[PATCH net] tcp: limit tcp_tso_rtt_log to a valid shift count

[Yingjie Wang <1075151112@qq.com>]

tcp_tso_autosize() right-shifts the u32 returned by tcp_min_rtt() by
sysctl_tcp_tso_rtt_log. The sysctl uses proc_dou8vec_minmax() without
an upper bound, so it accepts values through 255. Values above 31 make
the shift undefined.

A process with CAP_NET_ADMIN in the target network namespace can set the
per-netns sysctl. With CONFIG_UBSAN_SHIFT=y, setting it to 32 and sending
TCP data over loopback triggers:

  UBSAN: shift-out-of-bounds in net/ipv4/tcp_output.c:2264:30
  shift exponent 32 is too large for 32-bit type 'unsigned int'
  Call Trace:
   __ubsan_handle_shift_out_of_bounds
   tcp_tso_segs
   tcp_write_xmit
   __tcp_push_pending_frames
   tcp_push
   tcp_sendmsg_locked

Limit the sysctl to 31, the largest valid shift count for a u32, and
document the accepted range. Writes of larger values now fail with
-EINVAL.

On a UBSAN/KASAN build of the net tree, values 0 and 31 remained
accepted, values 32 and 255 returned -EINVAL, and TCP traffic with the
valid boundary values produced no sanitizer diagnostics.

Fixes: 65466904b015 ("tcp: adjust TSO packet sizes based on min_rtt")
Cc: stable@vger.kernel.org
Signed-off-by: Yingjie Wang <1075151112@qq.com>
---
 Documentation/networking/ip-sysctl.rst | 2 ++
 net/ipv4/sysctl_net_ipv4.c             | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 8edd2eb2a..66a54c510 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1128,6 +1128,8 @@ tcp_tso_rtt_log - INTEGER
 
 	If you want to use the old autosizing, set this sysctl to 0.
 
+	Possible values are [0, 31], inclusive.
+
 	Default: 9  (2^9 = 512 usec)
 
 tcp_pacing_ss_ratio - INTEGER
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e3760daa3..e95431140 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -26,6 +26,7 @@ static int ip_local_port_range_max[] = { 65535, 65535 };
 static int tcp_adv_win_scale_min = -31;
 static int tcp_adv_win_scale_max = 31;
 static int tcp_app_win_max = 31;
+static int tcp_tso_rtt_log_max = 31;
 static int tcp_min_snd_mss_min = TCP_MIN_SND_MSS;
 static int tcp_min_snd_mss_max = 65535;
 static int tcp_rto_max_max = TCP_RTO_MAX_SEC * MSEC_PER_SEC;
@@ -778,6 +779,7 @@ static const struct ctl_table ipv4_net_table[] = {
 		.maxlen		= sizeof(u8),
 		.mode		= 0644,
 		.proc_handler	= proc_dou8vec_minmax,
+		.extra2		= &tcp_tso_rtt_log_max,
 	},
 	{
 		.procname	= "ip_early_demux",

base-commit: f4377bf85887f57b59abb5479f7660a81ffe4449
-- 
2.43.0