From: Andrea Arcangeli aarcange@redhat.com Subject: coredump: fix race condition between collapse_huge_page() and core dumping
When fixing the race conditions between the coredump and the mmap_sem holders outside the context of the process, we focused on mmget_not_zero()/get_task_mm() callers in 04f5866e41fb70 ("coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping"), but those aren't the only cases where the mmap_sem can be taken outside of the context of the process as Michal Hocko noticed while backporting that commit to older -stable kernels.
If mmgrab() is called in the context of the process, but then the mm_count reference is transferred outside the context of the process, that can also be a problem if the mmap_sem has to be taken for writing through that mm_count reference.
khugepaged registration calls mmgrab() in the context of the process, but the mmap_sem for writing is taken later in the context of the khugepaged kernel thread.
collapse_huge_page() after taking the mmap_sem for writing doesn't modify any vma, so it's not obvious that it could cause a problem to the coredump, but it happens to modify the pmd in a way that breaks an invariant that pmd_trans_huge_lock() relies upon. collapse_huge_page() needs the mmap_sem for writing just to block concurrent page faults that call pmd_trans_huge_lock().
Specifically the invariant that "!pmd_trans_huge()" cannot become a "pmd_trans_huge()" doesn't hold while collapse_huge_page() runs.
The coredump will call __get_user_pages() without mmap_sem for reading, which eventually can invoke a lockless page fault which will need a functional pmd_trans_huge_lock().
So collapse_huge_page() needs to use mmget_still_valid() to check it's not running concurrently with the coredump... as long as the coredump can invoke page faults without holding the mmap_sem for reading.
This has "Fixes: khugepaged" to facilitate backporting, but in my view it's more a bug in the coredump code that will eventually have to be rewritten to stop invoking page faults without the mmap_sem for reading. So the long term plan is still to drop all mmget_still_valid().
Link: http://lkml.kernel.org/r/20190607161558.32104-1-aarcange@redhat.com Fixes: ba76149f47d8 ("thp: khugepaged") Signed-off-by: Andrea Arcangeli aarcange@redhat.com Reported-by: Michal Hocko mhocko@suse.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Oleg Nesterov oleg@redhat.com Cc: Jann Horn jannh@google.com Cc: Hugh Dickins hughd@google.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Peter Xu peterx@redhat.com Cc: Jason Gunthorpe jgg@mellanox.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
include/linux/sched/mm.h | 4 ++++ mm/khugepaged.c | 3 +++ 2 files changed, 7 insertions(+)
--- a/include/linux/sched/mm.h~coredump-fix-race-condition-between-collapse_huge_page-and-core-dumping +++ a/include/linux/sched/mm.h @@ -54,6 +54,10 @@ static inline void mmdrop(struct mm_stru * followed by taking the mmap_sem for writing before modifying the * vmas or anything the coredump pretends not to change from under it. * + * It also has to be called when mmgrab() is used in the context of + * the process, but then the mm_count refcount is transferred outside + * the context of the process to run down_write() on that pinned mm. + * * NOTE: find_extend_vma() called from GUP context is the only place * that can modify the "mm" (notably the vm_start/end) under mmap_sem * for reading and outside the context of the process, so it is also --- a/mm/khugepaged.c~coredump-fix-race-condition-between-collapse_huge_page-and-core-dumping +++ a/mm/khugepaged.c @@ -1004,6 +1004,9 @@ static void collapse_huge_page(struct mm * handled by the anon_vma lock + PG_lock. */ down_write(&mm->mmap_sem); + result = SCAN_ANY_PROCESS; + if (!mmget_still_valid(mm)) + goto out; result = hugepage_vma_revalidate(mm, address, &vma); if (result) goto out; _