salsauthd maxes cpu
| Affects | Status | Importance | Assigned to | Milestone | |
|---|---|---|---|---|---|
| cyrus-sasl2 (Ubuntu) |
Fix Released
|
Undecided
|
Unassigned | ||
| Precise |
Won't Fix
|
High
|
Unassigned | ||
| Trusty |
Invalid
|
High
|
Andreas Hasenack | ||
| Xenial |
Fix Released
|
High
|
Andreas Hasenack | ||
Bug Description
[Impact]
The rimap authentication mechanism in saslauthd can hit a condition where it will start spinning and using all available CPU. This condition can be easily encountered when an authentication is happening and the imap service is being restarted.
Furthermore, the saslauthd child process that picked up that authentication request and that is spinning now won't be reaped nor can it service further requests. If all children are left in this state, the authentication service as a whole won't be working anymore.
[Test Case]
This test can be performed in a LXD or VM.
* install the needed packages. mail-stack-delivery is used to have an imap server available on localhost that needs no further configuration. Accept the defaults for all debconf prompts:
sudo apt update
sudo apt install sasl2-bin mail-stack-delivery
* set the password "ubuntu" for the ubuntu user
echo ubuntu:ubuntu | sudo chpasswd
* start saslauthd like this, with just one child:
sudo /usr/sbin/saslauthd -a rimap -O localhost -r -n 1
* restart dovecot
sudo service dovecot restart
* test saslauthd authentication:
$ sudo testsaslauthd -u ubuntu -p ubuntu
0: OK "Success."
* Now let's break it. In one terminal watch the output of top:
top
* in another terminal, run the following:
sudo testsaslauthd -u ubuntu -p ubuntu & sleep 1; sudo service dovecot stop
* observe in the "top" terminal that saslauthd is consuming a lot of cpu. If that's not happening, try starting dovecot again and adjusting the sleep value in the previous test command, but 1s was enough in all my runs.
* start dovecot and repeat the authentication request. Since the only saslauthd child is now spinning, this will block:
sudo service dovecot start
$ sudo testsaslauthd -u ubuntu -p ubuntu
<blocks>
[Regression Potential]
This fix relies on read(2) returning zero bytes when the connection is dropped, and that is clearly documented in its manpage:
"On success, the number of bytes read is returned (zero indicates end of file),"
The select manpage also documents such a case being a valid case to indicate that a socket is ready to be read from, and that it won't block:
"The file descriptors listed in readfds will be watched to see if characters become available for reading (more precisely, to see if a read will not block; in particular, a file descriptor is also ready on end-of-file)"
This patch is what was used upstream, and is also present in bionic.
I can't think of regressions specific to this change, other than the usual risk of rebuilding a widely used library (sasl2) in an environment different from the one from when xenial was released, i.e., different libraries available, maybe different system-wide build options, etc.
[Other Info]
Trusty is still not accounting for read() returning zero being an end-of-file case, but the loop there has a counter and it eventually exits, not leading to a perpetual spin or high cpu usage (see comment #17 for a brief history on how this fix was dropped in the xenial package).
The fix is simple and could be applied there as well, if the SRU team prefers.
[Original Description]
sasl2-bin version 2.1.24~
We are running the daemon with the following options:
/usr/sbin/saslauthd -a rimap -O our.imap.server -r -m /var/spool/
We noticed that users were unable to send mail and that the saslauthd processes were using approximately 100% of each cpu core. An strace of one of the runaway process showed that it was stuck in the following behaviour:
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
select(9, [8], NULL, NULL, {0, 0}) = 1 (in [8], left {0, 0})
read(8, "", 940) = 0
.....
with further inspection showing that the file descriptor in question was a socket connected to our imap server in CLOSE_WAIT.
