| Message ID | 20190607161558.32104-1-aarcange@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/1] coredump: fix race condition between collapse_huge_page() and core dumping | expand |
On Fri, Jun 07, 2019 at 04:15:58PM +0000, Andrea Arcangeli wrote: > 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 commit > 04f5866e41fb70690e28397487d8bd8eea7d712a, 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(). > > Cc: <stable@vger.kernel.org> > Fixes: ba76149f47d8 ("thp: khugepaged") > Reported-by: Michal Hocko <mhocko@suse.com> > Acked-by: Michal Hocko <mhocko@suse.com> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index a3fda9f024c3..4a7944078cc3 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -54,6 +54,10 @@ static inline void mmdrop(struct mm_struct *mm) * 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 diff --git a/mm/khugepaged.c b/mm/khugepaged.c index a335f7c1fac4..0f7419938008 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1004,6 +1004,9 @@ static void collapse_huge_page(struct mm_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;
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 commit 04f5866e41fb70690e28397487d8bd8eea7d712a, 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(). Cc: <stable@vger.kernel.org> Fixes: ba76149f47d8 ("thp: khugepaged") Reported-by: Michal Hocko <mhocko@suse.com> Acked-by: Michal Hocko <mhocko@suse.com> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> --- include/linux/sched/mm.h | 4 ++++ mm/khugepaged.c | 3 +++ 2 files changed, 7 insertions(+)