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Registered in England and Wales under Company Registration No. 3798903 To: Christian Brauner 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 , 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 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-ID: <1637993.1744145165.1@warthog.procyon.org.uk> From: David Howells Date: Tue, 08 Apr 2025 21:46:29 +0100 Message-ID: <1638014.1744145189@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 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 cc: Marc Dionne 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; }