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SubjectRe: [PATCH v11 10/26] mm: protect VMA modifications using VMA sequence count
Hi Laurent,

On Thu, May 17, 2018 at 4:37 PM Laurent Dufour
<ldufour@linux.vnet.ibm.com> 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(-)
>
> struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_fault *vmf)
> @@ -665,9 +669,9 @@ 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
>

I have got a crash on 4.14 kernel with speculative page faults enabled
and here is my analysis of the problem.
The issue was reported only once.

[23409.303395] el1_da+0x24/0x84
[23409.303400] __radix_tree_lookup+0x8/0x90
[23409.303407] find_get_entry+0x64/0x14c
[23409.303410] pagecache_get_page+0x5c/0x27c
[23409.303416] __read_swap_cache_async+0x80/0x260
[23409.303420] swap_vma_readahead+0x264/0x37c
[23409.303423] swapin_readahead+0x5c/0x6c
[23409.303428] do_swap_page+0x128/0x6e4
[23409.303431] handle_pte_fault+0x230/0xca4
[23409.303435] __handle_speculative_fault+0x57c/0x7c8
[23409.303438] do_page_fault+0x228/0x3e8
[23409.303442] do_translation_fault+0x50/0x6c
[23409.303445] do_mem_abort+0x5c/0xe0
[23409.303447] el0_da+0x20/0x24

Process A accesses address ADDR (part of VMA A) and that results in a
translation fault.
Kernel enters __handle_speculative_fault to fix the fault.
Process A enters do_swap_page->swapin_readahead->swap_vma_readahead
from speculative path.
During this time, another process B which shares the same mm, does a
mprotect from another CPU which follows
mprotect_fixup->__split_vma, and it splits VMA A into VMAs A and B.
After the split, ADDR falls into VMA B, but process A is still using
VMA A.
Now ADDR is greater than VMA_A->vm_start and VMA_A->vm_end.
swap_vma_readahead->swap_ra_info uses start and end of vma to
calculate ptes and nr_pte, which goes wrong due to this and finally
resulting in wrong "entry" passed to
swap_vma_readahead->__read_swap_cache_async, and in turn causing
invalid swapper_space
being passed to __read_swap_cache_async->find_get_page, causing an abort.

The fix I have tried is to cache vm_start and vm_end also in vmf and
use it in swap_ra_clamp_pfn. Let me know your thoughts on this. I can
send
the patch I am a using if you feel that is the right thing to do.

Thanks,
Vinayak

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