My original intent was to replace the last non-upstream Arista's TCP-AO piece. That is per-netns procfs seqfile which lists AO keys. In my view an acceptable upstream alternative would be TCP-AO-diag uAPI.
So, I started by looking and reviewing TCP-MD5-diag code. And straight away I saw a bunch of issues:
1. Similarly to TCP_MD5SIG_EXT, which doesn't check tcpm_flags for unknown flags and so being non-extendable setsockopt(), the same way tcp_diag_put_md5sig() dumps md5 keys in an array of tcp_diag_md5sig, which makes it ABI non-extendable structure as userspace can't tolerate any new members in it.
2. Inet-diag allocates netlink message for sockets in inet_diag_dump_one_icsk(), which uses a TCP-diag callback .idiag_get_aux_size(), that pre-calculates the needed space for TCP-diag related information. But as neither socket lock nor rcu_readlock() are held between allocation and the actual TCP info filling, the TCP-related space requirement may change before reaching tcp_diag_put_md5sig(). I.e., the number of TCP-MD5 keys on a socket. Thankfully, TCP-MD5-diag won't overwrite the skb, but will return EMSGSIZE, triggering WARN_ON() in inet_diag_dump_one_icsk().
3. Inet-diag "do" request* can create skb of any message required size. But "dump" request* the skb size, since d35c99ff77ec ("netlink: do not enter direct reclaim from netlink_dump()") is limited by 32 KB. Having in mind that sizeof(struct tcp_diag_md5sig) = 100 bytes, dumps for sockets that have more than 327 keys are going to fail (not counting other diag infos, which lower this limit futher). That is much lower than the number of TCP-MD5 keys that can be allocated on a socket with the current default optmem_max limit (128Kb).
So, then I went and written selftests for TCP-MD5-diag and besides confirming that (2) and (3) are not theoretical issues, I also discovered another issues, that I didn't notice on code inspection:
4. nlattr::nla_len is __u16, which limits the largest netlink attibute by 64Kb or by 655 tcp_diag_md5sig keys in the diag array. What happens de-facto is that the netlink attribute gets u16 overflow, breaking the userspace parsing - RTA_NEXT(), that should point to the next attribute, points into the middle of md5 keys array.
In this patch set issues (2) and (4) are addressed. (2) by not returning EMSGSIZE when the dump raced with modifying TCP-MD5 keys on a socket, but mark the dump inconsistent by setting NLM_F_DUMP_INTR nlmsg flag. Which changes uAPI in situations where previously kernel did WARN() and errored the dump. (4) by artificially limiting the maximum attribute size by U16_MAX - 1.
In order to remove the new limit from (4) solution, my plan is to convert the dump of TCP-MD5 keys from an array to NL_ATTR_TYPE_NESTED_ARRAY (or alike), which should also address (1). And for (3), it's needed to teach tcp-diag how-to remember not only socket on which previous recvmsg() stopped, but potentially TCP-MD5 key as well.
I plan in the next part of patch set address (3), (1) and the new limit for (4), together with adding new TCP-AO-diag.
* Terminology from Documentation/userspace-api/netlink/intro.rst
Signed-off-by: Dmitry Safonov 0x7f454c46@gmail.com --- Dmitry Safonov (6): net/diag: Do not race on dumping MD5 keys with adding new MD5 keys net/diag: Warn only once on EMSGSIZE net/diag: Pre-allocate optional info only if requested net/diag: Always pre-allocate tcp_ulp info net/diag: Limit TCP-MD5-diag array by max attribute length net/netlink: Correct the comment on netlink message max cap
include/linux/inet_diag.h | 3 +- include/net/tcp.h | 1 - net/ipv4/inet_diag.c | 87 ++++++++++++++++++++++++++++++++++++++--------- net/ipv4/tcp_diag.c | 69 ++++++++++++++++++------------------- net/mptcp/diag.c | 20 ----------- net/netlink/af_netlink.c | 4 +-- net/tls/tls_main.c | 17 --------- 7 files changed, 109 insertions(+), 92 deletions(-) --- base-commit: 2e1b3cc9d7f790145a80cb705b168f05dab65df2 change-id: 20241106-tcp-md5-diag-prep-2f0dcf371d90
Best regards,
From: Dmitry Safonov 0x7f454c46@gmail.com
Inet-diag has two modes: (1) dumping information for a specific socket, for which kernel creates one netlink message with the information and (2) dumping information for multiple sockets (possibly with a filter), where for the reply kernel sends many messages, one for each matched socket.
Currently those two modes work differently as the information about a specific socket is never split between multiple messages. For (2), multi-socket dump for the reply kernel allocates up to 32Kb skb and fills that with as many socket dumps as possible. For (1), one-socket dump kernel pre-calculates the required space for the reply, allocates a new skb and nlmsg and only then starts filling the socket's details.
