linux-kernel - [PATCH] mm/hugetlb: __get_user_pages ignores certain follow_hugetlb

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Message-Id: <1500406795-58462-1-git-send-email-daniel.m.jordan@oracle.com>
Date:   Tue, 18 Jul 2017 12:39:55 -0700
From:   daniel.m.jordan@...cle.com
To:     linux-mm@...ck.org
Cc:     aarcange@...hat.com, akpm@...ux-foundation.org,
        aneesh.kumar@...ux.vnet.ibm.com, gerald.schaefer@...ibm.com,
        james.morse@....com, kirill.shutemov@...ux.intel.com,
        mhocko@...e.com, mike.kravetz@...cle.com,
        n-horiguchi@...jp.nec.com, punit.agrawal@....com,
        zhongjiang@...wei.com, linux-kernel@...r.kernel.org
Subject: [PATCH] mm/hugetlb: __get_user_pages ignores certain follow_hugetlb_page errors

Commit 9a291a7c9428 ("mm/hugetlb: report -EHWPOISON not -EFAULT when
FOLL_HWPOISON is specified") causes __get_user_pages to ignore certain
errors from follow_hugetlb_page.  After such error, __get_user_pages
subsequently calls faultin_page on the same VMA and start address that
follow_hugetlb_page failed on instead of returning the error immediately
as it should.

In follow_hugetlb_page, when hugetlb_fault returns a value covered under
VM_FAULT_ERROR, follow_hugetlb_page returns it without setting nr_pages
to 0 as __get_user_pages expects in this case, which causes the
following to happen in __get_user_pages: the "while (nr_pages)" check
succeeds, we skip the "if (!vma..." check because we got a VMA the last
time around, we find no page with follow_page_mask, and we call
faultin_page, which calls hugetlb_fault for the second time.

This issue also slightly changes how __get_user_pages works.  Before, it
only returned error if it had made no progress (i = 0).  But now,
follow_hugetlb_page can clobber "i" with an error code since its new
return path doesn't check for progress.  So if "i" is nonzero before a
failing call to follow_hugetlb_page, that indication of progress is lost
and __get_user_pages can return error even if some pages were
successfully pinned.

To fix this, change follow_hugetlb_page so that it updates nr_pages,
allowing __get_user_pages to fail immediately and restoring the "error
only if no progress" behavior to __get_user_pages.

Tested that __get_user_pages returns when expected on error from
hugetlb_fault in follow_hugetlb_page.

Fixes: 9a291a7c9428 ("mm/hugetlb: report -EHWPOISON not -EFAULT when FOLL_HWPOISON is specified")
Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc: James Morse <james.morse@....com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: Punit Agrawal <punit.agrawal@....com>
Cc: zhong jiang <zhongjiang@...wei.com>
---
 mm/hugetlb.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3eedb18..cc28993 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4095,6 +4095,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	unsigned long vaddr = *position;
 	unsigned long remainder = *nr_pages;
 	struct hstate *h = hstate_vma(vma);
+	int err = -EFAULT;

 	while (vaddr < vma->vm_end && remainder) {
 		pte_t *pte;
@@ -4170,11 +4171,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			}
 			ret = hugetlb_fault(mm, vma, vaddr, fault_flags);
 			if (ret & VM_FAULT_ERROR) {
-				int err = vm_fault_to_errno(ret, flags);
-
-				if (err)
-					return err;
-
+				err = vm_fault_to_errno(ret, flags);
 				remainder = 0;
 				break;
 			}
@@ -4229,7 +4226,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	 */
 	*position = vaddr;

-	return i ? i : -EFAULT;
+	return i ? i : err;
 }

 #ifndef __HAVE_ARCH_FLUSH_HUGETLB_TLB_RANGE
-- 
1.8.3.1