x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 289 insertions, 0 deletions

diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
new file mode 100644
index 000000000..b581cb1ee
--- /dev/null
+++ b/net/ipv6/syncookies.c
@@ -0,0 +1,289 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  IPv6 Syncookies implementation for the Linux kernel
+ *
+ *  Authors:
+ *  Glenn Griffin	<ggriffin.kernel@gmail.com>
+ *
+ *  Based on IPv4 implementation by Andi Kleen
+ *  linux/net/ipv4/syncookies.c
+ */
+
+#include <linux/tcp.h>
+#include <linux/random.h>
+#include <linux/siphash.h>
+#include <linux/kernel.h>
+#include <net/secure_seq.h>
+#include <net/ipv6.h>
+#include <net/tcp.h>
+#include <net/tcp_ecn.h>
+
+#define COOKIEBITS 24	/* Upper bits store count */
+#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
+
+static siphash_aligned_key_t syncookie6_secret[2];
+
+/* RFC 2460, Section 8.3:
+ * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
+ *
+ * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
+ * using higher values than ipv4 tcp syncookies.
+ * The other values are chosen based on ethernet (1500 and 9k MTU), plus
+ * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
+ */
+static __u16 const msstab[] = {
+	1280 - 60, /* IPV6_MIN_MTU - 60 */
+	1480 - 60,
+	1500 - 60,
+	9000 - 60,
+};
+
+static u32 cookie_hash(const struct in6_addr *saddr,
+		       const struct in6_addr *daddr,
+		       __be16 sport, __be16 dport, u32 count, int c)
+{
+	const struct {
+		struct in6_addr saddr;
+		struct in6_addr daddr;
+		u32 count;
+		__be16 sport;
+		__be16 dport;
+	} __aligned(SIPHASH_ALIGNMENT) combined = {
+		.saddr = *saddr,
+		.daddr = *daddr,
+		.count = count,
+		.sport = sport,
+		.dport = dport
+	};
+
+	net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
+	return siphash(&combined, offsetofend(typeof(combined), dport),
+		       &syncookie6_secret[c]);
+}
+
+static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
+				   const struct in6_addr *daddr,
+				   __be16 sport, __be16 dport, __u32 sseq,
+				   __u32 data)
+{
+	u32 count = tcp_cookie_time();
+	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
+		sseq + (count << COOKIEBITS) +
+		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
+		& COOKIEMASK));
+}
+
+static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
+				  const struct in6_addr *daddr, __be16 sport,
+				  __be16 dport, __u32 sseq)
+{
+	__u32 diff, count = tcp_cookie_time();
+
+	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
+
+	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
+	if (diff >= MAX_SYNCOOKIE_AGE)
+		return (__u32)-1;
+
+	return (cookie -
+		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
+		& COOKIEMASK;
+}
+
+u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
+			      const struct tcphdr *th, __u16 *mssp)
+{
+	int mssind;
+	const __u16 mss = *mssp;
+
+	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
+		if (mss >= msstab[mssind])
+			break;
+
+	*mssp = msstab[mssind];
+
+	return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
+				     th->dest, ntohl(th->seq), mssind);
+}
+EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
+
+__u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
+{
+	const struct ipv6hdr *iph = ipv6_hdr(skb);
+	const struct tcphdr *th = tcp_hdr(skb);
+
+	return __cookie_v6_init_sequence(iph, th, mssp);
+}
+
+int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th)
+{
+	__u32 cookie = ntohl(th->ack_seq) - 1;
+	__u32 seq = ntohl(th->seq) - 1;
+	__u32 mssind;
+
+	mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
+				      th->source, th->dest, seq);
+
+	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
+}
+EXPORT_SYMBOL_GPL(__cookie_v6_check);
+
+static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
+					     struct sk_buff *skb)
+{
+	struct tcp_options_received tcp_opt;
+	u32 tsoff = 0;
+	int mss;
+
+	if (tcp_synq_no_recent_overflow(sk))
+		goto out;
+
+	mss = __cookie_v6_check(ipv6_hdr(skb), tcp_hdr(skb));
+	if (!mss) {
+		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