Preallocating the needed size quite makes sense as most of the details are fix-sized and provided for each socket, see inet_sk_attr_size(). But there's an exception: .idiag_get_aux_size() which is optional for a socket. This is provided only for TCP sockets by tcp_diag.
For TCP-MD5 it calculates the memory needed to fill an array of (struct tcp_diag_md5sig). The issue here is that the amount of keys may change in inet_diag_dump_one_icsk() between inet_sk_attr_size() and sk_diag_fill() calls. As the code expects fix-sized information on any socket, it considers sk_diag_fill() failures by -EMSGSIZE reason as a bug, resulting in such WARN_ON():
[] ------------[ cut here ]------------ [] WARNING: CPU: 7 PID: 17420 at net/ipv4/inet_diag.c:586 inet_diag_dump_one_icsk+0x3c8/0x420 [] CPU: 7 UID: 0 PID: 17420 Comm: diag_ipv4 Tainted: G W 6.11.0-rc6-00022-gc9fd7a9f9aca-dirty #2 [] pc : inet_diag_dump_one_icsk+0x3c8/0x420 [] lr : inet_diag_dump_one_icsk+0x1d4/0x420 [] sp : ffff8000aef87460 ... [] Call trace: [] inet_diag_dump_one_icsk+0x3c8/0x420 [] tcp_diag_dump_one+0xa0/0xf0 [] inet_diag_cmd_exact+0x234/0x278 [] inet_diag_handler_cmd+0x16c/0x288 [] sock_diag_rcv_msg+0x1a8/0x550 [] netlink_rcv_skb+0x198/0x378 [] sock_diag_rcv+0x20/0x48 [] netlink_unicast+0x400/0x6a8 [] netlink_sendmsg+0x654/0xa58 [] __sys_sendto+0x1ec/0x330 [] __arm64_sys_sendto+0xc8/0x168 ... [] ---[ end trace 0000000000000000 ]---
One way to solve it would be to grab lock_sock() in inet_diag_dump_one_icsk(), but that may be costly and bring new lock dependencies. The alternative is to call tcp_diag_put_md5sig() as the last attribute of the netlink message and calculate how much space left after all previous attributes filled and translate it into (struct tcp_diag_md5sig)-sized units. If it turns out that there's not enough space for all TCP-MD5 keys, mark the dump as inconsistent by setting NLM_F_DUMP_INTR flag. Userspace may figure out that dumping raced with the socket properties change and retry again.
Currently it may be unexpected by userspace that netlink message for one socket may be inconsistent, but I believe we're on a safe side from breaking userspace as previously dump would fail and an ugly WARN was produced in dmesg. IOW, it is a clear improvement.
This is not a theoretical issue: I've written a test and that reproduces the issue I suspected (the backtrace above).
Fixes: c03fa9bcacd9 ("tcp_diag: report TCP MD5 signing keys and addresses") Cc: stable@vger.kernel.org Signed-off-by: Dmitry Safonov 0x7f454c46@gmail.com --- include/linux/inet_diag.h | 3 ++- net/ipv4/inet_diag.c | 8 +++---- net/ipv4/tcp_diag.c | 55 ++++++++++++++++++++++++++++------------------- 3 files changed, 39 insertions(+), 27 deletions(-)
diff --git a/include/linux/inet_diag.h b/include/linux/inet_diag.h index a9033696b0aad36ab9abd47e4b68e272053019d7..cb2ba672eba131986d0432dd628fc42bbf800886 100644 --- a/include/linux/inet_diag.h +++ b/include/linux/inet_diag.h @@ -22,7 +22,8 @@ struct inet_diag_handler {
int (*idiag_get_aux)(struct sock *sk, bool net_admin, - struct sk_buff *skb); + struct sk_buff *skb, + struct nlmsghdr *nlh);
size_t (*idiag_get_aux_size)(struct sock *sk, bool net_admin); diff --git a/net/ipv4/inet_diag.c b/net/ipv4/inet_diag.c index 67639309163d05c034fad80fc9a6096c3b79d42f..67b9cc4c0e47a596a4d588e793b7f13ee040a1e3 100644 --- a/net/ipv4/inet_diag.c +++ b/net/ipv4/inet_diag.c @@ -350,10 +350,6 @@ int inet_sk_diag_fill(struct sock *sk, struct inet_connection_sock *icsk,
handler->idiag_get_info(sk, r, info);
- if (ext & (1 << (INET_DIAG_INFO - 1)) && handler->idiag_get_aux) - if (handler->idiag_get_aux(sk, net_admin, skb) < 0) - goto errout; - if (sk->sk_state < TCP_TIME_WAIT) { union tcp_cc_info info; size_t sz = 0; @@ -368,6 +364,10 @@ int inet_sk_diag_fill(struct sock *sk, struct inet_connection_sock *icsk, goto errout; }
