linux-hardening - Re: [PATCH v3 0/3] Assorted improvements to usercopy

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Date:   Mon, 13 Dec 2021 15:47:33 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     William Kucharski <william.kucharski@...cle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v3 0/3] Assorted improvements to usercopy

On Mon, Dec 13, 2021 at 09:16:46PM +0000, Matthew Wilcox wrote:
> On Mon, Dec 13, 2021 at 12:47:58PM -0800, Kees Cook wrote:
> > On Mon, Dec 13, 2021 at 08:27:42PM +0000, Matthew Wilcox wrote:
> > > On Mon, Dec 13, 2021 at 07:18:57PM +0000, William Kucharski wrote:
> > > > I like these, but a quick question:
> > > > 
> > > > Since the usercopy_abort() calls are all because the offset exceeds the page
> > > > size, is there a reason why you don't specifically state that via the detail
> > > > parameter rather than just supply a NULL pointer?
> > > 
> > > Hmm ... I'd defer to Kees on this, because I'm not familiar with
> > > usercopy_abort() usage, but the only places which use the detail
> > > parameter today are slab/slub, which use it to pass the name of
> > > the slab.  I think the user is supposed to infer that we overran the
> > > end of the page based on the offset & length values.
> > 
> > I agree that leaving it NULL is best here. The "detail" is really about
> > adding more information about which thing it was, which for slab makes
> > sense, but most other stuff there isn't really anything to quickly
> > distinguish one from another (i.e. vmap is all vmap).
> 
> There _is_ a bit more information in the vmap case (not in the kmap
> or compound page case).  You can see it in /proc/vmallocinfo.  We
> could pass it in like this?
> 
>         if (is_vmalloc_addr(ptr)) {
>                 struct vm_struct *vm = find_vm_area(ptr);
> +               char sym[100];
>                 unsigned long offset;
> 
>                 if (!vm) {
> ..
> +               if (vm->caller)
> +                       snprintf(sym, sizeof(sym), "%pS", vm->caller);
>                 offset = ptr - vm->addr;
>                 if (offset + n > vm->size)
> -                       usercopy_abort("vmalloc", NULL, to_user, offset, n);
> +                       usercopy_abort("vmalloc", vm->caller ? sym : NULL,
> +                                       to_user, offset, n);

That is interesting, but I think we don't want to do it here; adding
to stack or making an allocation for this (even though it's slow-path)
doesn't seem like a good idea as far as keeping code size down.

-Kees

>                 return;
> 

-- 
Kees Cook