From: David Howells <dhowells@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: dhowells@redhat.com,
	syzbot+3b6c5c6a1d0119b687a1@syzkaller.appspotmail.com,
	syzbot+8245611446194a52150d@syzkaller.appspotmail.com,
	syzbot+1aa62e6852a6ad1c7944@syzkaller.appspotmail.com,
	syzbot+54e6c2176ba76c56217e@syzkaller.appspotmail.com,
	Marc Dionne <marc.dionne@auristor.com>,
	linux-afs@lists.infradead.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH] afs: Fix afs_dynroot_readdir() to not use the RCU read lock
Date: Tue, 08 Apr 2025 21:46:29 +0100	[thread overview]
Message-ID: <1638014.1744145189@warthog.procyon.org.uk> (raw)

afs_dynroot_readdir() uses the RCU read lock to walk the cell list whilst
emitting cell automount entries - but dir_emit() may write to a userspace
buffer, thereby causing a fault to occur and waits to happen.

Fix afs_dynroot_readdir() to get a shared lock on net->cells_lock instead.

This can be triggered by enabling lockdep, preconfiguring a number of
cells, doing "mount -t afs none /afs -o dyn" (or using the kafs-client
package with afs.mount systemd unit enabled) and then doing "ls /afs".

Fixes: 1d0b929fc070 ("afs: Change dynroot to create contents on demand")
Reported-by: syzbot+3b6c5c6a1d0119b687a1@syzkaller.appspotmail.com
Reported-by: syzbot+8245611446194a52150d@syzkaller.appspotmail.com
Reported-by: syzbot+1aa62e6852a6ad1c7944@syzkaller.appspotmail.com
Reported-by: syzbot+54e6c2176ba76c56217e@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
---
 fs/afs/dynroot.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 691e0ae607a1..8c6130789fde 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -348,9 +348,9 @@ static int afs_dynroot_readdir(struct file *file, struct dir_context *ctx)
 	}
 
 	if ((unsigned long long)ctx->pos <= AFS_MAX_DYNROOT_CELL_INO) {
-		rcu_read_lock();
+		down_read(&net->cells_lock);
 		ret = afs_dynroot_readdir_cells(net, ctx);
-		rcu_read_unlock();
+		up_read(&net->cells_lock);
 	}
 	return ret;
 }

next             reply	other threads:[~2025-04-08 20:46 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-04-08 20:46 David Howells [this message]
2025-04-09 10:13 ` [PATCH] afs: Fix afs_dynroot_readdir() to not use the RCU read lock Christian Brauner

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