Re: [PATCH/RFC] VFS: LOOKUP_MOUNTPOINT should used cached info whenever possible.

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-04-17 at 09:13 +1000, NeilBrown wrote:
> When performing a LOOKUP_MOUNTPOINT lookup we don't really want to
> engage with underlying systems at all.  Any mount point MUST be in the
> dcache with a complete direct path from the root to the mountpoint.
> That should be sufficient to find the mountpoint given a path name.
> 
> This becomes an issue when the filesystem changes unexpected, such as
> when a NFS server is changed to reject all access.  It then becomes
> impossible to unmount anything mounted on the filesystem which has
> changed.  We could simply lazy-unmount the changed filesystem and that
> will often be sufficient.  However if the target filesystem needs
> "umount -f" to complete the unmount properly, then the lazy unmount will
> leave it incompletely unmounted.  When "-f" is needed, we really need to
> be able to name the target filesyste
> 
> We NEVER want to revalidate anything.  We already avoid the revalidation
> of the mountpoint itself, be we won't need to revalidate anything on the
> path either as thay might affect the cache, and the cache is what we are
> really looking in.
> 
> Permission checks are a little less clear.  We currently allow any user
> to make the umount syscall and perform the path lookup and only reject
> unprivileged users once the target mount point has been found.  If we
> completely relax permission checks then an unprivileged user could probe
> inaccessible parts of the name space by examining the error returned
> from umount().
> 

That sounds pretty reasonable. Most umounts are done by root in some
fashion anyway.



> So we only relax permission check when the user has CAP_SYS_ADMIN
> (may_mount() succeeds).
> 
> Note that if the path given is not direct and for example uses symlinks
> or "..", then dentries or symlink content may not be cached and a remote
> server could cause problem.  We can only be certain of a safe unmount if
> a direct path is used.
> 

I guess we do still have to allow it to do a lookup due to symlinks. I
think this is still worthwhile though since it'd fix a lot of these
cases.

> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/namei.c | 46 ++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 40 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index edfedfbccaef..f2df1adae7c5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -498,8 +498,8 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
>   *
>   * When checking for MAY_APPEND, MAY_WRITE must also be set in @mask.
>   */
> -int inode_permission(struct mnt_idmap *idmap,
> -		     struct inode *inode, int mask)
> +int inode_permission_mp(struct mnt_idmap *idmap,
> +			struct inode *inode, int mask, bool mp)
>  {
>  	int retval;
>  
> @@ -523,7 +523,14 @@ int inode_permission(struct mnt_idmap *idmap,
>  			return -EACCES;
>  	}
>  
> -	retval = do_inode_permission(idmap, inode, mask);
> +	if (mp)
> +		/* We are looking for a mountpoint and so don't
> +		 * want to interact with the filesystem at all, just
> +		 * the dcache and icache.
> +		 */
> +		retval = generic_permission(idmap, inode, mask);
> +	else
> +		retval = do_inode_permission(idmap, inode, mask);
>  	if (retval)
>  		return retval;
>  
> @@ -533,6 +540,24 @@ int inode_permission(struct mnt_idmap *idmap,
>  
>  	return security_inode_permission(inode, mask);
>  }
> +
> +/**
> + * inode_permission - Check for access rights to a given inode
> + * @idmap:	idmap of the mount the inode was found from
> + * @inode:	Inode to check permission on
> + * @mask:	Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> + *
> + * Check for read/write/execute permissions on an inode.  We use fs[ug]id for
> + * this, letting us set arbitrary permissions for filesystem access without
> + * changing the "normal" UIDs which are used for other things.
> + *
> + * When checking for MAY_APPEND, MAY_WRITE must also be set in @mask.
> + */
> +int inode_permission(struct mnt_idmap *idmap,
> +		     struct inode *inode, int mask)
> +{
> +	return inode_permission_mp(idmap, inode, mask, false);
> +}
>  EXPORT_SYMBOL(inode_permission);
>  
>  /**
> @@ -589,6 +614,7 @@ struct nameidata {
>  #define ND_ROOT_PRESET 1
>  #define ND_ROOT_GRABBED 2
>  #define ND_JUMPED 4
> +#define ND_SYS_ADMIN 8
>  
>  static void __set_nameidata(struct nameidata *p, int dfd, struct filename *name)
>  {
> @@ -853,7 +879,8 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
>  
>  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
>  {
> -	if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE))
> +	if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) &&
> +	    likely(!(flags & LOOKUP_MOUNTPOINT)))
>  		return dentry->d_op->d_revalidate(dentry, flags);
>  	else
>  		return 1;
> @@ -1708,12 +1735,17 @@ static struct dentry *lookup_slow(const struct qstr *name,
>  static inline int may_lookup(struct mnt_idmap *idmap,
>  			     struct nameidata *nd)
>  {
> +	/* If we are looking for a mountpoint and we have the SYS_ADMIN
> +	 * capability, then we will by-pass the filesys for permission checks
> +	 * and just use generic_permission().
> +	 */
> +	bool mp = (nd->flags & LOOKUP_MOUNTPOINT) && (nd->state & ND_SYS_ADMIN);
>  	if (nd->flags & LOOKUP_RCU) {
> -		int err = inode_permission(idmap, nd->inode, MAY_EXEC|MAY_NOT_BLOCK);
> +		int err = inode_permission_mp(idmap, nd->inode, MAY_EXEC|MAY_NOT_BLOCK, mp);
>  		if (err != -ECHILD || !try_to_unlazy(nd))
>  			return err;
>  	}
> -	return inode_permission(idmap, nd->inode, MAY_EXEC);
> +	return inode_permission_mp(idmap, nd->inode, MAY_EXEC, mp);
>  }
>  
>  static int reserve_stack(struct nameidata *nd, struct path *link)
> @@ -2501,6 +2533,8 @@ int filename_lookup(int dfd, struct filename *name, unsigned flags,
>  	if (IS_ERR(name))
>  		return PTR_ERR(name);
>  	set_nameidata(&nd, dfd, name, root);
> +	if ((flags & LOOKUP_MOUNTPOINT) && may_mount())
> +		nd.state = ND_SYS_ADMIN;
>  	retval = path_lookupat(&nd, flags | LOOKUP_RCU, path);
>  	if (unlikely(retval == -ECHILD))
>  		retval = path_lookupat(&nd, flags, path);

This behavior looks right along the lines of what I was thinking.

Just for bisectability, it might be worthwhile to break this up along
conceptual lines: one patch to make it skip d_revalidate, one that
changes the permission checking, etc.

I'll plan to give this a try soon with Dave's reproducer.

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>