[Bug 202149] New: NULL Pointer Dereference in __split_huge_pmd on PPC64LE

[Matt Corallo]

Hey, sorry for the delay on this. I had some apparently-unrelated hangs that I believe were due to mpt3sas instability, and at the risk of speaking too soon for a bug I couldn't reliably reproduce, this patch appears to have resolved it, thanks!

> On Jan 21, 2019, at 07:35, Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com> wrote:
> 
> 
> Can you test this patch?
> 
> From e511e79af9a314854848ea8fda9dfa6d7e07c5e4 Mon Sep 17 00:00:00 2001
> From: "Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com>
> Date: Mon, 21 Jan 2019 16:43:17 +0530
> Subject: [PATCH] arch/powerpc/radix: Fix kernel crash with mremap
> 
> With support for split pmd lock, we use pmd page pmd_huge_pte pointer to store
> the deposited page table. In those config when we move page tables we need to
> make sure we move the depoisted page table to the right pmd page. Otherwise this
> can result in crash when we withdraw of deposited page table because we can find
> the pmd_huge_pte NULL.
> 
> c0000000004a1230 __split_huge_pmd+0x1070/0x1940
> c0000000004a0ff4 __split_huge_pmd+0xe34/0x1940 (unreliable)
> c0000000004a4000 vma_adjust_trans_huge+0x110/0x1c0
> c00000000042fe04 __vma_adjust+0x2b4/0x9b0
> c0000000004316e8 __split_vma+0x1b8/0x280
> c00000000043192c __do_munmap+0x13c/0x550
> c000000000439390 sys_mremap+0x220/0x7e0
> c00000000000b488 system_call+0x5c/0x70
> 
> Fixes: 675d995297d4 ("powerpc/book3s64: Enable split pmd ptlock.")
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> ---
> arch/powerpc/include/asm/book3s/64/pgtable.h | 2 --
> 1 file changed, 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 92eaea164700..86e62384256d 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -1262,8 +1262,6 @@ static inline int pmd_move_must_withdraw(struct spinlock *new_pmd_ptl,
>                     struct spinlock *old_pmd_ptl,
>                     struct vm_area_struct *vma)
> {
> -    if (radix_enabled())
> -        return false;
>    /*
>     * Archs like ppc64 use pgtable to store per pmd
>     * specific information. So when we switch the pmd,
> -- 
> 2.20.1
>