Re: [PATCH] mm: check VMA flags to avoid invalid PROT_NONE NUMA balancing

[Lorenzo Stoakes]

Just a quick ping on this, let me know if you need anything more from me!

Thanks, Lorenzo

On Sun, Sep 11, 2016 at 11:54:25PM +0100, Lorenzo Stoakes wrote:
> The NUMA balancing logic uses an arch-specific PROT_NONE page table flag defined
> by pte_protnone() or pmd_protnone() to mark PTEs or huge page PMDs respectively
> as requiring balancing upon a subsequent page fault. User-defined PROT_NONE
> memory regions which also have this flag set will not normally invoke the NUMA
> balancing code as do_page_fault() will send a segfault to the process before
> handle_mm_fault() is even called.
>
> However if access_remote_vm() is invoked to access a PROT_NONE region of memory,
> handle_mm_fault() is called via faultin_page() and __get_user_pages() without
> any access checks being performed, meaning the NUMA balancing logic is
> incorrectly invoked on a non-NUMA memory region.
>
> A simple means of triggering this problem is to access PROT_NONE mmap'd memory
> using /proc/self/mem which reliably results in the NUMA handling functions being
> invoked when CONFIG_NUMA_BALANCING is set.
>
> This issue was reported in bugzilla (issue 99101) which includes some simple
> repro code.
>
> There are BUG_ON() checks in do_numa_page() and do_huge_pmd_numa_page() added at
> commit c0e7cad to avoid accidentally provoking strange behaviour by attempting
> to apply NUMA balancing to pages that are in fact PROT_NONE. The BUG_ON()'s are
> consistently triggered by the repro.
>
> This patch moves the PROT_NONE check into mm/memory.c rather than invoking
> BUG_ON() as faulting in these pages via faultin_page() is a valid reason for
> reaching the NUMA check with the PROT_NONE page table flag set and is therefore
> not always a bug.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=99101
> Reported-by: Trevor Saunders <tbsaunde@xxxxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
>  mm/huge_memory.c |  3 ---
>  mm/memory.c      | 12 +++++++-----
>  2 files changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d76700d..954be55 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1198,9 +1198,6 @@ int do_huge_pmd_numa_page(struct fault_env *fe, pmd_t pmd)
>  	bool was_writable;
>  	int flags = 0;
>
> -	/* A PROT_NONE fault should not end up here */
> -	BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)));
> -
>  	fe->ptl = pmd_lock(vma->vm_mm, fe->pmd);
>  	if (unlikely(!pmd_same(pmd, *fe->pmd)))
>  		goto out_unlock;
> diff --git a/mm/memory.c b/mm/memory.c
> index 020226b..aebc04f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3351,9 +3351,6 @@ static int do_numa_page(struct fault_env *fe, pte_t pte)
>  	bool was_writable = pte_write(pte);
>  	int flags = 0;
>
> -	/* A PROT_NONE fault should not end up here */
> -	BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)));
> -
>  	/*
>  	* The "pte" at this point cannot be used safely without
>  	* validation through pte_unmap_same(). It's of NUMA type but
> @@ -3458,6 +3455,11 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
>  	return VM_FAULT_FALLBACK;
>  }
>
> +static inline bool vma_is_accessible(struct vm_area_struct *vma)
> +{
> +	return vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE);
> +}
> +
>  /*
>   * These routines also need to handle stuff like marking pages dirty
>   * and/or accessed for architectures that don't do it in hardware (most
> @@ -3524,7 +3526,7 @@ static int handle_pte_fault(struct fault_env *fe)
>  	if (!pte_present(entry))
>  		return do_swap_page(fe, entry);
>
> -	if (pte_protnone(entry))
> +	if (pte_protnone(entry) && vma_is_accessible(fe->vma))
>  		return do_numa_page(fe, entry);
>
>  	fe->ptl = pte_lockptr(fe->vma->vm_mm, fe->pmd);
> @@ -3590,7 +3592,7 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>
>  		barrier();
>  		if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) {
> -			if (pmd_protnone(orig_pmd))
> +			if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
>  				return do_huge_pmd_numa_page(&fe, orig_pmd);
>
>  			if ((fe.flags & FAULT_FLAG_WRITE) &&
> --
> 2.9.3