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    Subject[PATCH AUTOSEL 5.4 157/330] mm: avoid data corruption on CoW fault into PFN-mapped VMA
    Date
    From: "Kirill A. Shutemov" <kirill@shutemov.name>

    [ Upstream commit c3e5ea6ee574ae5e845a40ac8198de1fb63bb3ab ]

    Jeff Moyer has reported that one of xfstests triggers a warning when run
    on DAX-enabled filesystem:

    WARNING: CPU: 76 PID: 51024 at mm/memory.c:2317 wp_page_copy+0xc40/0xd50
    ...
    wp_page_copy+0x98c/0xd50 (unreliable)
    do_wp_page+0xd8/0xad0
    __handle_mm_fault+0x748/0x1b90
    handle_mm_fault+0x120/0x1f0
    __do_page_fault+0x240/0xd70
    do_page_fault+0x38/0xd0
    handle_page_fault+0x10/0x30

    The warning happens on failed __copy_from_user_inatomic() which tries to
    copy data into a CoW page.

    This happens because of race between MADV_DONTNEED and CoW page fault:

    CPU0 CPU1
    handle_mm_fault()
    do_wp_page()
    wp_page_copy()
    do_wp_page()
    madvise(MADV_DONTNEED)
    zap_page_range()
    zap_pte_range()
    ptep_get_and_clear_full()
    <TLB flush>
    __copy_from_user_inatomic()
    sees empty PTE and fails
    WARN_ON_ONCE(1)
    clear_page()

    The solution is to re-try __copy_from_user_inatomic() under PTL after
    checking that PTE is matches the orig_pte.

    The second copy attempt can still fail, like due to non-readable PTE, but
    there's nothing reasonable we can do about, except clearing the CoW page.

    Reported-by: Jeff Moyer <jmoyer@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Tested-by: Jeff Moyer <jmoyer@redhat.com>
    Cc: <stable@vger.kernel.org>
    Cc: Justin He <Justin.He@arm.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Link: http://lkml.kernel.org/r/20200218154151.13349-1-kirill.shutemov@linux.intel.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/memory.c | 35 +++++++++++++++++++++++++++--------
    1 file changed, 27 insertions(+), 8 deletions(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 9ea917e28ef4e..2157bb28117ac 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -2163,7 +2163,7 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
    bool ret;
    void *kaddr;
    void __user *uaddr;
    - bool force_mkyoung;
    + bool locked = false;
    struct vm_area_struct *vma = vmf->vma;
    struct mm_struct *mm = vma->vm_mm;
    unsigned long addr = vmf->address;
    @@ -2188,11 +2188,11 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
    * On architectures with software "accessed" bits, we would
    * take a double page fault, so mark it accessed here.
    */
    - force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf->orig_pte);
    - if (force_mkyoung) {
    + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
    pte_t entry;

    vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
    + locked = true;
    if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
    /*
    * Other thread has already handled the fault
    @@ -2216,18 +2216,37 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
    * zeroes.
    */
    if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
    + if (locked)
    + goto warn;
    +
    + /* Re-validate under PTL if the page is still mapped */
    + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
    + locked = true;
    + if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
    + /* The PTE changed under us. Retry page fault. */
    + ret = false;
    + goto pte_unlock;
    + }
    +
    /*
    - * Give a warn in case there can be some obscure
    - * use-case
    + * The same page can be mapped back since last copy attampt.
    + * Try to copy again under PTL.
    */
    - WARN_ON_ONCE(1);
    - clear_page(kaddr);
    + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
    + /*
    + * Give a warn in case there can be some obscure
    + * use-case
    + */
    +warn:
    + WARN_ON_ONCE(1);
    + clear_page(kaddr);
    + }
    }

    ret = true;

    pte_unlock:
    - if (force_mkyoung)
    + if (locked)
    pte_unmap_unlock(vmf->pte, vmf->ptl);
    kunmap_atomic(kaddr);
    flush_dcache_page(dst);
    --
    2.25.1
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