linux-kernel - [PATCH] afs: Fix afs_dynroot_readdir() to not use the RCU read lock

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

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@...kaller.appspotmail.com
Reported-by: syzbot+8245611446194a52150d@...kaller.appspotmail.com
Reported-by: syzbot+1aa62e6852a6ad1c7944@...kaller.appspotmail.com
Reported-by: syzbot+54e6c2176ba76c56217e@...kaller.appspotmail.com
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: linux-afs@...ts.infradead.org
cc: linux-fsdevel@...r.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;
 }