From: Andrea Arcangeli <aarca...@redhat.com> commit 59ea6d06cfa9247b586a695c21f94afa7183af74 upstream.
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-aarca...@redhat.com Fixes: ba76149f47d8 ("thp: khugepaged") Signed-off-by: Andrea Arcangeli <aarca...@redhat.com> Reported-by: Michal Hocko <mho...@suse.com> Acked-by: Michal Hocko <mho...@suse.com> Acked-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com> Cc: Oleg Nesterov <o...@redhat.com> Cc: Jann Horn <ja...@google.com> Cc: Hugh Dickins <hu...@google.com> Cc: Mike Rapoport <r...@linux.vnet.ibm.com> Cc: Mike Kravetz <mike.krav...@oracle.com> Cc: Peter Xu <pet...@redhat.com> Cc: Jason Gunthorpe <j...@mellanox.com> Cc: <sta...@vger.kernel.org> Signed-off-by: Andrew Morton <a...@linux-foundation.org> Signed-off-by: Linus Torvalds <torva...@linux-foundation.org> [Ajay: Just adjusted to apply on v4.9] Signed-off-by: Ajay Kaher <aka...@vmware.com> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- include/linux/mm.h | 4 ++++ mm/khugepaged.c | 3 +++ 2 files changed, 7 insertions(+) --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1197,6 +1197,10 @@ void unmap_vmas(struct mmu_gather *tlb, * 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 +++ b/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;