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SubjectRe: [PATCH v10 09/25] mm: protect VMA modifications using VMA sequence count
On Tue, Apr 17, 2018 at 04:33:15PM +0200, Laurent Dufour wrote:
> The VMA sequence count has been introduced to allow fast detection of
> VMA modification when running a page fault handler without holding
> the mmap_sem.
>
> This patch provides protection against the VMA modification done in :
> - madvise()
> - mpol_rebind_policy()
> - vma_replace_policy()
> - change_prot_numa()
> - mlock(), munlock()
> - mprotect()
> - mmap_region()
> - collapse_huge_page()
> - userfaultd registering services
>
> In addition, VMA fields which will be read during the speculative fault
> path needs to be written using WRITE_ONCE to prevent write to be split
> and intermediate values to be pushed to other CPUs.
>
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
> fs/proc/task_mmu.c | 5 ++++-
> fs/userfaultfd.c | 17 +++++++++++++----
> mm/khugepaged.c | 3 +++
> mm/madvise.c | 6 +++++-
> mm/mempolicy.c | 51 ++++++++++++++++++++++++++++++++++-----------------
> mm/mlock.c | 13 ++++++++-----
> mm/mmap.c | 22 +++++++++++++---------
> mm/mprotect.c | 4 +++-
> mm/swap_state.c | 8 ++++++--
> 9 files changed, 89 insertions(+), 40 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c486ad4b43f0..aeb417f28839 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1136,8 +1136,11 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
> goto out_mm;
> }
> for (vma = mm->mmap; vma; vma = vma->vm_next) {
> - vma->vm_flags &= ~VM_SOFTDIRTY;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags,
> + vma->vm_flags & ~VM_SOFTDIRTY);
> vma_set_page_prot(vma);
> + vm_write_end(vma);

trivial:

I think It's tricky to maintain that VMA fields to be read during SPF should be
(READ|WRITE_ONCE). I think we need some accessor to read/write them rather than
raw accessing like like vma_set_page_prot. Maybe spf prefix would be helpful.

vma_spf_set_value(vma, vm_flags, val);

We also add some markers in vm_area_struct's fileds to indicate that
people shouldn't access those fields directly.

Just a thought.


> }
> downgrade_write(&mm->mmap_sem);


> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index fe079756bb18..8a8a402ed59f 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -575,6 +575,10 @@ static unsigned long swapin_nr_pages(unsigned long offset)
> * the readahead.
> *
> * Caller must hold down_read on the vma->vm_mm if vmf->vma is not NULL.
> + * This is needed to ensure the VMA will not be freed in our back. In the case
> + * of the speculative page fault handler, this cannot happen, even if we don't
> + * hold the mmap_sem. Callees are assumed to take care of reading VMA's fields

I guess reader would be curious on *why* is safe with SPF.
Comment about the why could be helpful for reviewer.

> + * using READ_ONCE() to read consistent values.
> */
> struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_fault *vmf)
> @@ -668,9 +672,9 @@ static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
> unsigned long *start,
> unsigned long *end)
> {
> - *start = max3(lpfn, PFN_DOWN(vma->vm_start),
> + *start = max3(lpfn, PFN_DOWN(READ_ONCE(vma->vm_start)),
> PFN_DOWN(faddr & PMD_MASK));
> - *end = min3(rpfn, PFN_DOWN(vma->vm_end),
> + *end = min3(rpfn, PFN_DOWN(READ_ONCE(vma->vm_end)),
> PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
> }
>
> --
> 2.7.4
>

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