| Message ID | 20230125225358.2576151-1-zokeefe@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm/MADV_COLLAPSE: catch !none !huge !bad pmd lookups | expand |
On Wed, Jan 25, 2023 at 2:54 PM Zach O'Keefe <zokeefe@google.com> wrote: > > 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 Reviewed-by: Yang Shi <shy828301@gmail.com> > > --- > 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(). > > v1 -> v2: Fix typo > --- > mm/khugepaged.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 9548644bdb56..1face2ae5877 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -943,6 +943,10 @@ 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) > @@ -961,8 +965,12 @@ 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(pmde)) > + return SCAN_PMD_NULL; > if (pmd_bad(pmde)) > return SCAN_PMD_NULL; > return SCAN_SUCCEED; > -- > 2.39.1.456.gfc5497dd1b-goog >
On Wed, Jan 25, 2023 at 4:24 PM Yang Shi <shy828301@gmail.com> wrote: > > On Wed, Jan 25, 2023 at 2:54 PM Zach O'Keefe <zokeefe@google.com> wrote: > > > > 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 > > Reviewed-by: Yang Shi <shy828301@gmail.com> Thanks for your time as always, Yang! Best, Zach > > > > --- > > 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(). > > > > v1 -> v2: Fix typo > > --- > > mm/khugepaged.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 9548644bdb56..1face2ae5877 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -943,6 +943,10 @@ 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) > > @@ -961,8 +965,12 @@ 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(pmde)) > > + return SCAN_PMD_NULL; > > if (pmd_bad(pmde)) > > return SCAN_PMD_NULL; > > return SCAN_SUCCEED; > > -- > > 2.39.1.456.gfc5497dd1b-goog > >
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 9548644bdb56..1face2ae5877 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -943,6 +943,10 @@ 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) @@ -961,8 +965,12 @@ 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(pmde)) + return SCAN_PMD_NULL; if (pmd_bad(pmde)) return SCAN_PMD_NULL; return SCAN_SUCCEED;
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(). v1 -> v2: Fix typo --- mm/khugepaged.c | 8 ++++++++ 1 file changed, 8 insertions(+)