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    SubjectRe: [kernel-hardening] [PATCH v5 03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated
    On Fri, Jul 22, 2016 at 11:21 AM, Andy Lutomirski <luto@amacapital.net> wrote:
    > On Fri, Jul 22, 2016 at 3:21 AM, Ingo Molnar <mingo@kernel.org> wrote:
    >>
    >> * Andy Lutomirski <luto@kernel.org> wrote:
    >>
    >>> On 07/21/2016 09:43 PM, Valdis.Kletnieks@vt.edu wrote:
    >>> >On Mon, 11 Jul 2016 13:53:36 -0700, Andy Lutomirski said:
    >>> >>This avoids pointless races in which another CPU or task might see a
    >>> >>partially populated global pgd entry. These races should normally
    >>> >>be harmless, but, if another CPU propagates the entry via
    >>> >>vmalloc_fault and then populate_pgd fails (due to memory allocation
    >>> >>failure, for example), this prevents a use-after-free of the pgd
    >>> >>entry.
    >>> >>
    >>> >>Signed-off-by: Andy Lutomirski <luto@kernel.org>
    >>> >>---
    >>> >> arch/x86/mm/pageattr.c | 9 ++++++---
    >>> >> 1 file changed, 6 insertions(+), 3 deletions(-)
    >>> >
    >>> >I just bisected a failure to boot down to this patch. On my Dell Latitude
    >>> >laptop, it results in the kernel being loaded and then just basically sitting
    >>> >there dead in the water - as far as I can tell, it dies before the kernel
    >>> >ever gets going far enough to do any console I/O (even with ignore_loglevel).
    >>> >Nothing in /sys/fs/pstore either. I admit not understanding the VM code
    >>> >at all, so I don't have a clue *why* this causes indigestion...
    >>> >
    >>> >CPU is an Intel Core i5-3340M in case that matters....
    >>> >
    >>>
    >>> How much memory do you have and what's your config? My code is obviously
    >>> buggy, but I'm wondering why neither I nor the 0day bot caught this.
    >>>
    >>> The attached patch is compile-tested only. (Even Thunderbird doesn't want
    >>> to send non-flowed text right now, sigh.)
    >>>
    >>> --Andy
    >>
    >>> From 6589ddf69a1369e1ecb95f0af489d90b980e256e Mon Sep 17 00:00:00 2001
    >>> Message-Id: <6589ddf69a1369e1ecb95f0af489d90b980e256e.1469165371.git.luto@kernel.org>
    >>> From: Andy Lutomirski <luto@kernel.org>
    >>> Date: Thu, 21 Jul 2016 22:22:02 -0700
    >>> Subject: [PATCH] x86/mm: Fix populate_pgd()
    >>>
    >>> I make an obvious error in populate_pgd() -- it would fail to correctly
    >>> populate the page tables when it allocated a new pud page.
    >>
    >> JFYI, on allnoconfig it gives:
    >>
    >> arch/x86/mm/pageattr.c:1016:20: error: implicit declaration of function ‘pud_index’ [-Werror=implicit-function-declaration]
    >
    > As it happens, my fix interacts badly with the steaming pile of crap
    > that is Linux's support for <4 page table levels. Can you just revert
    > the offending patch and I'll redo it differently?

    No, don't revert it. The result doesn't work. I'll send something.

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