Re: 2.6.18-rc4-mm1

From: Ian Kent
Date: Wed Aug 16 2006 - 08:55:54 EST

On Wed, 2006-08-16 at 13:23 +0100, David Howells wrote:

> And saw the following appear in the kernel log around the problem bit for
> trash:/usr:
> | ...
> | SELinux: initialized (dev 0:18, type nfs), uses genfs_contexts
> | audit(1155729189.533:468): avc: denied { read } for pid=6472 comm="automount" name="" dev=hda2 ino=688243 scontext=root:system_r:automount_t:s0 tcontext=system_u:object_r:var_yp_t:s0 tclass=file
> | audit(1155729189.557:469): avc: denied { name_bind } for pid=6472 comm="automount" src=716 scontext=root:system_r:automount_t:s0 tcontext=system_u:object_r:reserved_port_t:s0 tclass=udp_socket
> Not sure what's going on here. The automounter tried to do bind a socket to a
> reserved port perhaps and was denied.

Possibly an RPC ping.
That's about the only thing I do that does net connects.

> | NFS: nfs_update_inode(0:18/2 ct=1 info=0x6)
> | NFS: permission(0:18/2), mask=0x1, res=0
> sys_mkdirat() calls do_path_lookup(), which checks MAY_EXEC on the dir.
> | NFS: permission(0:18/2), mask=0x1, res=0
> lookup_create() is called. This calls __lookup_hash(), which checks MAY_EXEC
> on the dir.
> | -->nfs_lookup(,usr,{200,80,44e3069a})
> __lookup_hash() then looks up the new dentry with intent to create:
> =============================== ===============================
> nd->flags LOOKUP_CREATE
> nd-> O_EXCL
> nd-> weird value, even in octal

I'm fairly sure there's a race in autofs for the create case. I've tried
to work a solution in the past but haven't been successful yet. In any
case autofs should not allow anyone else besides the daemon to do
anything in the autofs fs. It's been a while but I think this case leads
to a deadlock.

> This means that nfs_lookup() considers this to be "an exclusive create" of
> this node, and dispenses with the LOOKUP RPC call to the server.
> | NFS: lookup(/usr)
> | exlusive_create
> Just to confirm that the lookup is skipped.
> | <--nfs_lookup() = 00000000
> We return the dentry we were given, but don't return an error. The dentry we
> were given is left negative (on the assumption it's about to be created), but
> does get attached to the directory.
> | NFS: permission(0:18/2), mask=0x3, res=0
> vfs_mkdir() calls may_create() which checks that the directory has MAY_WRITE
> and MAY_EXEC permissions. This firstly calls nfs_permission, which grants
> permission.
> | audit(1155729189.605:470): avc: denied { write } for pid=6472 comm="automount" name="" dev=0:18 ino=2 scontext=root:system_r:automount_t:s0 tcontext=system_u:object_r:nfs_t:s0 tclass=dir
> And secondly calls security_inode_permission() though which SELinux which
> _denies_ permission.
> | NFS: dentry_delete(/usr, 0)
> vfs_mkdir() returns -ENOACCES to sys_mkdirat() which releases its hold on the
> dentry, but leaves the negative dentry attached to the directory.
> The negative dentry wouldn't normally be a problem, even though it's attached
> to its parent directory... except for the small matter that it's subsequently
> listed in a directory read operation.

Surely this dentry should also be unhashed at some point.
Wouldn't that be a sensible result of the failed operation?
It wouldn't then show up in a listing and the fs should normally be able
to deal with these.


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