mm, oom: fix potential data corruption when oom_reaper races with writer

Wenwei Tao has noticed that our current assumption that the oom victim
is dying and never doing any visible changes after it dies, and so the
oom_reaper can tear it down, is not entirely true.

__task_will_free_mem consider a task dying when SIGNAL_GROUP_EXIT is set
but do_group_exit sends SIGKILL to all threads _after_ the flag is set.
So there is a race window when some threads won't have
fatal_signal_pending while the oom_reaper could start unmapping the
address space.  Moreover some paths might not check for fatal signals
before each PF/g-u-p/copy_from_user.

We already have a protection for oom_reaper vs.  PF races by checking
MMF_UNSTABLE.  This has been, however, checked only for kernel threads
(use_mm users) which can outlive the oom victim.  A simple fix would be
to extend the current check in handle_mm_fault for all tasks but that
wouldn't be sufficient because the current check assumes that a kernel
thread would bail out after EFAULT from get_user*/copy_from_user and
never re-read the same address which would succeed because the PF path
has established page tables already.  This seems to be the case for the
only existing use_mm user currently (virtio driver) but it is rather
fragile in general.

This is even more fragile in general for more complex paths such as
generic_perform_write which can re-read the same address more times
(e.g.  iov_iter_copy_from_user_atomic to fail and then
iov_iter_fault_in_readable on retry).

Therefore we have to implement MMF_UNSTABLE protection in a robust way
and never make a potentially corrupted content visible.  That requires
to hook deeper into the PF path and check for the flag _every time_
before a pte for anonymous memory is established (that means all
!VM_SHARED mappings).

The corruption can be triggered artificially
(http://lkml.kernel.org/r/201708040646.v746kkhC024636@www262.sakura.ne.jp)
but there doesn't seem to be any real life bug report.  The race window
should be quite tight to trigger most of the time.

Link: http://lkml.kernel.org/r/20170807113839.16695-3-mhocko@kernel.org
Fixes: aac453635549 ("mm, oom: introduce oom reaper")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Wenwei Tao <wenwei.tww@alibaba-inc.com>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrea Argangeli <andrea@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/oom.h b/include/linux/oom.h
index 8a266e2..76aac4c 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -6,6 +6,8 @@
 #include <linux/types.h>
 #include <linux/nodemask.h>
 #include <uapi/linux/oom.h>
+#include <linux/sched/coredump.h> /* MMF_* */
+#include <linux/mm.h> /* VM_FAULT* */
 
 struct zonelist;
 struct notifier_block;
@@ -63,6 +65,26 @@
 	return tsk->signal->oom_mm;
 }
 
+/*
+ * Checks whether a page fault on the given mm is still reliable.
+ * This is no longer true if the oom reaper started to reap the
+ * address space which is reflected by MMF_UNSTABLE flag set in
+ * the mm. At that moment any !shared mapping would lose the content
+ * and could cause a memory corruption (zero pages instead of the
+ * original content).
+ *
+ * User should call this before establishing a page table entry for
+ * a !shared mapping and under the proper page table lock.
+ *
+ * Return 0 when the PF is safe VM_FAULT_SIGBUS otherwise.
+ */
+static inline int check_stable_address_space(struct mm_struct *mm)
+{
+	if (unlikely(test_bit(MMF_UNSTABLE, &mm->flags)))
+		return VM_FAULT_SIGBUS;
+	return 0;
+}
+
 extern unsigned long oom_badness(struct task_struct *p,
 		struct mem_cgroup *memcg, const nodemask_t *nodemask,
 		unsigned long totalpages);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 216114f..90731e3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -32,6 +32,7 @@
 #include <linux/userfaultfd_k.h>
 #include <linux/page_idle.h>
 #include <linux/shmem_fs.h>
+#include <linux/oom.h>
 
 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -550,6 +551,7 @@
 	struct mem_cgroup *memcg;
 	pgtable_t pgtable;
 	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	int ret = 0;
 
 	VM_BUG_ON_PAGE(!PageCompound(page), page);
 
@@ -561,9 +563,8 @@
 
 	pgtable = pte_alloc_one(vma->vm_mm, haddr);
 	if (unlikely(!pgtable)) {
-		mem_cgroup_cancel_charge(page, memcg, true);
-		put_page(page);
-		return VM_FAULT_OOM;
+		ret = VM_FAULT_OOM;
+		goto release;
 	}
 
 	clear_huge_page(page, haddr, HPAGE_PMD_NR);
@@ -576,13 +577,14 @@
 
 	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
 	if (unlikely(!pmd_none(*vmf->pmd))) {
-		spin_unlock(vmf->ptl);
-		mem_cgroup_cancel_charge(page, memcg, true);
-		put_page(page);
-		pte_free(vma->vm_mm, pgtable);
+		goto unlock_release;
 	} else {
 		pmd_t entry;
 
