linux-kernel - Re: [PATCH] powerpc/mm: fix mmap

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Date:   Mon, 6 Mar 2023 15:07:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, Sachin Sant <sachinp@...ux.ibm.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] powerpc/mm: fix mmap_lock bad unlock

On 06.03.23 14:55, Laurent Dufour wrote:
> When page fault is tried holding the per VMA lock, bad_access_pkey() and
> bad_access() should not be called because it is assuming the mmap_lock is
> held.
> In the case a bad access is detected, fall back to the default path,
> grabbing the mmap_lock to handle the fault and report the error.
> 
> Fixes: 169db3bb4609 ("powerc/mm: try VMA lock-based page fault handling first")
> Reported-by: Sachin Sant <sachinp@...ux.ibm.com>
> Link: https://lore.kernel.org/linux-mm/842502FB-F99C-417C-9648-A37D0ECDC9CE@linux.ibm.com
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
> ---
>   arch/powerpc/mm/fault.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index c7ae86b04b8a..e191b3ebd8d6 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -479,17 +479,13 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>   
>   	if (unlikely(access_pkey_error(is_write, is_exec,
>   				       (error_code & DSISR_KEYFAULT), vma))) {
> -		int rc = bad_access_pkey(regs, address, vma);
> -
>   		vma_end_read(vma);
> -		return rc;
> +		goto lock_mmap;
>   	}
>   
>   	if (unlikely(access_error(is_write, is_exec, vma))) {
> -		int rc = bad_access(regs, address);
> -
>   		vma_end_read(vma);
> -		return rc;
> +		goto lock_mmap;
>   	}
>   
>   	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);

IIUC, that commit is neither upstream not in mm-stable -- it's unstable. 
Maybe raise that as a review comment in reply to the original patch, so 
we can easily connect the dots and squash it into the original, 
problematic patch that is still under review.

-- 
Thanks,

David / dhildenb