linux-kernel - [PATCH v1] mm/madvise: report SIGBUS as -EFAULT for MADV_POPULATE

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Date:   Mon, 26 Jul 2021 17:49:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Linux API <linux-api@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>, Jann Horn <jannh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Hansen <dave.hansen@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>,
        Rolf Eike Beer <eike-kernel@...tec.de>,
        Ram Pai <linuxram@...ibm.com>, Shuah Khan <shuah@...nel.org>
Subject: [PATCH v1] mm/madvise: report SIGBUS as -EFAULT for MADV_POPULATE_(READ|WRITE)

Doing some extended tests and polishing the man page update for
MADV_POPULATE_(READ|WRITE), I realized that we end up converting also
SIGBUS (via -EFAULT) to -EINVAL, making it look like yet another
madvise() user error.

We want to report only problematic mappings and permission problems that
the user could have know as -EINVAL.

Let's not convert -EFAULT arising due to SIGBUS (or SIGSEGV) to
-EINVAL, but instead indicate -EFAULT to user space. While we could also
convert it to -ENOMEM, using -EFAULT looks more helpful when user space
might want to troubleshoot what's going wrong: MADV_POPULATE_(READ|WRITE)
is not part of an final Linux release and we can still adjust the behavior.

Fixes: 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables")
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Jann Horn <jannh@...gle.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Rik van Riel <riel@...riel.com>
Cc: Michael S. Tsirkin <mst@...hat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Richard Henderson <rth@...ddle.net>
Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc: Matt Turner <mattst88@...il.com>
Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc: "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>
Cc: Helge Deller <deller@....de>
Cc: Chris Zankel <chris@...kel.net>
Cc: Max Filippov <jcmvbkbc@...il.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Rolf Eike Beer <eike-kernel@...tec.de>
Cc: Ram Pai <linuxram@...ibm.com>
Cc: Shuah Khan <shuah@...nel.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/gup.c     | 7 +++++--
 mm/madvise.c | 4 +++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 42b8b1fa6521..b94717977d17 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1558,9 +1558,12 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
 		gup_flags |= FOLL_WRITE;
 
 	/*
-	 * See check_vma_flags(): Will return -EFAULT on incompatible mappings
-	 * or with insufficient permissions.
+	 * We want to report -EINVAL instead of -EFAULT for any permission
+	 * problems or incompatible mappings.
 	 */
+	if (check_vma_flags(vma, gup_flags))
+		return -EINVAL;
+
 	return __get_user_pages(mm, start, nr_pages, gup_flags,
 				NULL, NULL, locked);
 }
diff --git a/mm/madvise.c b/mm/madvise.c
index 6d3d348b17f4..5c065bc8b5f6 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -862,10 +862,12 @@ static long madvise_populate(struct vm_area_struct *vma,
 			switch (pages) {
 			case -EINTR:
 				return -EINTR;
-			case -EFAULT: /* Incompatible mappings / permissions. */
+			case -EINVAL: /* Incompatible mappings / permissions. */
 				return -EINVAL;
 			case -EHWPOISON:
 				return -EHWPOISON;
+			case -EFAULT: /* VM_FAULT_SIGBUS or VM_FAULT_SIGSEGV */
+				return -EFAULT;
 			default:
 				pr_warn_once("%s: unhandled return value: %ld\n",
 					     __func__, pages);
-- 
2.31.1