On Thu, 27 Dec 2018 11:24:31 -0800 Mike Kravetz mike.kravetz@oracle.com wrote:
On 12/27/18 3:44 AM, Colin Ian King wrote:
Hi,
Static analysis with CoverityScan on linux-next detected a potential null pointer dereference with the following commit:
From d8a1051ed4ba55679ef24e838a1942c9c40f0a14 Mon Sep 17 00:00:00 2001 From: Mike Kravetz mike.kravetz@oracle.com Date: Sat, 22 Dec 2018 10:55:57 +1100 Subject: [PATCH] hugetlbfs: use i_mmap_rwsem for more pmd sharing
The earlier check implies that "mapping" may be a null pointer:
var_compare_op: Comparing mapping to null implies that mapping might be null.
1008 if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping && 1009 mapping_cap_writeback_dirty(mapping)) {
..however later "mapper" is dereferenced when it may be potentially null:
1034 /* 1035 * For hugetlb pages, try_to_unmap could potentially call 1036 * huge_pmd_unshare. Because of this, take semaphore in 1037 * write mode here and set TTU_RMAP_LOCKED to indicate we 1038 * have taken the lock at this higer level. 1039 */ CID 1476097 (#1 of 1): Dereference after null check (FORWARD_NULL)
var_deref_model: Passing null pointer mapping to i_mmap_lock_write, which dereferences it.
1040 i_mmap_lock_write(mapping); 1041 unmap_success = try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED); 1042 i_mmap_unlock_write(mapping);
Thanks for the report.
The 'good news' is that mapping can not be null in the code path above. The reasons are:
- The page is locked upon entry to the routine
- Earlier in the routine there is the check: if (!page_mapped(hpage)) return true; For huge pages (which are processed in the else clause above), page_mapped implies page->mapping != null.
However, the routine hwpoison_user_mappings handles all page types. The page_mapped check is actually there to check for pages in the swap cache. It is just coincidence that it also implies mapping != null for huge pages.
It would be better to make an explicit check for mapping != null before calling i_mmap_lock_write/try_to_unmap. In this way, unrelated changes to code above will not potentially lead to the possibility of mapping == null.
I'm not sure what is the best way to handle this. Below is an updated version of the patch sent to Andrew. I can also provide a simple patch to the patch if that is easier.
Below is the delta. Please check it. It seems to do more than the above implies.
Also, I have notes here that
hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch and hugetlbfs-use-i_mmap_rwsem-to-fix-page-fault-truncate-race.patch
have additional updates pending. Due to emails such as
http://lkml.kernel.org/r/849f5202-2200-265f-7769-8363053e8373@oracle.com http://lkml.kernel.org/r/732c0b7d-5a4e-97a8-9677-30f3520893cb@oracle.com http://lkml.kernel.org/r/6b91dd42-b903-1f6c-729a-bd9f51273986@oracle.com
What's the status, please?
From: Mike Kravetz mike.kravetz@oracle.com Subject: hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization-fix
It would be better to make an explicit check for mapping != null before calling i_mmap_lock_write/try_to_unmap. In this way, unrelated changes to code above will not potentially lead to the possibility of mapping == null.
Signed-off-by: Mike Kravetz mike.kravetz@oracle.com Cc: Michal Hocko mhocko@kernel.org Cc: Hugh Dickins hughd@google.com Cc: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Cc: "Aneesh Kumar K . V" aneesh.kumar@linux.vnet.ibm.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: "Kirill A . Shutemov" kirill.shutemov@linux.intel.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: Prakash Sangappa prakash.sangappa@oracle.com Cc: Colin Ian King colin.king@canonical.com Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
--- a/mm/hugetlb.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization-fix +++ a/mm/hugetlb.c @@ -3250,6 +3250,14 @@ int copy_hugetlb_page_range(struct mm_st mmu_notifier_range_init(&range, src, vma->vm_start, vma->vm_end, MMU_NOTIFY_CLEAR); mmu_notifier_invalidate_range_start(&range); + } else { + /* + * For shared mappings i_mmap_rwsem must be held to call + * huge_pte_alloc, otherwise the returned ptep could go + * away if part of a shared pmd and another thread calls + * huge_pmd_unshare. + */ + i_mmap_lock_read(mapping); }
for (addr = vma->vm_start; addr < vma->vm_end; addr += sz) { @@ -3259,18 +3267,8 @@ int copy_hugetlb_page_range(struct mm_st if (!src_pte) continue;
- /* - * i_mmap_rwsem must be held to call huge_pte_alloc. - * Continue to hold until finished with dst_pte, otherwise - * it could go away if part of a shared pmd. - * - * Technically, i_mmap_rwsem is only needed in the non-cow - * case as cow mappings are not shared. - */ - i_mmap_lock_read(mapping); dst_pte = huge_pte_alloc(dst, addr, sz); if (!dst_pte) { - i_mmap_unlock_read(mapping); ret = -ENOMEM; break; } @@ -3285,10 +3283,8 @@ int copy_hugetlb_page_range(struct mm_st * after taking the lock below. */ dst_entry = huge_ptep_get(dst_pte); - if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) { - i_mmap_unlock_read(mapping); + if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) continue; - }
dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); @@ -3337,12 +3333,12 @@ int copy_hugetlb_page_range(struct mm_st } spin_unlock(src_ptl); spin_unlock(dst_ptl); - - i_mmap_unlock_read(mapping); }
if (cow) mmu_notifier_invalidate_range_end(&range); + else + i_mmap_unlock_read(mapping);
return ret; } --- a/mm/memory-failure.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization-fix +++ a/mm/memory-failure.c @@ -966,7 +966,7 @@ static bool hwpoison_user_mappings(struc enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS; struct address_space *mapping; LIST_HEAD(tokill); - bool unmap_success; + bool unmap_success = true; int kill = 1, forcekill; struct page *hpage = *hpagep; bool mlocked = PageMlocked(hpage); @@ -1030,7 +1030,7 @@ static bool hwpoison_user_mappings(struc
if (!PageHuge(hpage)) { unmap_success = try_to_unmap(hpage, ttu); - } else { + } else if (mapping) { /* * For hugetlb pages, try_to_unmap could potentially call * huge_pmd_unshare. Because of this, take semaphore in --- a/mm/rmap.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization-fix +++ a/mm/rmap.c @@ -25,6 +25,7 @@ * page->flags PG_locked (lock_page) * hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share) * mapping->i_mmap_rwsem + * hugetlb_fault_mutex (hugetlbfs specific page fault mutex) * anon_vma->rwsem * mm->page_table_lock or pte_lock * zone_lru_lock (in mark_page_accessed, isolate_lru_page) _