[Devel] [PATCH RHEL8 COMMIT] userfaultfd_release: always remove uffd flags and clear vm_userfaultfd_ctx

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-80.1.2.vz8.3.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-80.1.2.vz8.3.6
------>
commit ad76d820648d188623d94fd2ad2eca6618127e2f
Author: Oleg Nesterov <oleg at redhat.com>
Date:   Mon Apr 20 10:34:02 2020 +0300

    userfaultfd_release: always remove uffd flags and clear vm_userfaultfd_ctx
    
    userfaultfd_release() should clear vm_flags/vm_userfaultfd_ctx even if
    mm->core_state != NULL.
    
    Otherwise a page fault can see userfaultfd_missing() == T and use an
    already freed userfaultfd_ctx.
    
    Link: http://lkml.kernel.org/r/20190820160237.GB4983@redhat.com
    Fixes: 04f5866e41fb ("coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping")
    Signed-off-by: Oleg Nesterov <oleg at redhat.com>
    Reported-by: Kefeng Wang <wangkefeng.wang at huawei.com>
    Reviewed-by: Andrea Arcangeli <aarcange at redhat.com>
    Tested-by: Kefeng Wang <wangkefeng.wang at huawei.com>
    Cc: Peter Xu <peterx at redhat.com>
    Cc: Mike Rapoport <rppt at linux.ibm.com>
    Cc: Jann Horn <jannh at google.com>
    Cc: Jason Gunthorpe <jgg at mellanox.com>
    Cc: Michal Hocko <mhocko at suse.com>
    Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
    Cc: <stable at vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    
    https://jira.sw.ru/browse/PSBM-102938
    (cherry picked from commit 46d0b24c5ee10a15dfb25e20642f5a5ed59c5003)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/userfaultfd.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index df1e3d45aeb4..d126b4ae57ee 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -873,6 +873,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	/* len == 0 means wake all */
 	struct userfaultfd_wake_range range = { .len = 0, };
 	unsigned long new_flags;
+	bool still_valid;
 
 	WRITE_ONCE(ctx->released, true);
 
@@ -888,8 +889,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	 * taking the mmap_sem for writing.
 	 */
 	down_write(&mm->mmap_sem);
-	if (!mmget_still_valid(mm))
-		goto skip_mm;
+	still_valid = mmget_still_valid(mm);
 	prev = NULL;
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		cond_resched();
@@ -900,19 +900,20 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 			continue;
 		}
 		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
-		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
-				 new_flags, vma->anon_vma,
-				 vma->vm_file, vma->vm_pgoff,
-				 vma_policy(vma),
-				 NULL_VM_UFFD_CTX);
-		if (prev)
-			vma = prev;
-		else
-			prev = vma;
+		if (still_valid) {
+			prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
+					 new_flags, vma->anon_vma,
+					 vma->vm_file, vma->vm_pgoff,
+					 vma_policy(vma),
+					 NULL_VM_UFFD_CTX);
+			if (prev)
+				vma = prev;
+			else
+				prev = vma;
+		}
 		vma->vm_flags = new_flags;
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 	}
-skip_mm:
 	up_write(&mm->mmap_sem);
 	mmput(mm);
 wakeup: