Red Hat Bugzilla – Bug 1205258
Busy loop in recv(MSG_PEEK|MSG_WAITALL)
Last modified: 2015-11-19 16:48:24 EST
Description of problem: Kernel goes into a busy loop when it waits for more data in a recv(MSG_PEEK|MSG_WAITALL) call. E.g. ------ #include <stdlib.h> #include <netinet/ip.h> int main(void) { struct sockaddr_in addr = { .sin_family = AF_INET, .sin_port = htons(1234), .sin_addr = { INADDR_ANY } }; int conn; char buf[16]; int s = socket(AF_INET, SOCK_STREAM, 0); bind(s, (void *)&addr, sizeof addr); listen(s, 1); conn = accept(s, NULL, 0); recv(conn, buf, sizeof buf, MSG_PEEK|MSG_WAITALL); } ---- $ gcc x.c $ a.out & $ nc 127.0.0.1 1234 1234<enter> --> 'a.out' consumes 100% CPU 'a.out' stays alive and consumes CPU when the 'nc' connection is closed unclean (e.g. no TCP FIN/RST). This can be used for DDOS attacks. Version-Release number of selected component (if applicable): kernel-3.10.0-123.20.1.el7.x86_64 How reproducible: 100%
Upstream kernel behaves the same. MSG_WAITALL tells kernel to wait until whole buffer can be filled. Looks like the combination with MSG_PEEK is not handled properly in tcp_recvmsg: if (copied >= target) { /* Do not sleep, just process backlog. */ release_sock(sk); lock_sock(sk); } else sk_wait_data(sk, &timeo); In case both MSG_PEEK and MSG_WAITALL are there, sk_wait_data is not called.
lock_sock (the only lock taken at that moment) is preemptible in process context, so it should not lead to a DoS situation. Albeit maybe we can do better and handle the situation where both flags are set more intelligent?
What's the status of this bug? Is it being worked on? Is there any way a fix can be expedited?
This bug has been fixed upstream (in David Miller's net tree): https://git.kernel.org/cgit/linux/kernel/git/davem/net.git/commit/?id=dfbafc995304ebb9a9b03f65083e6e9cea143b20
Many thanks!
Patch(es) available on kernel-3.10.0-306.el7
Hi, thanks for the update. Will this patch go into the 3.13.0 kernel branch? I'm hoping to get Ubuntu to suck it into their LTS kernel packages which seem to be built from the 3.13.0 branch.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://rhn.redhat.com/errata/RHSA-2015-2152.html