[PATCH net v2 0/2] tcp: clamp return value of tcp_wmem_schedule() to non-negative

[PATCH net v2 0/2] tcp: clamp return value of tcp_wmem_schedule() to non-negative

[Octavian Purdila]

This series fixes a bug discovered in production in tcp_wmem_schedule()
where a negative sk->sk_forward_alloc under TCP memory pressure causes
tcp_wmem_schedule() to return a negative copy length when
sk->sk_wmem_queued already reaches or exceeds sysctl_tcp_wmem[0].

sk->sk_forward_alloc can transiently become negative when tcp_collapse()
re-packs partially read or high payload-to-truesize SKBs in the receive
queue and charges newly allocated SKBs via sk_mem_charge() before
freeing the original SKBs. When a subsequent send hits memory pressure
in sk_wmem_schedule() with sk->sk_wmem_queued >= sysctl_tcp_wmem[0],
tcp_wmem_schedule() returns min(copy, sk->sk_forward_alloc) < 0.
Callers such as tcp_sendmsg_locked() then pass that negative copy length
to skb_copy_to_page_nocache() -> copy_from_iter_full() ->
check_copy_size(), triggering WARN_ON_ONCE(bytes > INT_MAX) in
production and returning -EFAULT instead of waiting for memory or
returning -EAGAIN.

Patch 1 clamps the return value of tcp_wmem_schedule() to [0, copy] so
a negative sk->sk_forward_alloc is treated as 0 available bytes.
Patch 2 adds a packetdrill selftest that reproduces the negative
sk->sk_forward_alloc condition via tcp_collapse() under memory pressure
and verifies that a non-blocking write fails with EAGAIN rather than
EFAULT.

Changes in v2:
- Include production warning trace in Patch 1 commit message.
- Skip the packetdrill test when PAGE_SIZE != 4096 via
  --define=REQUIRES_PAGESIZE="4096" in ksft_runner.sh.
- Collect Reviewed-by tags from Kuniyuki Iwashima and Eric Dumazet.

Octavian Purdila (2):
  tcp: clamp return value of tcp_wmem_schedule() to non-negative
  selftests: net: packetdrill: add test for negative sk_forward_alloc in
    tcp_wmem_schedule()

 net/ipv4/tcp.c                                |   2 +-
 .../testing/selftests/net/packetdrill/config  |   1 +
 .../selftests/net/packetdrill/ksft_runner.sh  |   6 +
 .../tcp_wmem_schedule_negative_fwd_alloc.pkt  | 117 ++++++++++++++++++
 4 files changed, 125 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt

-- 
2.56.0.212.g129ede20d9-goog

[PATCH net v2 1/2] tcp: clamp return value of tcp_wmem_schedule() to non-negative

[Octavian Purdila]

When sk_wmem_schedule(sk, copy) fails under TCP memory pressure and
sk->sk_wmem_queued already reaches or exceeds sysctl_tcp_wmem[0],
tcp_wmem_schedule() returns min(copy, sk->sk_forward_alloc).

However, sk->sk_forward_alloc is a signed int that can transiently
become negative when tcp_collapse() re-packs partially read or high
payload-to-truesize SKBs in the receive queue and charges newly
allocated SKBs via sk_mem_charge() before freeing the original SKB.

If tcp_wmem_schedule() returns a negative value, tcp_sendmsg_locked()
passes that negative copy length to skb_copy_to_page_nocache() ->
copy_from_iter_full() -> check_copy_size(), where it is converted to an
unsigned size_t > INT_MAX, triggering a WARN_ON_ONCE(bytes > INT_MAX)
in production and returning -EFAULT instead of waiting for memory or
returning -EAGAIN:

  WARNING: CPU: 1 PID: 939331 at include/linux/thread_info.h:249 tcp_sendmsg+0x10c1/0x1530
  Call Trace:
   <TASK>
   __sys_sendmsg+0x292/0x490
   do_syscall_64+0x88/0x340
   entry_SYSCALL_64_after_hwframe+0x76/0x7e

Clamp the return value of tcp_wmem_schedule() to [0, copy] so callers
treat negative sk_forward_alloc as 0 available bytes.

Fixes: 849b425cd091 ("tcp: fix possible freeze in tx path under memory pressure")
Assisted-by: LLM
Signed-off-by: Octavian Purdila <tavip@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
---
v2:
- Add production warning trace to the commit message.

