CVE-2026-80527

In the Linux kernel, the following vulnerability has been resolved:

ceph: fix hanging _cephgetcaps() with stale mdswanted

A reader can hang forever in __cephgetcaps() when the client no longer holds FILE_RD, but local cap state still says that the capability is already wanted (via mds_wanted).

One way to trigger this is through MDS cap revocation. If another client performs a conflicting operation, the MDS can revoke FILE_RD from the reader; the next read then has to reacquire FILE_RD. If the cap update that should request FILE_RD never reaches the MDS after cap->mds_wanted was raised, the reader is left holding only non-file caps while local mds_wanted still includes the file read caps.

In that state, trygetcap_refs() sees need <= mds_wanted and returns 0, so _cephgetcaps() just waits on i_cap_wq. If the cap update that was supposed to request FILE_RD never reaches the MDS aftercap->mdswanted was` raised, no further request is sent and the waiter can sleep indefinitely until unrelated cap traffic happens to wake it up.

The ordering issue is that cap->mds_wanted is updated in _prepcap() before the CEPH_MSG_CLIENT_CAPS message is actually queued for send. That makes one field serve two different meanings at once: what this client wants, and what the client believes the MDS already knows it wants.

A proper fix would be to split those states and track whether a cap update is actually in flight or has been observed by the MDS. However, simply moving the cap->mds_wanted assignment later would not be sufficient: queueing the message in the messenger does not guarantee that the MDS processed that specific wanted set, and reconnect or message loss can still invalidate that assumption. Fixing that properly would require a larger rework of the cap state machine.

To allow simpler backports to stable kernels, this patch implements a simpler workaround:

• stop waiting forever in __cephgetcaps(); after a bounded wait, fall back to the renew path

• make cephrenewcaps() issue a synchronous OPEN request whenever the inode still does not actually hold the wanted caps, instead of only calling cephcheckcaps()

The extra issued-vs-wanted check in cephrenewcaps() is necessary because the previous test only checked whether the inode still had any real caps at all. That is not enough after revocation: the client can still hold something like pLs and yet be missing FILE_RD completely. In that case, falling back to cephcheckcaps() is not sufficient, because it still trusts cap->mds_wanted and may resend nothing. By requiring (issued & wanted) == wanted before taking the asynchronous path, the code only uses cephcheckcaps() when the wanted caps are already actually issued. Otherwise, it sends the synchronous OPEN renew.

This preserves the existing asynchronous fast path when the wanted caps are already issued, avoids changing cap-state semantics, and fixes the hang by guaranteeing that a stalled waiter eventually retries through a path that does not rely on the stale mds_wanted state.

[ idryomov: move CEPHGETCAPSWAITTIMEOUT from libceph.h to mds_client.h, formatting ]