Re: [PATCH net-next v2] rtnetlink: cap IFLA_VFINFO_LIST at a documented number of VFs

rtnl_fill_vf() emits one IFLA_VF_INFO per VF into the IFLA_VFINFO_LIST
nest and closes it with nla_nest_end(), which stores the accumulated
length into nla_len. That field is a u16, so a nest larger than 65535
bytes is written truncated modulo 65536.

Nothing catches it on the way. if_nlmsg_size() adds rtnl_vfinfo_size()
for every VF, so the skb really is large enough and none of the nla_put()
calls fails. Userspace then walks the message with RTA_NEXT(), which
advances by the stored length, so parsing resumes inside VF payload and
the attributes after the nest are read out of VF data: IFLA_VF_PORTS,
IFLA_XDP, IFLA_LINKINFO, IFLA_PERM_ADDRESS, IFLA_AF_SPEC. iproute2
prints "!!!Deficit" and strictly validating parsers reject the message.
On CONFIG_DEBUG_NET kernels nla_nest_end() also splats, via the
DEBUG_NET_WARN_ON_ONCE() added in commit 1346586a9ac9 ("netlink: add a
nla_nest_end_safe() helper").

Where the wrap falls depends on what was asked for and on the host. A VF
costs 196 bytes, 296 with statistics, 236 with GUIDs and 336 with both.
On a kernel without CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS the statistics
carry a padding attribute each and cost 32 bytes more, which makes those
two 328 and 368. So the nest overflows somewhere between 179 and 335
VFs. ice allows 256 VFs per PF
(ICE_MAX_SRIOV_VFS), which reaches it. Statistics are included unless the
request sets RTEXT_FILTER_SKIP_STATS, so the common case is the one that
wraps first.

A limit that moves with the requested attribute set and with the host's
alignment requirements is not something userspace can be told, so use
fixed numbers instead and document them as what the interface supports:
256 VFs, or 128 when statistics are included. Both stay well inside
U16_MAX even in the largest per-VF encoding, at 60416 and 47104 bytes
respectively. rtnl_vfinfo_size() applies the same limit so that the skb
is not sized for VFs that will not be emitted.

A device with more VFs than the limit reports a shorter
IFLA_VFINFO_LIST. IFLA_NUM_VF keeps carrying the real count, and
everything after the nest stays parsable, which is the part that is
broken today. An empty nest is already emitted for a PF with no VFs, so a
list shorter than IFLA_NUM_VF is not a new encoding.

Returning -EMSGSIZE instead, which is what nla_nest_end_safe() would
give, is not an option here: a nest that does not fit in a u16 will not
fit in a retried skb either, so it would turn a link dump on such a
device into a hard failure.

The other large nests in rtnl_fill_ifinfo() were audited and cannot
overflow. IFLA_AF_SPEC is bounded by a handful of address families at
about a kilobyte each, and IFLA_VF_PORTS would need more than 560 VFs,
which no in-tree driver allows.

Reported-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/netdev/16b289f6-b025-5dd3-443d-92d4c167e79c@intel.com/ (local)
Assisted-by: Claude:claude-fable-5
Signed-off-by: Artem Lytkin <redacted>
---
v2:
 - cap at fixed limits, 256 VFs and 128 when statistics are included, and
   document them in the netlink spec, rather than dropping whichever VF
   happens not to fit (Jakub)
 - drop the Fixes: tag, this is a long standing limitation of the
   interface and not a regression (Jakub)
 - retarget net-next for the same reason
 - changelog: corrected the per-VF sizes, which mixed two configurations,
   and dropped IFLA_PROP_LIST from the list of attributes that follow the
   nest, since it is emitted before it
v1: https://lore.kernel.org/netdev/20260725132236.88318-1-iprintercanon@gmail.com/ (local)

 Documentation/netlink/specs/rt-link.yaml |  5 +++++
 net/core/rtnetlink.c                     | 25 +++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/netlink/specs/rt-link.yaml b/Documentation/netlink/specs/rt-link.yaml
index 68c26a70bb649..b80c2ac3ac311 100644
--- a/Documentation/netlink/specs/rt-link.yaml
+++ b/Documentation/netlink/specs/rt-link.yaml

@@ -928,6 +928,11 @@ attribute-sets:
         name: vfinfo-list
         type: nest
         nested-attributes: vfinfo-list-attrs
+        doc: |
+          Per-VF details. The list holds at most 256 VFs, or 128 when
+          statistics are included, because it is one attribute and has to fit
+          in a u16 length. A device with more VFs than that reports a
+          truncated list; num-vf still carries the real count.
       -
         name: stats64
         type: binary

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 31c65a545a107..6395ea6701210 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c

@@ -1174,12 +1174,30 @@ static void copy_rtnl_link_stats(struct rtnl_link_stats *a,
 	a->rx_nohandler = b->rx_nohandler;
 }
 
+/* IFLA_VFINFO_LIST is one netlink attribute, so everything nested inside it
+ * has to fit in the u16 nla_len. Bound the number of VFs described there at a
+ * fixed value rather than at whatever happens to fit, so that the limit is a
+ * property of the interface instead of one of the requested attribute set and
+ * the host's alignment requirements. The largest per-VF encoding is 368 bytes
+ * with statistics and GUIDs and 236 bytes without statistics, so both limits
+ * keep the nest well inside U16_MAX.
+ */
+#define RTNL_VFINFO_MAX_VFS		256
+#define RTNL_VFINFO_MAX_VFS_STATS	128