[PATCH v2 0/2] userfaultfd: fix races around pmd_trans_huge() check

[Jann Horn]

The pmd_trans_huge() code in mfill_atomic() is wrong in three 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. As pointed out by Qi Zheng, pmd_trans_huge() is not sufficient for
   detecting PMDs that don't point to page tables.
   On older kernels (before 6.5), you'd just have to win a single fairly
   wide race to hit this.
   I've tested this on 6.1 stable by racing migration (with a mdelay()
   patched into try_to_migrate()) against UFFDIO_ZEROPAGE - on my x86
   VM, that causes a kernel oops in ptlock_ptr().
3. 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 (one fix for bugs 1+2,
one fix for bug 3), so that the first fix can be backported to kernels
affected by bugs 1+2.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Changes in v2:
- in patch 1/2:
  - change title
  - get rid of redundant early pmd_trans_huge() check
  - also check for swap PMDs and devmap PMDs (Qi Zheng)
- Link to v1: https://lore.kernel.org/r/20240812-uffd-thp-flip-fix-v1-0-4fc1db7ccdd0@xxxxxxxxxx

---
Jann Horn (2):
      userfaultfd: Fix checks for huge PMDs
      userfaultfd: Don't BUG_ON() if khugepaged yanks our page table

 mm/userfaultfd.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)
---
base-commit: d4560686726f7a357922f300fc81f5964be8df04
change-id: 20240812-uffd-thp-flip-fix-20f91f1151b9
-- 
Jann Horn <jannh@xxxxxxxxxx>