linux-kernel - [PATCH AUTOSEL 4.9 4/8] userfaultfd_release: always remove uffd flags and clear vm_userfaultfd

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Message-Id: <20190829105100.2649-4-sashal@kernel.org>
Date:   Thu, 29 Aug 2019 06:50:56 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jann Horn <jannh@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Michal Hocko <mhocko@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>, linux-fsdevel@...r.kernel.org
Subject: [PATCH AUTOSEL 4.9 4/8] userfaultfd_release: always remove uffd flags and clear vm_userfaultfd_ctx

From: Oleg Nesterov <oleg@...hat.com>

[ Upstream commit 46d0b24c5ee10a15dfb25e20642f5a5ed59c5003 ]

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@...hat.com>
Reported-by: Kefeng Wang <wangkefeng.wang@...wei.com>
Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
Tested-by: Kefeng Wang <wangkefeng.wang@...wei.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Mike Rapoport <rppt@...ux.ibm.com>
Cc: Jann Horn <jannh@...gle.com>
Cc: Jason Gunthorpe <jgg@...lanox.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/userfaultfd.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 8bf425a103f05..de63d4e2dfba1 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -464,6 +464,7 @@ 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;
 
 	ACCESS_ONCE(ctx->released) = true;
 
@@ -479,8 +480,7 @@ 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();
@@ -491,19 +491,20 @@ 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:
-- 
2.20.1