linux-kernel - [PATCH 1/1] userfaultfd: avoid huge_zero_page in UFFDIO

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Date: Thu, 11 Jan 2024 17:39:35 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: sfr@...b.auug.org.au, david@...hat.com, peterx@...hat.com, 
	aarcange@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Suren Baghdasaryan <surenb@...gle.com>, syzbot+705209281e36404998f6@...kaller.appspotmail.com
Subject: [PATCH 1/1] userfaultfd: avoid huge_zero_page in UFFDIO_MOVE

While testing UFFDIO_MOVE ioctl, syzbot triggered VM_BUG_ON_PAGE caused
by a call to PageAnonExclusive() with a huge_zero_page as a parameter.
UFFDIO_MOVE does not yet handle zeropages and returns EBUSY when one is
encountered. Add an early huge_zero_page check in the PMD move path
to avoid this situation.

Reported-by: syzbot+705209281e36404998f6@...kaller.appspotmail.com
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
Applies cleanly over linux-next, mm-stable and mm-unstable branches

 mm/userfaultfd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 216ab4c8621f..20e3b0d9cf7e 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1393,6 +1393,12 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
 				err = -ENOENT;
 				break;
 			}
+			/* Avoid moving zeropages for now */
+			if (is_huge_zero_pmd(*src_pmd)) {
+				spin_unlock(ptl);
+				err = -EBUSY;
+				break;
+			}
 
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
-- 
2.43.0.275.g3460e3d667-goog