Re: [syzbot] [afs?] BUG: sleeping function called from invalid context in __alloc_frozen_pages_noprof

[David Howells]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

commit db7a516159869b19f237c73bd75599bbe0bfcc4d
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Fri Mar 28 16:46:58 2025 +0000

    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.
    
    Fixes: 1d0b929fc070 ("afs: Change dynroot to create contents on demand")
    Reported-by: syzbot+3b6c5c6a1d0119b687a1@xxxxxxxxxxxxxxxxxxxxxxxxx
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
    cc: linux-afs@xxxxxxxxxxxxxxxxxxx
    cc: linux-fsdevel@xxxxxxxxxxxxxxx

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;
 }