From: Greg Kurz Date: Thu, 14 Jan 2021 17:04:12 +0100 Subject: 9pfs: Fully restart unreclaim loop (CVE-2021-20181) Git-commit: 89fbea8737e8f7b954745a1ffc4238d377055305 References: bsc#1182137 Depending on the client activity, the server can be asked to open a huge number of file descriptors and eventually hit RLIMIT_NOFILE. This is currently mitigated using a reclaim logic : the server closes the file descriptors of idle fids, based on the assumption that it will be able to re-open them later. This assumption doesn't hold of course if the client requests the file to be unlinked. In this case, we loop on the entire fid list and mark all related fids as unreclaimable (the reclaim logic will just ignore them) and, of course, we open or re-open their file descriptors if needed since we're about to unlink the file. This is the purpose of v9fs_mark_fids_unreclaim(). Since the actual opening of a file can cause the coroutine to yield, another client request could possibly add a new fid that we may want to mark as non-reclaimable as well. The loop is thus restarted if the re-open request was actually transmitted to the backend. This is achieved by keeping a reference on the first fid (head) before traversing the list. This is wrong in several ways: - a potential clunk request from the client could tear the first fid down and cause the reference to be stale. This leads to a use-after-free error that can be detected with ASAN, using a custom 9p client - fids are added at the head of the list : restarting from the previous head will always miss fids added by a some other potential request All these problems could be avoided if fids were being added at the end of the list. This can be achieved with a QSIMPLEQ, but this is probably too much change for a bug fix. For now let's keep it simple and just restart the loop from the current head. Fixes: CVE-2021-20181 Buglink: https://bugs.launchpad.net/qemu/+bug/1911666 Reported-by: Zero Day Initiative Reviewed-by: Christian Schoenebeck Reviewed-by: Stefano Stabellini Message-Id: <161064025265.1838153.15185571283519390907.stgit@bahia.lan> Signed-off-by: Greg Kurz Signed-off-by: Bruce Rogers --- hw/9pfs/9p.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c index 94df440fc74004bfa45b3fe30540..6026b51a1c04ee82d6366cb13d50 100644 --- a/hw/9pfs/9p.c +++ b/hw/9pfs/9p.c @@ -502,9 +502,9 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path) { int err; V9fsState *s = pdu->s; - V9fsFidState *fidp, head_fid; + V9fsFidState *fidp; - head_fid.next = s->fid_list; +again: for (fidp = s->fid_list; fidp; fidp = fidp->next) { if (fidp->path.size != path->size) { continue; @@ -524,7 +524,7 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path) * switched to the worker thread */ if (err == 0) { - fidp = &head_fid; + goto again; } } }