[0/2] userfaultfd: fix races around pmd_trans_huge() check

Message

Jann Horn Aug. 12, 2024, 4:42 p.m. UTC

The pmd_trans_huge() code in mfill_atomic() is wrong in two different
ways depending on kernel version:

1. The pmd_trans_huge() check is racy and can lead to a BUG_ON() (if you hit
   the right two race windows) - I've tested this in a kernel build with
   some extra mdelay() calls. See the commit message for a description
   of the race scenario.
   On older kernels (before 6.5), I think the same bug can even
   theoretically lead to accessing transhuge page contents as a page table
   if you hit the right 5 narrow race windows (I haven't tested this case).
2. On newer kernels (>=6.5), for shmem mappings, khugepaged is allowed
   to yank page tables out from under us (though I haven't tested that),
   so I think the BUG_ON() checks in mfill_atomic() are just wrong.

I decided to write two separate fixes for these, so that the first fix
can be backported to kernels affected by the first bug.

Signed-off-by: Jann Horn <[e-mail]>
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Jann Horn (2):
      userfaultfd: Fix pmd_trans_huge() recheck race
      userfaultfd: Don't BUG_ON() if khugepaged yanks our page table

 mm/userfaultfd.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)
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base-commit: d4560686726f7a357922f300fc81f5964be8df04
change-id: 20240812-uffd-thp-flip-fix-20f91f1151b9