 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 87ef6d5cbfeba..0e062fcd94a6c 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1037,7 +1037,7 @@ int tcp_wmem_schedule(struct sock *sk, int copy)
 	left = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_wmem[0]) - sk->sk_wmem_queued;
 	if (left > 0)
 		sk_forced_mem_schedule(sk, min(left, copy));
-	return min(copy, sk->sk_forward_alloc);
+	return clamp(sk->sk_forward_alloc, 0, copy);
 }
 
 void tcp_free_fastopen_req(struct tcp_sock *tp)
-- 
2.56.0.212.g129ede20d9-goog

[PATCH net v2 2/2] selftests: net: packetdrill: add test for negative sk_forward_alloc in tcp_wmem_schedule()

[Octavian Purdila]

Add a packetdrill test that drives sk->sk_forward_alloc negative via
tcp_collapse() on the receive queue while sk->sk_wmem_queued exceeds
sysctl_tcp_wmem[0], and verifies that a subsequent non-blocking write()
under TCP memory pressure fails with EAGAIN rather than returning a
negative copy length from tcp_wmem_schedule() and failing with EFAULT.

Assisted-by: LLM
Signed-off-by: Octavian Purdila <tavip@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
---
v2:
- Skip the test when PAGE_SIZE != 4096 via --define=REQUIRES_PAGESIZE="4096"
  in ksft_runner.sh (Eric Dumazet).

 .../testing/selftests/net/packetdrill/config  |   1 +
 .../selftests/net/packetdrill/ksft_runner.sh  |   6 +
 .../tcp_wmem_schedule_negative_fwd_alloc.pkt  | 117 ++++++++++++++++++
 3 files changed, 124 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt

diff --git a/tools/testing/selftests/net/packetdrill/config b/tools/testing/selftests/net/packetdrill/config
index b7df8bf309200..dc3cd72a8da8c 100644
--- a/tools/testing/selftests/net/packetdrill/config
+++ b/tools/testing/selftests/net/packetdrill/config
@@ -1,6 +1,7 @@
 CONFIG_HZ=1000
 CONFIG_HZ_1000=y
 CONFIG_IPV6=y
+CONFIG_MEMCG=y
 CONFIG_NET_NS=y
 CONFIG_NET_SCH_FIFO=y
 CONFIG_NET_SCH_FQ=y
diff --git a/tools/testing/selftests/net/packetdrill/ksft_runner.sh b/tools/testing/selftests/net/packetdrill/ksft_runner.sh
index 0a97d5ae34699..dcda2b93c76e0 100755
--- a/tools/testing/selftests/net/packetdrill/ksft_runner.sh
+++ b/tools/testing/selftests/net/packetdrill/ksft_runner.sh
@@ -52,6 +52,12 @@ if [[ -n "${KSFT_MACHINE_SLOW}" ]]; then
 	failfunc=ktap_test_xfail
 fi
 
