[PATCH 0/2] nfsd: proper fix for NULL deref in svc_pool_stats_start()

[PATCH 0/2] nfsd: proper fix for NULL deref in svc_pool_stats_start()

[NeilBrown]

A recent patch attempted to fix a NULL pointer deref but introduced a
different NULL pointer deref and a possible unbalance unlock.

This series fixes the bug correctly and reverts the faulty fix.

(Sorry for not reviewing this patch earlier)

NeilBrown

 [PATCH 1/2] nfsd: initialise nfsd_info.mutex early.
 [PATCH 2/2] Revert "nfsd: fix oops when reading pool_stats before

[PATCH 1/2] nfsd: initialise nfsd_info.mutex early.

[NeilBrown]

nfsd_info.mutex can be dereferenced by dsvc_pool_stats_state()
immediately after the the new netns is created.  Currently this can
trigger an oops.

Move the initialisation earlier before it can possibly be dereferenced.

Fixes: 7b207ccd9833 ("svc: don't hold reference for poolstats, only mutex.")
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Closes: https://lore.kernel.org/all/c2e9f6de-1ec4-4d3a-b18d-d5a6ec0814a0@linux.ibm.com/
Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfsd/nfsctl.c | 2 ++
 fs/nfsd/nfssvc.c | 1 -
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 533b65057e18..c848ebe5d08f 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -2169,6 +2169,8 @@ static __net_init int nfsd_net_init(struct net *net)
 	nn->nfsd_svcstats.program = &nfsd_program;
 	nn->nfsd_versions = NULL;
 	nn->nfsd4_minorversions = NULL;
+	nn->nfsd_info.mutex = &nfsd_mutex;
+	nn->nfsd_serv = NULL;
 	nfsd4_init_leases_net(nn);
 	get_random_bytes(&nn->siphash_key, sizeof(nn->siphash_key));
 	seqlock_init(&nn->writeverf_lock);
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index cd9a6a1a9fc8..89d7918de7b1 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -672,7 +672,6 @@ int nfsd_create_serv(struct net *net)
 		return error;
 	}
 	spin_lock(&nfsd_notifier_lock);
-	nn->nfsd_info.mutex = &nfsd_mutex;
 	nn->nfsd_serv = serv;
 	spin_unlock(&nfsd_notifier_lock);
 
-- 
2.44.0

[PATCH 2/2] Revert "nfsd: fix oops when reading pool_stats before server is started"

[NeilBrown]

This reverts commit 8e948c365d9c10b685d1deb946bd833d6a9b43e0.

The reverted commit moves a test on a field protected by a mutex outside
of the protection of that mutex, and so is obviously racey.

Depending on how the race goes, si->serv might be NULL when dereferenced
in svc_pool_stats_start(), or svc_pool_stats_stop() might unlock a mutex
that hadn't been locked.

This bug that the commit tried to fix has been addressed by initialising
->mutex earlier.

Fixes: 8e948c365d9c ("nfsd: fix oops when reading pool_stats before server is started")
Signed-off-by: NeilBrown <neilb@suse.de>
---
 net/sunrpc/svc_xprt.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 49a3bea33f9d..dd86d7f1e97e 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -1421,13 +1421,12 @@ static void *svc_pool_stats_start(struct seq_file *m, loff_t *pos)
 
 	dprintk("svc_pool_stats_start, *pidx=%u\n", pidx);
 
-	if (!si->serv)
-		return NULL;
-
 	mutex_lock(si->mutex);
 
 	if (!pidx)
 		return SEQ_START_TOKEN;
+	if (!si->serv)
+		return NULL;
 	return pidx > si->serv->sv_nrpools ? NULL
 		: &si->serv->sv_pools[pidx - 1];
 }
@@ -1459,8 +1458,7 @@ static void svc_pool_stats_stop(struct seq_file *m, void *p)
 {
 	struct svc_info *si = m->private;
 
-	if (si->serv)
-		mutex_unlock(si->mutex);
+	mutex_unlock(si->mutex);
 }
 
 static int svc_pool_stats_show(struct seq_file *m, void *p)
-- 
2.44.0

Re: [PATCH 0/2] nfsd: proper fix for NULL deref in svc_pool_stats_start()

[Jeff Layton]

On Tue, 2024-06-25 at 09:04 +1000, NeilBrown wrote:
> A recent patch attempted to fix a NULL pointer deref but introduced a
> different NULL pointer deref and a possible unbalance unlock.
> 
> This series fixes the bug correctly and reverts the faulty fix.
> 
> (Sorry for not reviewing this patch earlier)
> 
> NeilBrown
> 
>  [PATCH 1/2] nfsd: initialise nfsd_info.mutex early.
>  [PATCH 2/2] Revert "nfsd: fix oops when reading pool_stats before

Yes, this looks like a superior fix. Thanks, Neil!

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

Re: [PATCH 0/2] nfsd: proper fix for NULL deref in svc_pool_stats_start()

[Chuck Lever]

On Tue, Jun 25, 2024 at 09:04:55AM +1000, NeilBrown wrote:
> A recent patch attempted to fix a NULL pointer deref but introduced a
> different NULL pointer deref and a possible unbalance unlock.
> 
> This series fixes the bug correctly and reverts the faulty fix.
> 
> (Sorry for not reviewing this patch earlier)
> 
> NeilBrown
> 
>  [PATCH 1/2] nfsd: initialise nfsd_info.mutex early.
>  [PATCH 2/2] Revert "nfsd: fix oops when reading pool_stats before

Applied to nfsd-fixes (for 6.10-rc). Thanks!

-- 
Chuck Lever