Re: [PATCH] fs: improve dump_mapping() robustness

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 16, 2024 at 03:53:35PM +0800, Baolin Wang wrote:

> With checking the 'dentry.parent' and 'dentry.d_name.name' used by
> dentry_name(), I can see dump_mapping() will output the invalid dentry
> instead of crashing the system when this issue is reproduced again.

>  	dentry_ptr = container_of(dentry_first, struct dentry, d_u.d_alias);
> -	if (get_kernel_nofault(dentry, dentry_ptr)) {
> +	if (get_kernel_nofault(dentry, dentry_ptr) ||
> +	    !dentry.d_parent || !dentry.d_name.name) {
>  		pr_warn("aops:%ps ino:%lx invalid dentry:%px\n",
>  				a_ops, ino, dentry_ptr);
>  		return;

That's nowhere near enough.  Your ->d_name.name can bloody well be pointing
to an external name that gets freed right under you.  Legitimately so.

Think what happens if dentry has a long name (longer than would fit into
the embedded array) and gets renamed name just after you copy it into
a local variable.  Old name will get freed.  Yes, freeing is RCU-delayed,
but I don't see anything that would prevent your thread losing CPU
and not getting it back until after the sucker's been freed.