linux-kernel - [PATCH 3.16 141/157] floppy: fix out-of-bounds read in copy

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Message-ID: <lsq.1565469607.528093703@decadent.org.uk>
Date:   Sat, 10 Aug 2019 21:40:07 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Willy Tarreau" <w@....eu>, "Denis Efremov" <efremov@...ras.ru>
Subject: [PATCH 3.16 141/157] floppy: fix out-of-bounds read in copy_buffer

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Denis Efremov <efremov@...ras.ru>

commit da99466ac243f15fbba65bd261bfc75ffa1532b6 upstream.

This fixes a global out-of-bounds read access in the copy_buffer
function of the floppy driver.

The FDDEFPRM ioctl allows one to set the geometry of a disk.  The sect
and head fields (unsigned int) of the floppy_drive structure are used to
compute the max_sector (int) in the make_raw_rw_request function.  It is
possible to overflow the max_sector.  Next, max_sector is passed to the
copy_buffer function and used in one of the memcpy calls.

An unprivileged user could trigger the bug if the device is accessible,
but requires a floppy disk to be inserted.

The patch adds the check for the .sect * .head multiplication for not
overflowing in the set_geometry function.

The bug was found by syzkaller.

Signed-off-by: Denis Efremov <efremov@...ras.ru>
Tested-by: Willy Tarreau <w@....eu>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/block/floppy.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -3236,8 +3236,10 @@ static int set_geometry(unsigned int cmd
 	int cnt;

 	/* sanity checking for parameters. */
-	if (g->sect <= 0 ||
-	    g->head <= 0 ||
+	if ((int)g->sect <= 0 ||
+	    (int)g->head <= 0 ||
+	    /* check for overflow in max_sector */
+	    (int)(g->sect * g->head) <= 0 ||
 	    /* check for zero in F_SECT_PER_TRACK */
 	    (unsigned char)((g->sect << 2) >> FD_SIZECODE(g)) == 0 ||
 	    g->track <= 0 || g->track > UDP->tracks >> STRETCH(g) ||