linux-kernel - [PATCH 3/3] prctl_set_mm: downgrade mmap

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Date:   Tue, 30 Apr 2019 10:18:44 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     gorcunov@...il.com
Cc:     akpm@...ux-foundation.org, arunks@...eaurora.org, brgl@...ev.pl,
        geert+renesas@...der.be, ldufour@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mguzik@...hat.com, mhocko@...nel.org, mkoutny@...e.com,
        rppt@...ux.ibm.com, vbabka@...e.cz, ktkhai@...tuozzo.com
Subject: [PATCH 3/3] prctl_set_mm: downgrade mmap_sem to read lock

Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
arg_start|end and env_start|end in mm_struct") we use arg_lock for
boundaries modifications. Synchronize prctl_set_mm with this lock and
keep mmap_sem for reading only (analogous to what we already do in
prctl_set_mm_map).

v2: call find_vma without arg_lock held

CC: Cyrill Gorcunov <gorcunov@...il.com>
CC: Laurent Dufour <ldufour@...ux.ibm.com>
Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
 kernel/sys.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index e1acb444d7b0..641fda756575 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2123,9 +2123,14 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = -EINVAL;
 
-	down_write(&mm->mmap_sem);
+	/*
+	 * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
+	 * a) concurrent sys_brk, b) finding VMA for addr validation.
+	 */
+	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, addr);
 
+	spin_lock(&mm->arg_lock);
 	prctl_map.start_code	= mm->start_code;
 	prctl_map.end_code	= mm->end_code;
 	prctl_map.start_data	= mm->start_data;
@@ -2213,7 +2218,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = 0;
 out:
-	up_write(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
+	up_read(&mm->mmap_sem);
 	return error;
 }
 
-- 
2.16.4