+pagesize=$(grep -E '^--define=REQUIRES_PAGESIZE=' $script | cut -d '=' -f 3 | tr -d '"')
+if [[ -n "$pagesize" && "$(getconf PAGESIZE)" != "$pagesize" ]]; then
+	ktap_skip_all "requires ${pagesize}B PAGE_SIZE"
+	exit "$KSFT_SKIP"
+fi
+
 ip_versions=$(grep -E '^--ip_version=' $script | cut -d '=' -f 2)
 if [[ -z $ip_versions ]]; then
 	ip_versions="ipv4 ipv6 ipv4-mapped-ipv6"
diff --git a/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt b/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt
new file mode 100644
index 0000000000000..96a5d60a52cb1
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test that tcp_wmem_schedule() clamps negative sk->sk_forward_alloc to 0
+// when sk_wmem_schedule() fails under memory pressure and
+// sk->sk_wmem_queued >= sysctl_tcp_wmem[0], returning EAGAIN rather than EFAULT.
+
+--define=REQUIRES_PAGESIZE="4096"
+--mtu=65535
+--tolerance_usecs=100000
+
+`set -e
+./defaults.sh
+ip link set dev tun0 mtu 65535
+sysctl -q net.ipv4.tcp_rmem="4096 131072 6291456"
+sysctl -q net.ipv4.tcp_wmem="4096 16384 4194304"
+cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+[ -n "$cgroot" ]
+echo +memory > "${cgroot}/cgroup.subtree_control"
+mkdir -p "${cgroot}/pkt_${PPID}"
+echo $PPID > "${cgroot}/pkt_${PPID}/cgroup.procs"
+`
+
+// Create listening socket in pkt_${PPID} memcg and establish connection
+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
++0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
++0.000 bind(3, ..., ...) = 0
++0.000 listen(3, 1) = 0
+
++0.000 < S 0:0(0) win 65535 <mss 36000,nop,wscale 7>
++0.000 > S. 0:0(0) ack 1 <...>
++0.000 < . 1:1(0) ack 1 win 512
++0.000 accept(3, ..., ...) = 4
+
+// Move packetdrill back to its original cgroup so only the sockets remain charged to pkt_${PPID}
++0.000 `cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+runner_pid=$(awk '/^PPid:/ {print $2}' /proc/$PPID/status)
+orig_cg=$(sed -n 's/^0:://p' /proc/$runner_pid/cgroup)
+echo $PPID > "${cgroot}${orig_cg}/cgroup.procs"`
+
++0.000 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [2097152], 4) = 0
++0.000 setsockopt(4, SOL_SOCKET, SO_RCVBUF, [145536], 4) = 0
+
+// 1. Warm up rcv_ssthresh to full window (~256 KB)
++0.000 < P. 1:36001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 36001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 36001:72001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 72001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 72001:108001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 108001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 108001:144001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 144001
++0.000 read(4, ..., 36000) = 36000
+
+// 2. Queue 500 KB on TX so sk_wmem_queued >= sysctl_tcp_wmem[0]
++0.000 write(4, ..., 500000) = 500000
+
+// 3a. Inject 6 high-density 36 KB segments (216 KB payload, 222.7 KB truesize)
+//     and read 100 bytes from the head SKB so before(skb->seq, tp->copied_seq) is true.
++0.000 < P. 144001:180001(36000) ack 1 win 512
++0.000 read(4, ..., 100) = 100
+
++0.000 < P. 180001:216001(36000) ack 1 win 512
++0.000 < P. 216001:252001(36000) ack 1 win 512
++0.000 < P. 252001:288001(36000) ack 1 win 512
++0.000 < P. 288001:324001(36000) ack 1 win 512
++0.000 < P. 324001:360001(36000) ack 1 win 512
+
+// 3b. Inject 22 smaller 1000-byte segments into the remaining advertised rcv_wnd.
+//     Their higher truesize pushes sk_rmem_alloc > sk_rcvbuf (291072),
+//     triggering tcp_prune_queue -> tcp_collapse on the 216 KB of 36 KB SKBs
+//     and driving sk->sk_forward_alloc negative (-32768).
++0.000 < P. 360001:361001(1000) ack 1 win 512
++0.000 < P. 361001:362001(1000) ack 1 win 512
++0.000 < P. 362001:363001(1000) ack 1 win 512
++0.000 < P. 363001:364001(1000) ack 1 win 512
++0.000 < P. 364001:365001(1000) ack 1 win 512
++0.000 < P. 365001:366001(1000) ack 1 win 512
++0.000 < P. 366001:367001(1000) ack 1 win 512
++0.000 < P. 367001:368001(1000) ack 1 win 512
++0.000 < P. 368001:369001(1000) ack 1 win 512
++0.000 < P. 369001:370001(1000) ack 1 win 512
++0.000 < P. 370001:371001(1000) ack 1 win 512
++0.000 < P. 371001:372001(1000) ack 1 win 512
++0.000 < P. 372001:373001(1000) ack 1 win 512
++0.000 < P. 373001:374001(1000) ack 1 win 512
++0.000 < P. 374001:375001(1000) ack 1 win 512
++0.000 < P. 375001:376001(1000) ack 1 win 512
++0.000 < P. 376001:377001(1000) ack 1 win 512
++0.000 < P. 377001:378001(1000) ack 1 win 512
++0.000 < P. 378001:379001(1000) ack 1 win 512
++0.000 < P. 379001:380001(1000) ack 1 win 512
++0.000 < P. 380001:381001(1000) ack 1 win 512
++0.000 < P. 381001:382001(1000) ack 1 win 512
+
+// 4. Cap the socket memcg at its current usage and expect -EAGAIN on non-blocking write.
++0.010 `cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+echo 0 > "${cgroot}/pkt_${PPID}/memory.high"
+cat "${cgroot}/pkt_${PPID}/memory.current" > "${cgroot}/pkt_${PPID}/memory.max"`
++0.000 fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
++0.000 write(4, ..., 16384) = -1 EAGAIN (Resource temporarily unavailable)
+
++0.000 close(4) = 0
++0.000 close(3) = 0
+
+// Untimed cleanup command executed by packetdrill on both pass and early failure.
+`cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+[ -n "$cgroot" ] || exit 0
+runner_pid=$(awk '/^PPid:/ {print $2}' /proc/$PPID/status)
+orig_cg=$(sed -n 's/^0:://p' /proc/$runner_pid/cgroup)
+echo $PPID > "${cgroot}${orig_cg}/cgroup.procs" 2>/dev/null || true
+echo $$ > "${cgroot}${orig_cg}/cgroup.procs" 2>/dev/null || true
+rmdir "${cgroot}/pkt_${PPID}"`
-- 
2.56.0.212.g129ede20d9-goog