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 Link: https://lore.kernel.org/netdev/16b289f6-b025-5dd3-443d-92d4c167e79c@intel.com/ Assisted-by: Claude:claude-fable-5 Signed-off-by: Artem Lytkin --- 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/ 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 + +static int rtnl_vfinfo_max_vfs(u32 ext_filter_mask) +{ + return ext_filter_mask & RTEXT_FILTER_SKIP_STATS ? + RTNL_VFINFO_MAX_VFS : RTNL_VFINFO_MAX_VFS_STATS; +} + /* All VF info */ static inline int rtnl_vfinfo_size(const struct net_device *dev, u32 ext_filter_mask) { if (dev->dev.parent && (ext_filter_mask & RTEXT_FILTER_VF)) { - int num_vfs = dev_num_vf(dev->dev.parent); + int num_vfs = min(dev_num_vf(dev->dev.parent), + rtnl_vfinfo_max_vfs(ext_filter_mask)); size_t size = nla_total_size(0); size += num_vfs * (nla_total_size(0) + @@ -1717,6 +1735,11 @@ static noinline_for_stack int rtnl_fill_vf(struct sk_buff *skb, if (!vfinfo) return -EMSGSIZE; + /* IFLA_NUM_VF above stays the device's VF count; the list itself is + * capped so that its length cannot overflow nla_len. + */ + num_vfs = min(num_vfs, rtnl_vfinfo_max_vfs(ext_filter_mask)); + for (i = 0; i < num_vfs; i++) { if (rtnl_fill_vfinfo(skb, dev, i, ext_filter_mask)) { nla_nest_cancel(skb, vfinfo); base-commit: bc03e080390a1b8dd6252b598aa5ec801fc5e13d -- 2.43.0