On Mon, Jan 18, 2016 at 11:39:34AM +0100, Michal Privoznik wrote:
So I can observe this crasher that with freshly started daemon (and virtlogd enabled) I am trying to startup a domain that immediately dies (because it's said to use huge pages but I haven't allocated a single one in the pool). But what's interesting is that the daemon crashes too. Hardly reproducible with -O0 or under valgrind. But I just got lucky: ==20469== Invalid write of size 8 ==20469== at 0x4C2E99B: memcpy@GLIBC_2.2.5 (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so) ==20469== by 0x217EDD07: qemuProcessReadLog (qemu_process.c:1670) ==20469== by 0x217EDE1D: qemuProcessReportLogError (qemu_process.c:1696) ==20469== by 0x217EE8C1: qemuProcessWaitForMonitor (qemu_process.c:1957) ==20469== by 0x217F6636: qemuProcessLaunch (qemu_process.c:4955) ==20469== by 0x217F71A4: qemuProcessStart (qemu_process.c:5152) ==20469== by 0x21846582: qemuDomainObjStart (qemu_driver.c:7396) ==20469== by 0x218467DE: qemuDomainCreateWithFlags (qemu_driver.c:7450) ==20469== by 0x21846845: qemuDomainCreate (qemu_driver.c:7468) ==20469== by 0x5611CD0: virDomainCreate (libvirt-domain.c:6753) ==20469== by 0x125D9A: remoteDispatchDomainCreate (remote_dispatch.h:3613) ==20469== by 0x125CB7: remoteDispatchDomainCreateHelper (remote_dispatch.h:3589) ==20469== Address 0x27a52ad0 is 0 bytes after a block of size 5,584 alloc'd ==20469== at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so) ==20469== by 0x9B8D1DB: xdr_string (in /lib64/libc-2.21.so) ==20469== by 0x563B39C: xdr_virLogManagerProtocolNonNullString (log_protocol.c:24) ==20469== by 0x563B6B7: xdr_virLogManagerProtocolDomainReadLogFileRet (log_protocol.c:123) ==20469== by 0x164B34: virNetMessageDecodePayload (virnetmessage.c:407) ==20469== by 0x5682360: virNetClientProgramCall (virnetclientprogram.c:379) ==20469== by 0x563B30E: virLogManagerDomainReadLogFile (log_manager.c:272) ==20469== by 0x217CD613: qemuDomainLogContextRead (qemu_domain.c:2485) ==20469== by 0x217EDC76: qemuProcessReadLog (qemu_process.c:1660) ==20469== by 0x217EDE1D: qemuProcessReportLogError (qemu_process.c:1696) ==20469== by 0x217EE8C1: qemuProcessWaitForMonitor (qemu_process.c:1957) ==20469== by 0x217F6636: qemuProcessLaunch (qemu_process.c:4955) This points to memmove() in qemuProcessReadLog(). And in fact after few moments and a couple of papers (to debug the code) I can observe the bug. Imagine we just read the following string from qemu: "abc\n2016-01-18T09:40:44.022744Z qemu-system-x86_64: Error\n" After the first pass of the while() loop in the qemuProcessReadLog() @buf still points to the beginning of the string, @filter_next points to the beginning of the second line. So we start second iteration because there is yet another newline character at the end. In this iteration @eol points to it actually. Now, the control gets inside true branch of if(). Just to remind you: got = 58 filter_next = buf + 5, eol = buf + 58.
So several weeks ago I spent non-negligible amount of hours on this, with papers written all over and everything. I couldn't find a problem in it, but I didn't consider the fact that in first iteration we can go with the if condition being false, hence taking the else branch which forgets to update 'got'. I always set got -= skip. Good catch, but please mention that in the first iteration we take the false branch, thanks.
Therefore skip = 54 which is correct. The message we want to skip
is 54 bytes long. However:
memmove(filter_next, eol + 1, (got - skip) +1);
which is
memmove(filter_next, eol + 1, 5)
is obviously wrong as there is only one byte we can access, not 5!
Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
---
src/qemu/qemu_process.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 05cbda2..f85afd5 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -1667,7 +1667,7 @@ qemuProcessReadLog(qemuDomainLogContextPtr logCtxt, char **msg)
if (virLogProbablyLogMessage(filter_next) ||
STRPREFIX(filter_next, "char device redirected to")) {
size_t skip = (eol + 1) - filter_next;
- memmove(filter_next, eol + 1, (got - skip) + 1);
+ memmove(filter_next, eol + 1, buf + got - eol);
I wonder why do we even deal with this when qemuDomainLogContextRead() always returns buffer that is zero-terminated, but that's a question for another day, I guess. ACK and we could also probably backport to maint branches. Martin
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