[PATCH net] rtnetlink: truncate IFLA_VFINFO_LIST instead of overflowing its length

rtnl_fill_vf() closes the IFLA_VFINFO_LIST nest 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.

The skb is sized by if_nlmsg_size(), which adds rtnl_vfinfo_size() for
every VF, so the buffer really is large enough for the whole list and
none of the nla_put() calls in rtnl_fill_vfinfo() fails. The overflow is
therefore silent. Userspace then walks the message with RTA_NEXT(),
which advances by the stored length, so parsing resumes inside VF
payload and the top-level attributes that follow the nest are read out
of VF data. Those are IFLA_VF_PORTS, IFLA_XDP, IFLA_LINKINFO,
IFLA_PERM_ADDRESS and IFLA_PROP_LIST. iproute2 prints "!!!Deficit", and
strictly validating parsers reject the message outright.

rtnl_fill_vfinfo() emits 296 bytes per VF on a 64-bit kernel with
efficient unaligned access, so the nest wraps at 222 VFs; without
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS it is 328 bytes and 200 VFs, and
with ndo_get_vf_guid 336 bytes and 196 VFs. ice supports up to 256 VFs
per PF (ICE_MAX_SRIOV_VFS), so this is reachable on shipping hardware.
Requests that set RTEXT_FILTER_SKIP_STATS need 335 VFs, which the 256-VF
limit puts out of reach, so plain "ip link show" is fine today while
"ip -s link show" is not.

On CONFIG_DEBUG_NET kernels nla_nest_end() now also splats via
DEBUG_NET_WARN_ON_ONCE(), added along with nla_nest_end_safe() in commit
1346586a9ac9 ("netlink: add a nla_nest_end_safe() helper").

Using nla_nest_end_safe() here would not help: a nest that does not fit
in a u16 will not fit in a retried skb either, so returning -EMSGSIZE
would turn "ip link show" on such a device into a hard failure. Bound
the nest in the writer instead, as suggested when this was last
discussed: drop the VF that would push it past U16_MAX and end the list
there. Userspace sees IFLA_NUM_VF unchanged and a shorter
IFLA_VFINFO_LIST, and everything after the nest stays parsable. 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.

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.

Fixes: c02db8c6290b ("rtnetlink: make SR-IOV VF interface symmetric")
Reported-by: Jacob Keller <jacob.e.keller@intel.com>
Closes: 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>
---
 net/core/rtnetlink.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 12aa3aa1688b1..bd1d65dcf42fd 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c

@@ -1687,10 +1687,22 @@ static noinline_for_stack int rtnl_fill_vf(struct sk_buff *skb,
 		return -EMSGSIZE;
 
 	for (i = 0; i < num_vfs; i++) {
+		unsigned char *mark = skb_tail_pointer(skb);
+
 		if (rtnl_fill_vfinfo(skb, dev, i, ext_filter_mask)) {
 			nla_nest_cancel(skb, vfinfo);
 			return -EMSGSIZE;
 		}
+
+		/* An attribute length is a u16, so the nest cannot describe
+		 * more than U16_MAX bytes. Drop the VF that would overflow it
+		 * and stop: a truncated list keeps the rest of the message
+		 * parsable, whereas a wrapped nest length does not.
+		 */
+		if (skb_tail_pointer(skb) - (unsigned char *)vfinfo > U16_MAX) {
+			nlmsg_trim(skb, mark);
+			break;
+		}
 	}
 
 	nla_nest_end(skb, vfinfo);

base-commit: 248951ddc14de84de3910f9b13f51491a8cd91df
-- 
2.43.0