+		ret = check_stable_address_space(vma->vm_mm);
+		if (ret)
+			goto unlock_release;
+
 		/* Deliver the page fault to userland */
 		if (userfaultfd_missing(vma)) {
 			int ret;
@@ -610,6 +612,15 @@
 	}
 
 	return 0;
+unlock_release:
+	spin_unlock(vmf->ptl);
+release:
+	if (pgtable)
+		pte_free(vma->vm_mm, pgtable);
+	mem_cgroup_cancel_charge(page, memcg, true);
+	put_page(page);
+	return ret;
+
 }
 
 /*
@@ -688,7 +699,10 @@
 		ret = 0;
 		set = false;
 		if (pmd_none(*vmf->pmd)) {
-			if (userfaultfd_missing(vma)) {
+			ret = check_stable_address_space(vma->vm_mm);
+			if (ret) {
+				spin_unlock(vmf->ptl);
+			} else if (userfaultfd_missing(vma)) {
 				spin_unlock(vmf->ptl);
 				ret = handle_userfault(vmf, VM_UFFD_MISSING);
 				VM_BUG_ON(ret & VM_FAULT_FALLBACK);
diff --git a/mm/memory.c b/mm/memory.c
index c717b5b..fe2fba2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -68,6 +68,7 @@
 #include <linux/debugfs.h>
 #include <linux/userfaultfd_k.h>
 #include <linux/dax.h>
+#include <linux/oom.h>
 
 #include <asm/io.h>
 #include <asm/mmu_context.h>
@@ -2893,6 +2894,7 @@
 	struct vm_area_struct *vma = vmf->vma;
 	struct mem_cgroup *memcg;
 	struct page *page;
+	int ret = 0;
 	pte_t entry;
 
 	/* File mapping without ->vm_ops ? */
@@ -2925,6 +2927,9 @@
 				vmf->address, &vmf->ptl);
 		if (!pte_none(*vmf->pte))
 			goto unlock;
+		ret = check_stable_address_space(vma->vm_mm);
+		if (ret)
+			goto unlock;
 		/* Deliver the page fault to userland, check inside PT lock */
 		if (userfaultfd_missing(vma)) {
 			pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -2959,6 +2964,10 @@
 	if (!pte_none(*vmf->pte))
 		goto release;
 
+	ret = check_stable_address_space(vma->vm_mm);
+	if (ret)
+		goto release;
+
 	/* Deliver the page fault to userland, check inside PT lock */
 	if (userfaultfd_missing(vma)) {
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -2978,7 +2987,7 @@
 	update_mmu_cache(vma, vmf->address, vmf->pte);
 unlock:
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
-	return 0;
+	return ret;
 release:
 	mem_cgroup_cancel_charge(page, memcg, false);
 	put_page(page);
@@ -3252,7 +3261,7 @@
 int finish_fault(struct vm_fault *vmf)
 {
 	struct page *page;
-	int ret;
+	int ret = 0;
 
 	/* Did we COW the page? */
 	if ((vmf->flags & FAULT_FLAG_WRITE) &&
@@ -3260,7 +3269,15 @@
 		page = vmf->cow_page;
 	else
 		page = vmf->page;
-	ret = alloc_set_pte(vmf, vmf->memcg, page);
+
+	/*
+	 * check even for read faults because we might have lost our CoWed
+	 * page
+	 */
+	if (!(vmf->vma->vm_flags & VM_SHARED))
+		ret = check_stable_address_space(vmf->vma->vm_mm);
+	if (!ret)
+		ret = alloc_set_pte(vmf, vmf->memcg, page);
 	if (vmf->pte)
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
 	return ret;
@@ -3900,29 +3917,6 @@
 			mem_cgroup_oom_synchronize(false);
 	}
 
-	/*
-	 * This mm has been already reaped by the oom reaper and so the
-	 * refault cannot be trusted in general. Anonymous refaults would
-	 * lose data and give a zero page instead e.g. This is especially
-	 * problem for use_mm() because regular tasks will just die and
-	 * the corrupted data will not be visible anywhere while kthread
-	 * will outlive the oom victim and potentially propagate the date
-	 * further.
-	 */
-	if (unlikely((current->flags & PF_KTHREAD) && !(ret & VM_FAULT_ERROR)
-				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags))) {
-
-		/*
-		 * We are going to enforce SIGBUS but the PF path might have
-		 * dropped the mmap_sem already so take it again so that
-		 * we do not break expectations of all arch specific PF paths
-		 * and g-u-p
-		 */
-		if (ret & VM_FAULT_RETRY)
-			down_read(&vma->vm_mm->mmap_sem);
-		ret = VM_FAULT_SIGBUS;
-	}
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(handle_mm_fault);