netdev - Re: [PATCH v2] vmalloc: Fix issues with flush flag

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Date:   Tue, 21 May 2019 01:59:54 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "davem@...emloft.net" <davem@...emloft.net>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mroos@...ux.ee" <mroos@...ux.ee>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "namit@...are.com" <namit@...are.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v2] vmalloc: Fix issues with flush flag

On Mon, 2019-05-20 at 18:43 -0700, David Miller wrote:
> From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
> Date: Tue, 21 May 2019 01:20:33 +0000
> 
> > Should it handle executing an unmapped page gracefully? Because
> > this
> > change is causing that to happen much earlier. If something was
> > relying
> > on a cached translation to execute something it could find the
> > mapping
> > disappear.
> 
> Does this work by not mapping any kernel mappings at the beginning,
> and then filling in the BPF mappings in response to faults?
No, nothing too fancy. It just flushes the vm mapping immediatly in
vfree for execute (and RO) mappings. The only thing that happens around
allocation time is setting of a new flag to tell vmalloc to do the
flush.

The problem before was that the pages would be freed before the execute
mapping was flushed. So then when the pages got recycled, random,
sometimes coming from userspace, data would be mapped as executable in
the kernel by the un-flushed tlb entries.