linux-kernel - [PATCH 4.11 090/115] ksm: prevent crash after write_protect

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Date:   Mon,  5 Jun 2017 18:18:03 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
        Federico Simoncelli <fsimonce@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.11 090/115] ksm: prevent crash after write_protect_page fails

4.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrea Arcangeli <aarcange@...hat.com>

commit a7306c3436e9c8e584a4b9fad5f3dc91be2a6076 upstream.

"err" needs to be left set to -EFAULT if split_huge_page succeeds.
Otherwise if "err" gets clobbered with zero and write_protect_page
fails, try_to_merge_one_page() will succeed instead of returning -EFAULT
and then try_to_merge_with_ksm_page() will continue thinking kpage is a
PageKsm when in fact it's still an anonymous page.  Eventually it'll
crash in page_add_anon_rmap.

This has been reproduced on Fedora25 kernel but I can reproduce with
upstream too.

The bug was introduced in commit f765f540598a ("ksm: prepare to new THP
semantics") introduced in v4.5.

    page:fffff67546ce1cc0 count:4 mapcount:2 mapping:ffffa094551e36e1 index:0x7f0f46673
    flags: 0x2ffffc0004007c(referenced|uptodate|dirty|lru|active|swapbacked)
    page dumped because: VM_BUG_ON_PAGE(!PageLocked(page))
    page->mem_cgroup:ffffa09674bf0000
    ------------[ cut here ]------------
    kernel BUG at mm/rmap.c:1222!
    CPU: 1 PID: 76 Comm: ksmd Not tainted 4.9.3-200.fc25.x86_64 #1
    RIP: do_page_add_anon_rmap+0x1c4/0x240
    Call Trace:
      page_add_anon_rmap+0x18/0x20
      try_to_merge_with_ksm_page+0x50b/0x780
      ksm_scan_thread+0x1211/0x1410
      ? prepare_to_wait_event+0x100/0x100
      ? try_to_merge_with_ksm_page+0x780/0x780
      kthread+0xd9/0xf0
      ? kthread_park+0x60/0x60
      ret_from_fork+0x25/0x30

Fixes: f765f54059 ("ksm: prepare to new THP semantics")
Link: http://lkml.kernel.org/r/20170513131040.21732-1-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
Reported-by: Federico Simoncelli <fsimonce@...hat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/ksm.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1028,8 +1028,7 @@ static int try_to_merge_one_page(struct
 		goto out;

 	if (PageTransCompound(page)) {
-		err = split_huge_page(page);
-		if (err)
+		if (split_huge_page(page))
 			goto out_unlock;
 	}