+ if (ext & (1 << (INET_DIAG_INFO - 1)) && handler->idiag_get_aux) + if (handler->idiag_get_aux(sk, net_admin, skb, nlh) < 0) + goto errout; + /* Keep it at the end for potential retry with a larger skb, * or else do best-effort fitting, which is only done for the * first_nlmsg. diff --git a/net/ipv4/tcp_diag.c b/net/ipv4/tcp_diag.c index f428ecf9120f2f596e1d67db2b2a0d0d0e211905..36606a19b451f059e32c58c0d76a878dc9be5ff0 100644 --- a/net/ipv4/tcp_diag.c +++ b/net/ipv4/tcp_diag.c @@ -53,29 +53,39 @@ static void tcp_diag_md5sig_fill(struct tcp_diag_md5sig *info, }
static int tcp_diag_put_md5sig(struct sk_buff *skb, - const struct tcp_md5sig_info *md5sig) + const struct tcp_md5sig_info *md5sig, + struct nlmsghdr *nlh) { + size_t key_size = sizeof(struct tcp_diag_md5sig); + unsigned int attrlen, md5sig_count; const struct tcp_md5sig_key *key; struct tcp_diag_md5sig *info; struct nlattr *attr; - int md5sig_count = 0;
+ /* + * Userspace doesn't like to see zero-filled key-values, so + * allocating too large attribute is bad. + */ hlist_for_each_entry_rcu(key, &md5sig->head, node) md5sig_count++; if (md5sig_count == 0) return 0;
- attr = nla_reserve(skb, INET_DIAG_MD5SIG, - md5sig_count * sizeof(struct tcp_diag_md5sig)); + attrlen = skb_availroom(skb) - NLA_HDRLEN; + md5sig_count = min(md5sig_count, attrlen / key_size); + attr = nla_reserve(skb, INET_DIAG_MD5SIG, md5sig_count * key_size); if (!attr) return -EMSGSIZE;
info = nla_data(attr); - memset(info, 0, md5sig_count * sizeof(struct tcp_diag_md5sig)); + memset(info, 0, md5sig_count * key_size); hlist_for_each_entry_rcu(key, &md5sig->head, node) { - tcp_diag_md5sig_fill(info++, key); - if (--md5sig_count == 0) + /* More keys on a socket than pre-allocated space available */ + if (md5sig_count-- == 0) { + nlh->nlmsg_flags |= NLM_F_DUMP_INTR; break; + } + tcp_diag_md5sig_fill(info++, key); }
return 0; @@ -110,25 +120,11 @@ static int tcp_diag_put_ulp(struct sk_buff *skb, struct sock *sk, }
static int tcp_diag_get_aux(struct sock *sk, bool net_admin, - struct sk_buff *skb) + struct sk_buff *skb, struct nlmsghdr *nlh) { struct inet_connection_sock *icsk = inet_csk(sk); int err = 0;
-#ifdef CONFIG_TCP_MD5SIG - if (net_admin) { - struct tcp_md5sig_info *md5sig; - - rcu_read_lock(); - md5sig = rcu_dereference(tcp_sk(sk)->md5sig_info); - if (md5sig) - err = tcp_diag_put_md5sig(skb, md5sig); - rcu_read_unlock(); - if (err < 0) - return err; - } -#endif - if (net_admin) { const struct tcp_ulp_ops *ulp_ops;
@@ -138,6 +134,21 @@ static int tcp_diag_get_aux(struct sock *sk, bool net_admin, if (err) return err; } + +#ifdef CONFIG_TCP_MD5SIG + if (net_admin) { + struct tcp_md5sig_info *md5sig; + + rcu_read_lock(); + md5sig = rcu_dereference(tcp_sk(sk)->md5sig_info); + if (md5sig) + err = tcp_diag_put_md5sig(skb, md5sig, nlh); + rcu_read_unlock(); + if (err < 0) + return err; + } +#endif + return 0; }
Hi Dmitry,
kernel test robot noticed the following build warnings:
[auto build test WARNING on 2e1b3cc9d7f790145a80cb705b168f05dab65df2]
url: https://github.com/intel-lab-lkp/linux/commits/Dmitry-Safonov-via-B4-Relay/n... base: 2e1b3cc9d7f790145a80cb705b168f05dab65df2 patch link: https://lore.kernel.org/r/20241106-tcp-md5-diag-prep-v1-1-d62debf3dded%40gma... patch subject: [PATCH net 1/6] net/diag: Do not race on dumping MD5 keys with adding new MD5 keys config: arm64-randconfig-003-20241107 (https://download.01.org/0day-ci/archive/20241107/202411071218.G7g6a8JG-lkp@i...) compiler: clang version 20.0.0git (https://github.com/llvm/llvm-project 592c0fe55f6d9a811028b5f3507be91458ab2713) reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241107/202411071218.G7g6a8JG-lkp@i...)
