Hello, On 11/05/2014 03:21 PM, bstroesser@xxxxxxxxxxxxxx wrote: > Hello, > > I'm sending a small set of 3 patches for a problem, that I have > reported a few weeks ago. > rpc.mountd can be blocked by a bad client, that sends lots of > RPC requests, but never reads the replies from the socket either > intentionally or e.g. caused by a wrong configured MTU. > > While looking for a possible solution, I found another weakness > in rpc.mountd if it is used "multithreaded" (-t nn). > > The first two patches fix that weakness in the case of !HAVE_LIBTIRPC > and HAVE_LIBTIRPC. > The third patch more a kind of suggestion how the main problem could > be fixed. I don't know whether we can set MAXREC without causing > new troubles. When this patch is used, a further patch for libtirpc > also should be used. You can find it here: > http://sourceforge.net/p/libtirpc/mailman/libtirpc-devel/?viewmonth=201409 Over the weekend I took a good hard look at these 3 patches and the one for libtirpc with the reproducer Bodo supplied me. (Bodo, thanks all the help getting things set up!). I agree with the libtirpc patch so its been committed, but other three made me nervous so I wanted to take a good hard look at them. (Neil's question, "do we really what to make these non-block" haunted me ;-) ). It turns out, at least in my setup, these patches do not and can not stop the mountd DOS that Bodo's reproducer creates. Here is why they do not work: The fd that the write() is getting hung on, is not be created by the 3 nfs_svc_create() calls in mountd:main(). Its being created by the accept() when the tcp connection is created (via SVC_RECV()), so making those fds coming out of the nfs_svc_create() non-blocking does nothing. Here is the hang can't be stop (for now): Here is the stack: Stack trace of thread 4150: #0 0x00000036816f0e90 write (libc.so.6) #1 0x00007fd53837da6d write_vc (libtirpc.so.1) #2 0x00007fd53838103f flush_out (libtirpc.so.1) #3 0x00007fd53837dd81 svc_vc_reply (libtirpc.so.1) #4 0x00007fd53837b096 svcerr_noprog (libtirpc.so.1) #5 0x00007fd53837b360 svc_getreq_common (libtirpc.so.1) #6 0x00000000004086d9 my_svc_getreqset (mountd) #7 0x0000000000403d80 main (mountd) #8 0x000000368161ffe0 __libc_start_main (libc.so.6) #9 0x00000000004040f5 _start (mountd) The fd that created by the SVC_RECV() is never given back to the app (in case mountd) to make it non-blocking. Its used in the error path where it get hung up in the write(). The app has no control over that. Now I definitely see a problem, but these patches (with the exception of the libtirpc patch) don't address the problem I am seeing. So unless I'm not seeing something, I'm not in favor of taking these 3 patches. thoughts? steved. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