Browsing saslauthd/
while( select (fds, &perm, NULL, NULL, &timeout ) >0 ) {
if ( FD_ISSET(s, &perm) ) {
ret = read(s, rbuf+rc, sizeof(rbuf)-rc);
if ( ret<0 ) {
rc = ret;
} else {
rc += ret;
}
}
}
It looks like this loop is expected to run until a read error is encountered or the timeout of 1 second is reached. There is no test to check that 0 bytes were read, indicating that the connection was closed by the remote peer. Since select() will immediately return the size of the set of the partially closed descriptor (1, which is >0), and calls to read() will always yield 0 bytes, there's the potential for execution to get stuck in this non blocking loop and I'm presuming that that's what's happening here.
We've not performed any further analysis to prove that this is really what's happening but if my intuition is correct then our IMAP server (an nginx imap proxy) most liklely closes the connection at an unexpected time under as yet undetermined conditions.
Related branches
- Christian Ehrhardt (community): Approve
- Canonical Server: Pending requested
- Diff: 0 lines
| Launchpad Janitor (janitor) wrote | #1 |
| Changed in cyrus-sasl2 (Ubuntu): | |
| status: | New → Confirmed |
| Fufroma (fufroma-a) wrote | #2 |
| Fufroma (fufroma-a) wrote | #3 |
| Robie Basak (racb) wrote | #4 |
| Changed in cyrus-sasl2 (Ubuntu): | |
| status: | Confirmed → Fix Released |
| Daniel (pada) wrote | #5 |
| Robie Basak (racb) wrote | #6 |
| Robie Basak (racb) wrote | #8 |
| Daniel (pada) wrote | #9 |
| Changed in cyrus-sasl2 (Ubuntu Precise): | |
| status: | New → Triaged |
| importance: | Undecided → High |
| Daniel (pada) wrote | #10 |
| Roberto Suarez (robe-allenta) wrote | #11 |
| Roberto Suarez (robe-allenta) wrote | #12 |
| Andreas Hasenack (ahasenack) wrote | #13 |
| Changed in cyrus-sasl2 (Ubuntu Precise): | |
| status: | Triaged → Won't Fix |
| Andreas Hasenack (ahasenack) wrote | #14 |
| tags: |
added: server-next removed: precise |
| Changed in cyrus-sasl2 (Ubuntu Trusty): | |
| status: | New → Triaged |
| Changed in cyrus-sasl2 (Ubuntu Xenial): | |
| status: | New → Triaged |
| Andreas Hasenack (ahasenack) wrote | #15 |
| Changed in cyrus-sasl2 (Ubuntu Xenial): | |
| assignee: | nobody → Andreas Hasenack (ahasenack) |
| importance: | Undecided → High |
| Changed in cyrus-sasl2 (Ubuntu Trusty): | |
| importance: | Undecided → High |
| assignee: | nobody → Andreas Hasenack (ahasenack) |
| description: | updated |
| description: | updated |
| description: | updated |
| description: | updated |
| Andreas Hasenack (ahasenack) wrote | #16 |
| Andreas Hasenack (ahasenack) wrote | #17 |
| Changed in cyrus-sasl2 (Ubuntu Trusty): | |
| status: | Triaged → Invalid |
| description: | updated |
| description: | updated |
| Changed in cyrus-sasl2 (Ubuntu Xenial): | |
| status: | Triaged → In Progress |
| Brian Murray (brian-murray) wrote Please test proposed package | #18 |
| Changed in cyrus-sasl2 (Ubuntu Xenial): | |
| status: | In Progress → Fix Committed |
| tags: | added: verification-needed verification-needed-xenial |
| Andreas Hasenack (ahasenack) wrote | #19 |
| Andreas Hasenack (ahasenack) wrote | #20 |
| tags: |
added: verification-done-xenial removed: verification-needed-xenial |
| Launchpad Janitor (janitor) wrote | #21 |
| Changed in cyrus-sasl2 (Ubuntu Xenial): | |
| status: | Fix Committed → Fix Released |
| Łukasz Zemczak (sil2100) wrote Update Released | #22 |
Status changed to 'Confirmed' because the bug affects multiple users.