linux-kernel - [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL

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Date:   Wed, 19 Aug 2020 23:01:00 +1200
From:   Barry Song <song.bao.hua@...ilicon.com>
To:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     <linuxarm@...wei.com>, Barry Song <song.bao.hua@...ilicon.com>,
        "John Hubbard" <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Christoph Hellwig" <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Dave Chinner" <david@...morbit.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "Jonathan Corbet" <corbet@....net>, Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>, Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL_LONGTERM

gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
insensible.

since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
users from calling get_user_pages() with FOLL_LONGTERM while not with
FOLL_PIN.

mm/gup_benchmark.c used to be the only user who did this improperly.
But it has been fixed by moving to use pin_user_pages().

Cc: John Hubbard <jhubbard@...dia.com>
Cc: Jan Kara <jack@...e.cz>
Cc: Jérôme Glisse <jglisse@...hat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Dave Chinner <david@...morbit.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>
Cc: Jonathan Corbet <corbet@....net>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Shuah Khan <shuah@...nel.org>
Cc: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
---
 mm/gup.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ae096ea7583f..4da669f79566 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1789,6 +1789,25 @@ static long __get_user_pages_remote(struct mm_struct *mm,
 				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
 }
 
+static bool is_valid_gup_flags(unsigned int gup_flags)
+{
+	/*
+	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
+	 * never directly by the caller, so enforce that with an assertion:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+		return false;
+	/*
+	 * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying
+	 * that is, FOLL_LONGTERM is a specific case, more restrictive case of
+	 * FOLL_PIN.
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
+		return false;
+
+	return true;
+}
+
 /**
  * get_user_pages_remote() - pin user pages in memory
  * @mm:		mm_struct of target mm
@@ -1854,11 +1873,7 @@ long get_user_pages_remote(struct mm_struct *mm,
 		unsigned int gup_flags, struct page **pages,
 		struct vm_area_struct **vmas, int *locked)
 {
-	/*
-	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
-	 * never directly by the caller, so enforce that with an assertion:
-	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+	if (!is_valid_gup_flags(gup_flags))
 		return -EINVAL;
 
 	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
@@ -1904,11 +1919,7 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
 		struct vm_area_struct **vmas)
 {
-	/*
-	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
-	 * never directly by the caller, so enforce that with an assertion:
-	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+	if (!is_valid_gup_flags(gup_flags))
 		return -EINVAL;
 
 	return __gup_longterm_locked(current->mm, start, nr_pages,
@@ -2810,11 +2821,7 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
 int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
 {
-	/*
-	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
-	 * never directly by the caller, so enforce that:
-	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+	if (!is_valid_gup_flags(gup_flags))
 		return -EINVAL;
 
 	/*
-- 
2.27.0