linux-kernel - [PATCH] vt: don't hardcode the mem allocation upper bound

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Message-ID: <nycvar.YSQ.7.76.2003281702410.2671@knanqh.ubzr>
Date:   Sat, 28 Mar 2020 17:32:42 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     gregkh@...uxfoundation.org
cc:     Adam Borowski <kilobyte@...band.pl>,
        Chen Wandun <chenwandun@...wei.com>, jslaby@...e.com,
        daniel.vetter@...ll.ch, sam@...nborg.org, b.zolnierkie@...sung.com,
        lukas@...ner.de, ghalat@...hat.com, linux-kernel@...r.kernel.org
Subject: [PATCH] vt: don't hardcode the mem allocation upper bound

The code in vc_do_resize() bounds the memory allocation size to avoid
exceeding MAX_ORDER down the kzalloc() call chain and generating a 
runtime warning triggerable from user space. However, not only is it
unwise to use a literal value here, but MAX_ORDER may also be
configurable based on CONFIG_FORCE_MAX_ZONEORDER.
Let's use KMALLOC_MAX_SIZE instead.

Note that prior commit bb1107f7c605 ("mm, slab: make sure that 
KMALLOC_MAX_SIZE will fit into MAX_ORDER") the KMALLOC_MAX_SIZE value
could not be relied upon.

Signed-off-by: Nicolas Pitre <nico@...xnic.net>
Cc: <stable@...r.kernel.org> # v4.10+

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 15d2769805..37c5f21490 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1193,7 +1193,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 	if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
 		return 0;

-	if (new_screen_size > (4 << 20))
+	if (new_screen_size > KMALLOC_MAX_SIZE)
 		return -EINVAL;
 	newscreen = kzalloc(new_screen_size, GFP_USER);
 	if (!newscreen)