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 <jannh@xxxxxxxxxx>
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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(-)
---
base-commit: d4560686726f7a357922f300fc81f5964be8df04
change-id: 20240812-uffd-thp-flip-fix-20f91f1151b9
--
Jann Horn <jannh@xxxxxxxxxx>
