It is possible to deadlock libvirt by having a domain with XML longer than PIPE_BUF, and by writing a hook script that closes stdin early. This is because libvirt was keeping a copy of the child's stdin read fd open, which means the write fd in the parent will never see EPIPE (remember, libvirt should always be run with SIGPIPE ignored, so we should never get a SIGPIPE signal). Since there is no error, libvirt blocks waiting for a write to complete, even though the only reader is also libvirt. The solution is to ensure that only the child can act as a reader before the parent does any writes; and then dealing with the fallout of dealing with EPIPE. Thankfully, this is not a security hole - since the only way to trigger the deadlock is to install a custom hook script, anyone that already has privileges to install a hook script already has privileges to do any number of other equally disruptive things to libvirt; it would only be a security hole if an unprivileged user could install a hook script to DoS a privileged user. * src/util/command.c (virCommandRun): Close parent's copy of child read fd earlier. (virCommandProcessIO): Don't let EPIPE be fatal; the child may be done parsing input. * tests/commandhelper.c (main): Set up a SIGPIPE situation. * tests/commandtest.c (test20): Trigger it. * tests/commanddata/test20.log: New file. --- I tested this by using just the changes to 'tests/', and was able to trigger deadlock in 100% of my testing with the buffer lengths shown here. src/util/command.c | 22 +++++++++++++--------- tests/commanddata/test20.log | 13 +++++++++++++ tests/commandhelper.c | 6 ++++++ tests/commandtest.c | 42 ++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 74 insertions(+), 9 deletions(-) create mode 100644 tests/commanddata/test20.log diff --git a/src/util/command.c b/src/util/command.c index ba43078..52a457c 100644 --- a/src/util/command.c +++ b/src/util/command.c @@ -1738,7 +1738,7 @@ virCommandProcessIO(virCommandPtr cmd, int *inpipe) break; if (poll(fds, nfds, -1) < 0) { - if ((errno == EAGAIN) || (errno == EINTR)) + if (errno == EAGAIN || errno == EINTR) continue; virReportSystemError(errno, "%s", _("unable to poll on child")); @@ -1797,8 +1797,13 @@ virCommandProcessIO(virCommandPtr cmd, int *inpipe) done = write(infd, cmd->inbuf + inoff, inlen - inoff); if (done < 0) { - if (errno != EINTR && - errno != EAGAIN) { + if (errno == EPIPE) { + VIR_DEBUG("child closed stdin early, ignoring EPIPE " + "on fd %d", infd); + if (VIR_CLOSE(*inpipe) < 0) + VIR_DEBUG("ignoring failed close on fd %d", infd); + infd = -1; + } else if (errno != EINTR && errno != EAGAIN) { virReportSystemError(errno, "%s", _("unable to write to child input")); goto cleanup; @@ -1885,6 +1890,7 @@ virCommandRun(virCommandPtr cmd, int *exitstatus) bool string_io; bool async_io = false; char *str; + int tmpfd; if (!cmd ||cmd->has_error == ENOMEM) { virReportOOMError(); @@ -1965,7 +1971,7 @@ virCommandRun(virCommandPtr cmd, int *exitstatus) cmd->flags |= VIR_EXEC_RUN_SYNC; if (virCommandRunAsync(cmd, NULL) < 0) { if (cmd->inbuf) { - int tmpfd = infd[0]; + tmpfd = infd[0]; if (VIR_CLOSE(infd[0]) < 0) VIR_DEBUG("ignoring failed close on fd %d", tmpfd); tmpfd = infd[1]; @@ -1976,6 +1982,9 @@ virCommandRun(virCommandPtr cmd, int *exitstatus) return -1; } + tmpfd = infd[0]; + if (VIR_CLOSE(infd[0]) < 0) + VIR_DEBUG("ignoring failed close on fd %d", tmpfd); if (string_io) ret = virCommandProcessIO(cmd, &infd[1]); @@ -1994,15 +2003,11 @@ virCommandRun(virCommandPtr cmd, int *exitstatus) /* Reset any capturing, in case caller runs * this identical command again */ if (cmd->inbuf) { - int tmpfd = infd[0]; - if (VIR_CLOSE(infd[0]) < 0) - VIR_DEBUG("ignoring failed close on fd %d", tmpfd); tmpfd = infd[1]; if (VIR_CLOSE(infd[1]) < 0) VIR_DEBUG("ignoring failed close on fd %d", tmpfd); } if (cmd->outbuf == &outbuf) { - int tmpfd ATTRIBUTE_UNUSED; tmpfd = cmd->outfd; if (VIR_CLOSE(cmd->outfd) < 0) VIR_DEBUG("ignoring failed close on fd %d", tmpfd); @@ -2011,7 +2016,6 @@ virCommandRun(virCommandPtr cmd, int *exitstatus) VIR_FREE(outbuf); } if (cmd->errbuf == &errbuf) { - int tmpfd ATTRIBUTE_UNUSED; tmpfd = cmd->errfd; if (VIR_CLOSE(cmd->errfd) < 0) VIR_DEBUG("ignoring failed close on fd %d", tmpfd); diff --git a/tests/commanddata/test20.log b/tests/commanddata/test20.log new file mode 100644 index 0000000..7d48121 --- /dev/null +++ b/tests/commanddata/test20.log @@ -0,0 +1,13 @@ +ARG:--close-stdin +ENV:DISPLAY=:0.0 +ENV:HOME=/home/test +ENV:HOSTNAME=test +ENV:LANG=C +ENV:LOGNAME=testTMPDIR=/tmp +ENV:PATH=/usr/bin:/bin +ENV:USER=test +FD:0 +FD:1 +FD:2 +DAEMON:no +CWD:/tmp diff --git a/tests/commandhelper.c b/tests/commandhelper.c index a80d191..ec0c458 100644 --- a/tests/commandhelper.c +++ b/tests/commandhelper.c @@ -113,6 +113,12 @@ int main(int argc, char **argv) { VIR_FORCE_FCLOSE(log); + if (argc > 1 && STREQ(argv[1], "--close-stdin")) { + if (freopen("/dev/null", "r", stdin) != stdin) + goto error; + usleep(100*1000); + } + char buf[1024]; ssize_t got; diff --git a/tests/commandtest.c b/tests/commandtest.c index 30e7efb..fa445d3 100644 --- a/tests/commandtest.c +++ b/tests/commandtest.c @@ -36,6 +36,9 @@ #include "command.h" #include "virfile.h" #include "virpidfile.h" +#include "virterror_internal.h" + +#define VIR_FROM_THIS VIR_FROM_NONE #ifdef WIN32 @@ -784,6 +787,44 @@ cleanup: return ret; } +/* + * Run program, no args, inherit all ENV, keep CWD. + * Ignore huge stdin data, to provoke SIGPIPE or EPIPE in parent. + */ +static int test20(const void *unused ATTRIBUTE_UNUSED) +{ + virCommandPtr cmd = virCommandNewArgList(abs_builddir "/commandhelper", + "--close-stdin", NULL); + char *buf; + int ret = -1; + + struct sigaction sig_action; + + sig_action.sa_handler = SIG_IGN; + sig_action.sa_flags = 0; + sigemptyset(&sig_action.sa_mask); + + sigaction(SIGPIPE, &sig_action, NULL); + + if (virAsprintf(&buf, "1\n%100000d\n", 2) < 0) { + virReportOOMError(); + goto cleanup; + } + virCommandSetInputBuffer(cmd, buf); + + if (virCommandRun(cmd, NULL) < 0) { + virErrorPtr err = virGetLastError(); + printf("Cannot run child %s\n", err->message); + goto cleanup; + } + + ret = checkoutput("test20"); +cleanup: + virCommandFree(cmd); + VIR_FREE(buf); + return ret; +} + static const char *const newenv[] = { "PATH=/usr/bin:/bin", "HOSTNAME=test", @@ -868,6 +909,7 @@ mymain(void) DO_TEST(test17); DO_TEST(test18); DO_TEST(test19); + DO_TEST(test20); return ret==0 ? EXIT_SUCCESS : EXIT_FAILURE; } -- 1.7.7.6