Re: [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork

[John Hubbard <jhubbard@nvidia.com>]

On 10/23/20 5:19 PM, Jason Gunthorpe wrote:
> Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during
> fork() for ptes") pages under a FOLL_PIN will not be write protected
> during COW for fork. This means that pages returned from
> pin_user_pages(FOLL_WRITE) should not become write protected while the pin
> is active.
> 
> However, there is a small race where get_user_pages_fast(FOLL_PIN) can
> establish a FOLL_PIN at the same time copy_present_page() is write
> protecting it:
> 
>          CPU 0                             CPU 1
>     get_user_pages_fast()
>      internal_get_user_pages_fast()
>                                         copy_page_range()
>                                           pte_alloc_map_lock()
>                                             copy_present_page()
>                                               atomic_read(has_pinned) == 0
> 					     page_maybe_dma_pinned() == false
>       atomic_set(has_pinned, 1);
>       gup_pgd_range()
>        gup_pte_range()
>         pte_t pte = gup_get_pte(ptep)
>         pte_access_permitted(pte)
>         try_grab_compound_head()
>                                               pte = maybe_mkwrite()

Are you sure you didn't reverse this part? That makes it writeable, not
write-protected. And it doesn't actually happen as shown, either: if
page_maybe_dma_pinned() returns false, then copy_present_page() returns
early and does *not* do the maybe_mkwrite(), but instead does:

     pte = pte_wrprotect(pte);

...so probably you want to write that line, instead of what you have.

> 	                                     set_pte_at();
>                                           pte_unmap_unlock()
>        // GUP now returns with a write protected page
> 
> The first attempt to resolve this by using the write protect caused
> problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid
> early COW write protect games during fork()")
> 
> Instead wrap copy_p4d_range() with the write side of something like a
> seqcount and check the read side around gup_pgd_range(). If there is a
> collision then get_user_pages_fast() fails and falls back to slow GUP.
> 
> Slow GUP is safe against this race because copy_page_range() is only
> called while holding the write side of the mmap_lock on the src mm_struct.
> 
> Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()")
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>


OK, I've *finally* convinced myself that the seqlock-like thing and the
memory barriers are correct. :) And the overall locking story also looks good.

Aside from the minor commit log question above, the diffs look correct, so
either way,

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> ---
>   include/linux/mm_types.h |  6 ++++++
>   kernel/fork.c            |  1 +
>   mm/gup.c                 | 19 +++++++++++++++++++
>   mm/memory.c              | 16 +++++++++++++++-
>   4 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5a9238f6caad97..8c7c9de476c4f8 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -446,6 +446,12 @@ struct mm_struct {
>   		 */
>   		atomic_t has_pinned;
>   
> +		/**
> +		 * @write_protecet_seq: Odd when any thread is write
> +		 * protecting pages in this mm, for instance during fork().
> +		 */
> +		unsigned long write_protect_seq;
> +
>   #ifdef CONFIG_MMU
>   		atomic_long_t pgtables_bytes;	/* PTE page table pages */
>   #endif
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 32083db7a2a23e..342243f621c742 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1007,6 +1007,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>   	mm->vmacache_seqnum = 0;
>   	atomic_set(&mm->mm_users, 1);
>   	atomic_set(&mm->mm_count, 1);
> +	mm->write_protect_seq = 0;
>   	mmap_init_lock(mm);
>   	INIT_LIST_HEAD(&mm->mmlist);
>   	mm->core_state = NULL;
> diff --git a/mm/gup.c b/mm/gup.c
> index ecbe1639ea2af7..2c1a1e0555479e 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2677,12 +2677,19 @@ static unsigned int lockless_pages_from_mm(unsigned long addr,
>   					   struct page **pages)
>   {
>   	unsigned long flags;
> +	unsigned long seq;
>   	int nr_pinned = 0;
>   
>   	if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
>   	    !gup_fast_permitted(addr, end))
>   		return 0;
>   
> +	if (gup_flags & FOLL_PIN) {
> +		seq = smp_load_acquire(&current->mm->write_protect_seq);
> +		if (seq & 1)
> +			return 0;
> +	}
> +
>   	/*
>   	 * Disable interrupts. The nested form is used, in order to allow full,
>   	 * general purpose use of this routine.
> @@ -2697,6 +2704,18 @@ static unsigned int lockless_pages_from_mm(unsigned long addr,
>   	local_irq_save(flags);
>   	gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned);
>   	local_irq_restore(flags);
> +
> +	/*
> +	 * When pinning pages for DMA there could be a concurrent write protect
> +	 * from fork() via copy_page_range(), in this case always fail fast GUP.
> +	 */
> +	if (gup_flags & FOLL_PIN) {
> +		smp_rmb();
> +		if (READ_ONCE(current->mm->write_protect_seq) != seq) {
> +			unpin_user_pages(pages, nr_pinned);
> +			return 0;
> +		}
> +	}
>   	return nr_pinned;
>   }
>   
> diff --git a/mm/memory.c b/mm/memory.c
> index c48f8df6e50268..e2f959cce8563d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1171,6 +1171,17 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>   		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
>   					0, src_vma, src_mm, addr, end);
>   		mmu_notifier_invalidate_range_start(&range);
> +		/*
> +		 * This is like a seqcount where the mmap_lock provides
> +		 * serialization for the write side. However, unlike seqcount
> +		 * the read side falls back to obtaining the mmap_lock rather
> +		 * than spinning. For this reason none of the preempt related
> +		 * machinery in seqcount is desired here.
> +		 */
> +		mmap_assert_write_locked(src_mm);
> +		WRITE_ONCE(src_mm->write_protect_seq,
> +			   src_mm->write_protect_seq + 1);
> +		smp_wmb();
>   	}
>   
>   	ret = 0;
> @@ -1187,8 +1198,11 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>   		}
>   	} while (dst_pgd++, src_pgd++, addr = next, addr != end);
>   
> -	if (is_cow)
> +	if (is_cow) {
> +		smp_store_release(&src_mm->write_protect_seq,
> +				  src_mm->write_protect_seq + 1);
>   		mmu_notifier_invalidate_range_end(&range);
> +	}
>   	return ret;
>   }
>   
>