From ebd7c72c63acc3d00944c88cbf96143a8cae179f Mon Sep 17 00:00:00 2001 From: "J. Bruce Fields" Date: Mon, 12 Sep 2016 16:00:47 -0400 Subject: [PATCH] nfsd: randomize SETCLIENTID reply to help distinguish servers NFSv4.1 has built-in trunking support that allows a client to determine whether two connections to two different IP addresses are actually to the same server. NFSv4.0 does not, but RFC 7931 attempts to provide clients a means to do this, basically by performing a SETCLIENTID to one address and confirming it with a SETCLIENTID_CONFIRM to the other. Linux clients since 05f4c350ee02 "NFS: Discover NFSv4 server trunking when mounting" implement a variation on this suggestion. It is possible that other clients do too. This depends on the clientid and verifier not being accepted by an unrelated server. Since both are 64-bit values, that would be very unlikely if they were random numbers. But they aren't: knfsd generates the 64-bit clientid by concatenating the 32-bit boot time (in seconds) and a counter. This makes collisions between clientids generated by the same server extremely unlikely. But collisions are very likely between clientids generated by servers that boot at the same time, and it's quite common for multiple servers to boot at the same time. The verifier is a concatenation of the SETCLIENTID time (in seconds) and a counter, so again collisions between different servers are likely if multiple SETCLIENTIDs are done at the same time, which is a common case. Therefore recent NFSv4.0 clients may decide two different servers are really the same, and mount a filesystem from the wrong server. Fortunately the Linux client, since 55b9df93ddd6 "nfsv4/v4.1: Verify the client owner id during trunking detection", only does this when given the non-default "migration" mount option. The fault is really with RFC 7931, and needs a client fix, but in the meantime we can mitigate the chance of these collisions by randomizing the starting value of the counters used to generate clientids and verifiers. Reported-by: Frank Sorenson Reviewed-by: Jeff Layton Signed-off-by: J. Bruce Fields --- fs/nfsd/nfsctl.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c index 65ad0165a94f..36b2af931e06 100644 --- a/fs/nfsd/nfsctl.c +++ b/fs/nfsd/nfsctl.c @@ -1216,6 +1216,8 @@ static __net_init int nfsd_init_net(struct net *net) goto out_idmap_error; nn->nfsd4_lease = 90; /* default lease time */ nn->nfsd4_grace = 90; + nn->clverifier_counter = prandom_u32(); + nn->clientid_counter = prandom_u32(); return 0; out_idmap_error: -- 2.20.1