshitha Ramamurthy.
v2: https://lore.kernel.org/netdev/20261102112613.18334-1-amaan.cheval@gmail.com/
 - Refill when the bias reaches 1 instead of 0 so the driver keeps a
   reference of its own across every handoff. At bias 0 a consumer that
   returns its reference synchronously, such as an AF_XDP socket in copy
   mode on an XDP_REDIRECT from DQO-QPL, freed a page still registered in
   the QPL.
 - Commit message: explain the refill point, drop the build notes.
 - Re-tested on 6.12.113 (c3-standard-8, DQO-RDA, which still uses the
   bias on 6.12): no EIO/EFAULT from splice/tee and no NIC reset after
   applying the patch. DQO-QPL is untested; I have no access to that
   hardware.
v1: https://lore.kernel.org/netdev/20261031173449.2439047-1-amaan.cheval@gmail.com/

Built allmodconfig W=1, no new warnings.

 drivers/net/ethernet/google/gve/gve.h                |  2 +-
 .../net/ethernet/google/gve/gve_buffer_mgmt_dqo.c    |  4 ++--
 drivers/net/ethernet/google/gve/gve_rx.c             |  8 ++++----
 drivers/net/ethernet/google/gve/gve_utils.c          | 12 ++++--------
 drivers/net/ethernet/google/gve/gve_utils.h          |  2 +-
 5 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/drivers/net/ethernet/google/gve/gve.h b/drivers/net/ethernet/google/gve/gve.h
index b7ce6e8632db..18d7edbf4b3b 100644
--- a/drivers/net/ethernet/google/gve/gve.h
+++ b/drivers/net/ethernet/google/gve/gve.h
@@ -119,7 +119,7 @@ struct gve_rx_slot_page_info {
 	void *page_address;
 	u32 page_offset; /* offset to write to in page */
 	unsigned int buf_size;
-	int pagecnt_bias; /* expected pagecnt if only the driver has a ref */
+	u16 pagecnt_bias; /* expected pagecnt if only the driver has a ref */
 	u16 pad; /* adjustment for rx padding */
 	u8 can_flip; /* tracks if the networking stack is using the page */
 };
diff --git a/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c b/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
index 6880d1531ba5..52223baf0303 100644
--- a/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
@@ -149,8 +149,8 @@ int gve_alloc_qpl_page_dqo(struct gve_rx_ring *rx,
 	buf_state->last_single_ref_offset = 0;
 
 	/* The page already has 1 ref. */
-	page_ref_add(buf_state->page_info.page, INT_MAX - 1);
-	buf_state->page_info.pagecnt_bias = INT_MAX;
+	page_ref_add(buf_state->page_info.page, USHRT_MAX - 1);
+	buf_state->page_info.pagecnt_bias = USHRT_MAX;
 
 	return 0;
 }
diff --git a/drivers/net/ethernet/google/gve/gve_rx.c b/drivers/net/ethernet/google/gve/gve_rx.c
index 81ea800e66e9..3bed19866376 100644
--- a/drivers/net/ethernet/google/gve/gve_rx.c
+++ b/drivers/net/ethernet/google/gve/gve_rx.c
@@ -152,8 +152,8 @@ static void gve_setup_rx_buffer(struct gve_rx_ring *rx,
 	page_info->buf_size = rx->packet_buffer_size;
 	*slot_addr = cpu_to_be64(addr);
 	/* The page already has 1 ref */
-	page_ref_add(page, INT_MAX - 1);
-	page_info->pagecnt_bias = INT_MAX;
+	page_ref_add(page, USHRT_MAX - 1);
+	page_info->pagecnt_bias = USHRT_MAX;
 }
 
 static int gve_rx_alloc_buffer(struct gve_priv *priv, struct device *dev,
@@ -230,8 +230,8 @@ static int gve_rx_prefill_pages(struct gve_rx_ring *rx,
 			rx->qpl_copy_pool[j].buf_size = rx->packet_buffer_size;
 
 			/* The page already has 1 ref. */
-			page_ref_add(page, INT_MAX - 1);
-			rx->qpl_copy_pool[j].pagecnt_bias = INT_MAX;
+			page_ref_add(page, USHRT_MAX - 1);
+			rx->qpl_copy_pool[j].pagecnt_bias = USHRT_MAX;
 		}
 	}
 
diff --git a/drivers/net/ethernet/google/gve/gve_utils.c b/drivers/net/ethernet/google/gve/gve_utils.c
index caed38cd5f2b..930555ac8fe9 100644
--- a/drivers/net/ethernet/google/gve/gve_utils.c
+++ b/drivers/net/ethernet/google/gve/gve_utils.c
@@ -93,15 +93,11 @@ void gve_dec_pagecnt_bias(struct gve_rx_slot_page_info *page_info)
 {
 	page_info->pagecnt_bias--;
 	if (page_info->pagecnt_bias == 0) {
-		int pagecount = page_count(page_info->page);
-
-		/* If we have run out of bias - set it back up to INT_MAX
-		 * minus the existing refs.
+		/* Out of bias: take another batch. USHRT_MAX keeps this rare
+		 * while leaving page_count() far below INT_MAX.
 		 */
-		page_info->pagecnt_bias = INT_MAX - pagecount;
-
-		/* Set pagecount back up to max. */
-		page_ref_add(page_info->page, INT_MAX - pagecount);
+		page_info->pagecnt_bias = USHRT_MAX;
+		page_ref_add(page_info->page, USHRT_MAX);
 	}
 }
 
diff --git a/drivers/net/ethernet/google/gve/gve_utils.h b/drivers/net/ethernet/google/gve/gve_utils.h
index bf2e9a0adb36..c686142d37ea 100644
--- a/drivers/net/ethernet/google/gve/gve_utils.h
+++ b/drivers/net/ethernet/google/gve/gve_utils.h
@@ -25,7 +25,7 @@ struct sk_buff *gve_rx_copy_data(struct net_device *dev, struct napi_struct *nap
 struct sk_buff *gve_rx_copy(struct net_device *dev, struct napi_struct *napi,
 			    struct gve_rx_slot_page_info *page_info, u16 len);
 
-/* Decrement pagecnt_bias. Set it back to INT_MAX if it reached zero. */
+/* Decrement pagecnt_bias. Top it up by USHRT_MAX if it reached zero. */
 void gve_dec_pagecnt_bias(struct gve_rx_slot_page_info *page_info);
 
 void gve_add_napi(struct gve_priv *priv, int ntfy_idx,
-- 
2.47.3