+		goto out;
+	}
+
+	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
+
+	/* check for timestamp cookie support */
+	memset(&tcp_opt, 0, sizeof(tcp_opt));
+	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
+
+	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
+		union tcp_seq_and_ts_off st;
+
+		st = secure_tcpv6_seq_and_ts_off(net,
+						 ipv6_hdr(skb)->daddr.s6_addr32,
+						 ipv6_hdr(skb)->saddr.s6_addr32,
+						 tcp_hdr(skb)->dest,
+						 tcp_hdr(skb)->source);
+		tsoff = st.ts_off;
+		tcp_opt.rcv_tsecr -= tsoff;
+	}
+
+	if (!cookie_timestamp_decode(net, &tcp_opt))
+		goto out;
+
+	return cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb,
+				      &tcp_opt, mss, tsoff);
+out:
+	return ERR_PTR(-EINVAL);
+}
+
+struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
+{
+	const struct tcphdr *th = tcp_hdr(skb);
+	struct ipv6_pinfo *np = inet6_sk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_request_sock *ireq;
+	struct net *net = sock_net(sk);
+	struct request_sock *req;
+	struct dst_entry *dst;
+	struct sock *ret = sk;
+	__u8 rcv_wscale;
+	int full_space;
+	SKB_DR(reason);
+
+	if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
+	    !th->ack || th->rst)
+		goto out;
+
+	if (cookie_bpf_ok(skb)) {
+		req = cookie_bpf_check(sk, skb);
+	} else {
+		req = cookie_tcp_check(net, sk, skb);
+		if (IS_ERR(req))
+			goto out;
+	}
+	if (!req) {
+		SKB_DR_SET(reason, NO_SOCKET);
+		goto out_drop;
+	}
+
+	ireq = inet_rsk(req);
+
+	ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
+	ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
+
+	if (security_inet_conn_request(sk, skb, req)) {
+		SKB_DR_SET(reason, SECURITY_HOOK);
+		goto out_free;
+	}
+
+	if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
+	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
+	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
+		refcount_inc(&skb->users);
+		ireq->pktopts = skb;
+	}
+
+	/* So that link locals have meaning */
+	if (!sk->sk_bound_dev_if &&
+	    ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
+		ireq->ir_iif = tcp_v6_iif(skb);
+
+	tcp_ao_syncookie(sk, skb, req, AF_INET6);
+
+	/*
+	 * We need to lookup the dst_entry to get the correct window size.
+	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
+	 * me if there is a preferred way.
+	 */
+	{
+		struct in6_addr *final_p, final;
+		struct flowi6 fl6;
+		memset(&fl6, 0, sizeof(fl6));
+		fl6.flowi6_proto = IPPROTO_TCP;
+		fl6.daddr = ireq->ir_v6_rmt_addr;
+		final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
+		fl6.saddr = ireq->ir_v6_loc_addr;
+		fl6.flowi6_oif = ireq->ir_iif;
+		fl6.flowi6_mark = ireq->ir_mark;
+		fl6.fl6_dport = ireq->ir_rmt_port;
+		fl6.fl6_sport = inet_sk(sk)->inet_sport;
+		fl6.flowi6_uid = sk_uid(sk);
+		security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
+
+		ip6_ecmp_set_mp_hash(net, &fl6, tcp_rsk(req)->txhash);
+
+		dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
+		if (IS_ERR(dst)) {
+			SKB_DR_SET(reason, IP_OUTNOROUTES);
+			goto out_free;
+		}
+	}
+
+	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :dst_metric(dst, RTAX_WINDOW);
+	/* limit the window selection if the user enforce a smaller rx buffer */
+	full_space = tcp_full_space(sk);
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
+		req->rsk_window_clamp = full_space;
+
+	tcp_select_initial_window(sk, full_space, req->mss,
+				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
+				  ireq->wscale_ok, &rcv_wscale,
+				  dst_metric(dst, RTAX_INITRWND));
+
+	/* req->syncookie is set true only if ACK is validated
+	 * by BPF kfunc, then, rcv_wscale is already configured.
+	 */
+	if (!req->syncookie)
+		ireq->rcv_wscale = rcv_wscale;
+	ireq->ecn_ok &= cookie_ecn_ok(net, dst);
+	tcp_rsk(req)->accecn_ok = ireq->ecn_ok && cookie_accecn_ok(th);
+
+	ret = tcp_get_cookie_sock(sk, skb, req, dst);
+	if (!ret) {
+		SKB_DR_SET(reason, NO_SOCKET);
+		goto out_drop;
+	}
+out:
+	return ret;
+out_free:
+	reqsk_free(req);
+out_drop:
+	sk_skb_reason_drop(sk, skb, reason);
+	return NULL;
+}