On Thu, 2024-10-17 at 18:24 -0400, Olga Kornievskaia wrote:
> There is a race between laundromat handling of revoked delegations
> and a client sending free_stateid operation. Laundromat thread
> finds that delegation has expired and needs to be revoked so it
> marks the delegation stid revoked and it puts it on a reaper list
> but then it unlock the state lock and the actual delegation revocation
> happens without the lock. Once the stid is marked revoked a racing
> free_stateid processing thread does the following (1) it calls
> list_del_init() which removes it from the reaper list and (2) frees
> the delegation stid structure. The laundromat thread ends up not
> calling the revoke_delegation() function for this particular delegation
> but that means it will no release the lock lease that exists on
> the file.
>
> Now, a new open for this file comes in and ends up finding that
> lease list isn't empty and calls nfsd_breaker_owns_lease() which ends
> up trying to derefence a freed delegation stateid. Leading to the
> followint use-after-free KASAN warning:
>
> kernel: ==================================================================
> kernel: BUG: KASAN: slab-use-after-free in nfsd_breaker_owns_lease+0x140/0x160 [nfsd]
> kernel: Read of size 8 at addr ffff0000e73cd0c8 by task nfsd/6205
> kernel:
> kernel: CPU: 2 UID: 0 PID: 6205 Comm: nfsd Kdump: loaded Not tainted 6.11.0-rc7+ #9
> kernel: Hardware name: Apple Inc. Apple Virtualization Generic Platform, BIOS 2069.0.0.0.0 08/03/2024
> kernel: Call trace:
> kernel: dump_backtrace+0x98/0x120
> kernel: show_stack+0x1c/0x30
> kernel: dump_stack_lvl+0x80/0xe8
> kernel: print_address_description.constprop.0+0x84/0x390
> kernel: print_report+0xa4/0x268
> kernel: kasan_report+0xb4/0xf8
> kernel: __asan_report_load8_noabort+0x1c/0x28
> kernel: nfsd_breaker_owns_lease+0x140/0x160 [nfsd]
> kernel: leases_conflict+0x68/0x370
> kernel: __break_lease+0x204/0xc38
> kernel: nfsd_open_break_lease+0x8c/0xf0 [nfsd]
> kernel: nfsd_file_do_acquire+0xb3c/0x11d0 [nfsd]
> kernel: nfsd_file_acquire_opened+0x84/0x110 [nfsd]
> kernel: nfs4_get_vfs_file+0x634/0x958 [nfsd]
> kernel: nfsd4_process_open2+0xa40/0x1a40 [nfsd]
> kernel: nfsd4_open+0xa08/0xe80 [nfsd]
> kernel: nfsd4_proc_compound+0xb8c/0x2130 [nfsd]
> kernel: nfsd_dispatch+0x22c/0x718 [nfsd]
> kernel: svc_process_common+0x8e8/0x1960 [sunrpc]
> kernel: svc_process+0x3d4/0x7e0 [sunrpc]
> kernel: svc_handle_xprt+0x828/0xe10 [sunrpc]
> kernel: svc_recv+0x2cc/0x6a8 [sunrpc]
> kernel: nfsd+0x270/0x400 [nfsd]
> kernel: kthread+0x288/0x310
> kernel: ret_from_fork+0x10/0x20
>
> This patch proposes a fix that's based on adding 2 new additional
> stid's sc_status values that help coordinate between the laundromat
> and other operations (nfsd4_free_stateid() and nfsd4_delegreturn()).
>
> First to make sure, that once the stid is marked revoked, it is not
> removed by the nfsd4_free_stateid(), the laundromat take a reference
> on the stateid. Then, coordinating whether the stid has been put
> on the cl_revoked list or we are processing FREE_STATEID and need to
> make sure to remove it from the list, each check that state and act
> accordingly. If laundromat has added to the cl_revoke list before
> the arrival of FREE_STATEID, then nfsd4_free_stateid() knows to remove
> it from the list. If nfsd4_free_stateid() finds that operations arrived
> before laundromat has placed it on cl_revoke list, it marks the state
> freed and then laundromat will no longer add it to the list.
