Re: [PATCH 1/1] nfsd: fix race between laundromat and free_stateid

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2024-10-10 at 18:18 -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
> 
> Proposing to have laundromat thread hold the state_lock over both
> marking thru revoking the delegation as well as making free_stateid
> acquire state_lock before accessing the list. Making sure that
> revoke_delegation() (ie kernel_setlease(unlock)) is called for
> every delegation that was revoked and added to the reaper list.
> 

Nice detective work!

> CC: stable@vger.kernel.org
> Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
> 
> --- I can't figure out the Fixes: tag. Laundromat's behaviour has
> been like that forever. But the free_stateid bits wont apply before
> the 1e3577a4521e ("SUNRPC: discard sv_refcnt, and svc_get/svc_put").
> But we used that fixes tag already with a previous fix for a different
> problem.
> ---
>  fs/nfsd/nfs4state.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 9c2b1d251ab3..c97907d7fb38 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -6605,13 +6605,13 @@ nfs4_laundromat(struct nfsd_net *nn)
>  		unhash_delegation_locked(dp, SC_STATUS_REVOKED);
>  		list_add(&dp->dl_recall_lru, &reaplist);
>  	}
> -	spin_unlock(&state_lock);
>  	while (!list_empty(&reaplist)) {
>  		dp = list_first_entry(&reaplist, struct nfs4_delegation,
>  					dl_recall_lru);
>  		list_del_init(&dp->dl_recall_lru);
>  		revoke_delegation(dp);
>  	}
> +	spin_unlock(&state_lock);
>  
>  	spin_lock(&nn->client_lock);
>  	while (!list_empty(&nn->close_lru)) {
> @@ -7213,7 +7213,9 @@ 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);
> +			spin_lock(&state_lock);
>  			list_del_init(&dp->dl_recall_lru);
> +			spin_unlock(&state_lock);
>  			spin_unlock(&cl->cl_lock);
>  			nfs4_put_stid(s);
>  			ret = nfs_ok;


I'm not thrilled with this patch, but it does seem like it would fix
the problem. Long term, I think we need to get rid of the state_lock,
but I don't have a real plan for that just yet as it's not 100% clear
what it still protects.

As far as a Fixes tag, this bug is likely very old. I'd say it probably
got introduced here:

    2d4a532d385f nfsd: ensure that clp->cl_revoked list is protected by clp->cl_lock

In any case, let's go with this patch until we can come up with a
better plan for delegation handling.

Reviewed-by: Jeff Layton <jlayton@kernel.org>