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

From: Matthew Wilcox <willy@infradead.org>
To: Kees Cook <keescook@chromium.org>
Cc: William Kucharski <william.kucharski@oracle.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"linux-hardening@vger.kernel.org"
	<linux-hardening@vger.kernel.org>
Subject: Re: [PATCH v3 0/3] Assorted improvements to usercopy
Date: Mon, 13 Dec 2021 21:16:46 +0000	[thread overview]
Message-ID: <Ybe4PoG2qQyn8zCX@casper.infradead.org> (raw)
In-Reply-To: <202112131246.4BBFBA69A@keescook>

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);
                return;