[PATCH 2/2] mm/MADV_COLLAPSE: catch !none !huge !bad pmd lookups

Zach O'Keefe
2023-01-25T01:58:17Z

In commit 34488399fa08 ("mm/madvise: add file and shmem support to
MADV_COLLAPSE") we make the following change to find_pmd_or_thp_or_none():

- if (!pmd_present(pmde))
- return SCAN_PMD_NULL;
+ if (pmd_none(pmde))
+ return SCAN_PMD_NONE;

This was for-use by MADV_COLLAPSE file/shmem codepaths, where MADV_COLLAPSE
might identify a pte-mapped hugepage, only to have khugepaged race-in, free
the pte table, and clear the pmd. Such codepaths include:

A) If we find a suitably-aligned compound page of order HPAGE_PMD_ORDER
already in the pagecache.
B) In retract_page_tables(), if we fail to grab mmap_lock for the target
mm/address.

In these cases, collapse_pte_mapped_thp() really does expect a none (not
just !present) pmd, and we want to suitably identify that case separate
from the case where no pmd is found, or it's a bad-pmd (of course, many
things could happen once we drop mmap_lock, and the pmd could plausibly
undergo multiple transitions due to intervening fault, split, etc).
Regardless, the code is prepared install a huge-pmd only when the existing
pmd entry is either a genuine pte-table-mapping-pmd, or the none-pmd.

However, the commit introduces a logical hole; namely, that we've allowed
!none- && !huge- && !bad-pmds to be classified as genuine
pte-table-mapping-pmds. One such example that could leak through are swap
entries. The pmd values aren't checked again before use in
pte_offset_map_lock(), which is expecting nothing less than a genuine
pte-table-mapping-pmd.

We want to put back the !pmd_present() check (below the pmd_none() check),
but need to be careful to deal with subtleties in pmd transitions and
treatments by various arch.

The issue is that __split_huge_pmd_locked() temporarily clears the present
bit (or otherwise marks the entry as invalid), but pmd_present()
and pmd_trans_huge() still need to return true while the pmd is in this
transitory state. For example, x86's pmd_present() also checks the
_PAGE_PSE , riscv's version also checks the _PAGE_LEAF bit, and arm64 also
checks a PMD_PRESENT_INVALID bit.

Covering all 4 cases for x86 (all checks done on the same pmd value):

1) pmd_present() && pmd_trans_huge()
All we actually know here is that the PSE bit is set. Either:
a) We aren't racing with __split_huge_page(), and PRESENT or PROTNONE
is set.
=> huge-pmd
b) We are currently racing with __split_huge_page(). The danger here
is that we proceed as-if we have a huge-pmd, but really we are
looking at a pte-mapping-pmd. So, what is the risk of this
danger?

The only relevant path is:

madvise_collapse() -> collapse_pte_mapped_thp()

Where we might just incorrectly report back "success", when really
the memory isn't pmd-backed. This is fine, since split could
happen immediately after (actually) successful madvise_collapse().
So, it should be safe to just assume huge-pmd here.

2) pmd_present() && !pmd_trans_huge()
Either:
a) PSE not set and either PRESENT or PROTNONE is.
=> pte-table-mapping pmd (or PROT_NONE)
b) devmap. This routine can be called immediately after
unlocking/locking mmap_lock -- or called with no locks held (see
khugepaged_scan_mm_slot()), so previous VMA checks have since been
invalidated.

3) !pmd_present() && pmd_trans_huge()
Not possible.

4) !pmd_present() && !pmd_trans_huge()
Neither PRESENT nor PROTNONE set
=> not present

I've checked all archs that implement pmd_trans_huge() (arm64, riscv,
powerpc, longarch, x86, mips, s390) and this logic roughly translates
(though devmap treatment is unique to x86 and powerpc, and (3) doesn't
necessarily hold in general -- but that doesn't matter since !pmd_present()
always takes failure path).

Also, add a comment above find_pmd_or_thp_or_none() to help future
travelers reason about the validity of the code; namely, the possible
mutations that might happen out from under us, depending on how
mmap_lock is held (if at all).

Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: stable@vger.kernel.org

---
Request that this be pulled into stable since it's theoretically
possible (though I have no reproducer) that while mmap_lock is dropped,
racing thp migration installs a pmd migration entry which then has a path to
be consumed, unchecked, by pte_offset_map().
---
mm/khugepaged.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fa38cae240b9..7ea668bbea70 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -941,6 +941,10 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
return SCAN_SUCCEED;
}

+/*
+ * See pmd_trans_unstable() for how the result may change out from
+ * underneath us, even if we hold mmap_lock in read.
+ */
static int find_pmd_or_thp_or_none(struct mm_struct *mm,
unsigned long address,
pmd_t **pmd)
@@ -959,8 +963,12 @@ static int find_pmd_or_thp_or_none(struct mm_struct *mm,
#endif
if (pmd_none(pmde))
return SCAN_PMD_NONE;
+ if (!pmd_present(pmde))
+ return SCAN_PMD_NULL;
if (pmd_trans_huge(pmde))
return SCAN_PMD_MAPPED;
+ if (pmd_devmap(pmd))
+ return SCAN_PMD_NULL;
if (pmd_bad(pmde))
return SCAN_PMD_NULL;
return SCAN_SUCCEED;
--
2.39.1.405.gd4c25cc71f-goog