>
> Also, for nfsd4_delegreturn() when looking for the specified stid,
> we need to access stid that are marked removed or freeable, it means
> the laundromat has started processing it but hasn't finished and this
> delegreturn needs to return nfserr_deleg_revoked and not
> nfserr_bad_stateid. The latter will not trigger a FREE_STATEID and the
> lack of it will leave this stid on the cl_revoked list indefinitely.
>
> Fixes: 2d4a532d385f ("nfsd: ensure that clp->cl_revoked list is
> protected by clp->cl_lock")
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Olga Kornievskaia <okorniev@xxxxxxxxxx>
> ---
> fs/nfsd/nfs4state.c | 15 ++++++++++++---
> fs/nfsd/state.h | 2 ++
> 2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index ac1859c7cc9d..cb989802e896 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1370,10 +1370,16 @@ static void revoke_delegation(struct nfs4_delegation *dp)
> if (dp->dl_stid.sc_status &
> (SC_STATUS_REVOKED | SC_STATUS_ADMIN_REVOKED)) {
Now that I look, we'll never call revoke_delegation() without one of
these bits set. Maybe we should turn that if statement into a
WARN_ON_ONCE check, and then just do the rest unconditionally.
> spin_lock(&clp->cl_lock);
> - refcount_inc(&dp->dl_stid.sc_count);
If you're going to remove this, then I think you also need to add a
recount_inc() to nfs4_revoke_states() before the call to
revoke_delegation().
> + if (dp->dl_stid.sc_status & SC_STATUS_FREED) {
> + list_del_init(&dp->dl_recall_lru);
> + spin_unlock(&clp->cl_lock);
> + goto out;
> + }
The FREEABLE/FREED dance is pretty complex. It'd be nice to have some
documentation around it. Maybe consider adding a kerneldoc header over
this function that explains that, and the requirement that you need to
take an extra reference to the stateid before calling this.
> list_add(&dp->dl_recall_lru, &clp->cl_revoked);
> + dp->dl_stid.sc_status |= SC_STATUS_FREEABLE;
> spin_unlock(&clp->cl_lock);
> }
> +out:
I'd just move the spin_unlock to here and get rid of the two above.
> destroy_unhashed_deleg(dp);
> }
>
> @@ -6545,6 +6551,7 @@ nfs4_laundromat(struct nfsd_net *nn)
> dp = list_entry (pos, struct nfs4_delegation, dl_recall_lru);
> if (!state_expired(<, dp->dl_time))
> break;
> + refcount_inc(&dp->dl_stid.sc_count);
> unhash_delegation_locked(dp, SC_STATUS_REVOKED);
> list_add(&dp->dl_recall_lru, &reaplist);
> }
> @@ -7156,7 +7163,9 @@ nfsd4_free_stateid(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> if (s->sc_status & SC_STATUS_REVOKED) {
> spin_unlock(&s->sc_lock);
> dp = delegstateid(s);
> - list_del_init(&dp->dl_recall_lru);
> + if (s->sc_status & SC_STATUS_FREEABLE)
> + list_del_init(&dp->dl_recall_lru);
> + s->sc_status |= SC_STATUS_FREED;
> spin_unlock(&cl->cl_lock);
> nfs4_put_stid(s);
> ret = nfs_ok;
> @@ -7486,7 +7495,7 @@ nfsd4_delegreturn(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> if ((status = fh_verify(rqstp, &cstate->current_fh, S_IFREG, 0)))
> return status;
>
> - status = nfsd4_lookup_stateid(cstate, stateid, SC_TYPE_DELEG, 0, &s, nn);
> + status = nfsd4_lookup_stateid(cstate, stateid, SC_TYPE_DELEG, SC_STATUS_REVOKED|SC_STATUS_FREEABLE, &s, nn);
> if (status)
> goto out;
> dp = delegstateid(s);
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 79c743c01a47..35b3564c065f 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -114,6 +114,8 @@ struct nfs4_stid {
> /* For a deleg stateid kept around only to process free_stateid's: */
> #define SC_STATUS_REVOKED BIT(1)
> #define SC_STATUS_ADMIN_REVOKED BIT(2)
> +#define SC_STATUS_FREEABLE BIT(3)
> +#define SC_STATUS_FREED BIT(4)
> unsigned short sc_status;
>
> struct list_head sc_cp_list;
Other than the problems above, this looks reasonable. Nice work!
--
Jeff Layton <jlayton@xxxxxxxxxx>