If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot lkp@intel.com | Closes: https://lore.kernel.org/oe-kbuild-all/202411071218.G7g6a8JG-lkp@intel.com/
All warnings (new ones prefixed by >>):
In file included from net/ipv4/tcp_diag.c:9: In file included from include/linux/net.h:24: In file included from include/linux/mm.h:2213: include/linux/vmstat.h:504:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion] 504 | return vmstat_text[NR_VM_ZONE_STAT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~ ^ 505 | item]; | ~~~~ include/linux/vmstat.h:511:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion] 511 | return vmstat_text[NR_VM_ZONE_STAT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~ ^ 512 | NR_VM_NUMA_EVENT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~~ include/linux/vmstat.h:518:36: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion] 518 | return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_" | ~~~~~~~~~~~ ^ ~~~ include/linux/vmstat.h:524:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion] 524 | return vmstat_text[NR_VM_ZONE_STAT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~ ^ 525 | NR_VM_NUMA_EVENT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~~
net/ipv4/tcp_diag.c:70:3: warning: variable 'md5sig_count' is uninitialized when used here [-Wuninitialized]
70 | md5sig_count++; | ^~~~~~~~~~~~ net/ipv4/tcp_diag.c:60:36: note: initialize the variable 'md5sig_count' to silence this warning 60 | unsigned int attrlen, md5sig_count; | ^ | = 0 5 warnings generated.
vim +/md5sig_count +70 net/ipv4/tcp_diag.c
c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 54 c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 55 static int tcp_diag_put_md5sig(struct sk_buff *skb, 4a6144fbc706b3c Dmitry Safonov 2024-11-06 56 const struct tcp_md5sig_info *md5sig, 4a6144fbc706b3c Dmitry Safonov 2024-11-06 57 struct nlmsghdr *nlh) c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 58 { 4a6144fbc706b3c Dmitry Safonov 2024-11-06 59 size_t key_size = sizeof(struct tcp_diag_md5sig); 4a6144fbc706b3c Dmitry Safonov 2024-11-06 60 unsigned int attrlen, md5sig_count; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 61 const struct tcp_md5sig_key *key; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 62 struct tcp_diag_md5sig *info; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 63 struct nlattr *attr; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 64 4a6144fbc706b3c Dmitry Safonov 2024-11-06 65 /* 4a6144fbc706b3c Dmitry Safonov 2024-11-06 66 * Userspace doesn't like to see zero-filled key-values, so 4a6144fbc706b3c Dmitry Safonov 2024-11-06 67 * allocating too large attribute is bad. 4a6144fbc706b3c Dmitry Safonov 2024-11-06 68 */ c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 69 hlist_for_each_entry_rcu(key, &md5sig->head, node) c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 @70 md5sig_count++; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 71 if (md5sig_count == 0) c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 72 return 0; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 73 4a6144fbc706b3c Dmitry Safonov 2024-11-06 74 attrlen = skb_availroom(skb) - NLA_HDRLEN; 4a6144fbc706b3c Dmitry Safonov 2024-11-06 75 md5sig_count = min(md5sig_count, attrlen / key_size); 4a6144fbc706b3c Dmitry Safonov 2024-11-06 76 attr = nla_reserve(skb, INET_DIAG_MD5SIG, md5sig_count * key_size); c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 77 if (!attr) c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 78 return -EMSGSIZE; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 79 c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 80 info = nla_data(attr); 4a6144fbc706b3c Dmitry Safonov 2024-11-06 81 memset(info, 0, md5sig_count * key_size); c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 82 hlist_for_each_entry_rcu(key, &md5sig->head, node) { 4a6144fbc706b3c Dmitry Safonov 2024-11-06 83 /* More keys on a socket than pre-allocated space available */ 4a6144fbc706b3c Dmitry Safonov 2024-11-06 84 if (md5sig_count-- == 0) { 4a6144fbc706b3c Dmitry Safonov 2024-11-06 85 nlh->nlmsg_flags |= NLM_F_DUMP_INTR; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 86 break; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 87 } 4a6144fbc706b3c Dmitry Safonov 2024-11-06 88 tcp_diag_md5sig_fill(info++, key); 4a6144fbc706b3c Dmitry Safonov 2024-11-06 89 } c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 90 c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 91 return 0; c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 92 } c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 93 #endif c03fa9bcacd9ac0 Ivan Delalande 2017-08-31 94
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