Re: [PATCH 1/2] userfaultfd: Fix pmd_trans_huge() recheck race

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Hi Jann,

On 2024/8/13 00:42, Jann Horn wrote:
The following race can occur:

   mfill_atomic                other thread
   ============                ============
                               <zap PMD>
   pmdp_get_lockless() [reads none pmd]
   <bail if trans_huge>
   <if none:>
                               <pagefault creates transhuge zeropage>
     __pte_alloc [no-op]
                               <zap PMD>
   <bail if pmd_trans_huge(*dst_pmd)>
   BUG_ON(pmd_none(*dst_pmd))

I have experimentally verified this in a kernel with extra mdelay() calls;
the BUG_ON(pmd_none(*dst_pmd)) triggers.

On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow
pte_offset_map[_lock]() to fail"), this can't lead to anything worse than
a BUG_ON(), since the page table access helpers are actually designed to
deal with page tables concurrently disappearing; but on older kernels
(<=6.4), I think we could probably theoretically race past the two BUG_ON()
checks and end up treating a hugepage as a page table.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
  mm/userfaultfd.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e54e5c8907fa..ec3750467aa5 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -801,7 +801,8 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
  			break;
  		}
  		/* If an huge pmd materialized from under us fail */
-		if (unlikely(pmd_trans_huge(*dst_pmd))) {
+		dst_pmdval = pmdp_get_lockless(dst_pmd);
+		if (unlikely(pmd_none(dst_pmdval) || pmd_trans_huge(dst_pmdval))) {

Before commit 0d940a9b270b, should we also check for
is_pmd_migration_entry(), pmd_devmap() and pmd_bad() here?

Thanks,
Qi

  			err = -EFAULT;
  			break;
  		}





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