linux-kernel - Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE

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Date:   Fri, 22 Apr 2022 17:10:27 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "npiggin@...il.com" <npiggin@...il.com>
CC:     "songliubraving@...com" <songliubraving@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "hch@...radead.org" <hch@...radead.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "Kernel-team@...com" <Kernel-team@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "song@...nel.org" <song@...nel.org>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

On Fri, 2022-04-22 at 14:31 +1000, Nicholas Piggin wrote:
> Any other concerns with the fix?

I had another concern with the vmalloc huge pages. Possibly getting
into paranoid territory here, but it's around the new behavior of
freeing vmalloc page tables. At least on x86 this doesn't happen very
often. AFAICT it all happens during boot or during memory hot unplug.

The risk this entails is racing against all the software page table
walks and walking a freed table. At least on x86 the walks of the
kernel tables are done with no locks, which works because the PTE
updates are atomic and pretty much never freed. Some of the kernel page
table walks in the fault handler are actually triggerable from
userspace.

So it's in the category of "already there is a bug" or a little trouble
someone could cause with a lot of effort. At least until PKS vmalloc
memory shows up. Anyway, as long as we are going over this all again I
thought it was worth bringing up.