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Date:   Sun, 12 Apr 2020 17:41:50 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Naresh Kamboju <naresh.kamboju@linaro.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        lkft-triage@lists.linaro.org, Shuah Khan <shuah@kernel.org>,
        Anders Roxell <anders.roxell@linaro.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Andrew Morton <akpm@linux-foundation.org>,
        Leonardo Bras <leonardo@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Jules Irenge <jbi.octave@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
        Rik van Riel <riel@surriel.com>,
        Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>, Mel Gorman <mgorman@suse.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Mateusz Nosek <mateusznosek0@gmail.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Kees Cook <keescook@chromium.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Hillf Danton <hdanton@sina.com>
Subject: Re: i386: selftests: vm: compaction_test: BUG: kernel NULL pointer
 dereference, address: 00000000
Message-ID: <20200412214150.GB38470@xz-x1>
References: <CA+G9fYsRGvkqtpdGv_aVr+Hn17KgYq04Q=EE=pB774qVxRqOeg@mail.gmail.com>
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Hi, Naresh,

On Sun, Apr 12, 2020 at 12:24:57PM +0530, Naresh Kamboju wrote:
> On Linux mainline kernel 5.6.0 running kselftest vm compaction_test on i386
>  kernel running on x86_64 devices we have noticed this Kernel BUG followed by
> Kernel panic.
> 
> BUG: kernel NULL pointer dereference, address: 00000000
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
> 
> Steps to reproduce:
> --------------------------
> # cd /opt/kselftests/default-in-kernel/vm/
> # ./compaction_test
> 
> [   45.955706] kauditd_printk_skb: 15 callbacks suppressed
> [   45.955707] audit: type=1334 audit(1586671685.974:25): prog-id=13 op=UNLOAD
> [   45.968082] audit: type=1334 audit(1586671685.974:26): prog-id=12 op=UNLOAD
> [   46.008163] get_swap_device: Bad swap file entry 7007504a
> [   46.013647] BUG: kernel NULL pointer dereference, address: 00000000

I suspect this is the same issue with:

https://lore.kernel.org/lkml/20200410002518.GG8179@shao2-debian/

And since this one is very easy to reproduce, I finally noticed that
we have wrongly enabled uffd-wp on x86_32, which is definely not going
to work... Because we'll use bit 2 of swap entry assuming that's the
uffd-wp bit, while that's part of swp offset on 32bit systems.

Naresh, could you try whether below change fixes the issue for you?

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 8d078642b4be..7ac524d1316e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -149,7 +149,7 @@ config X86
        select HAVE_ARCH_TRACEHOOK
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
-       select HAVE_ARCH_USERFAULTFD_WP         if USERFAULTFD
+       select HAVE_ARCH_USERFAULTFD_WP         if X86_64 && USERFAULTFD
        select HAVE_ARCH_VMAP_STACK             if X86_64
        select HAVE_ARCH_WITHIN_STACK_FRAMES
        select HAVE_ASM_MODVERSIONS

Thanks,

-- 
Peter Xu