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From:   Siddh Raman Pant <code@siddh.me>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        syzbot+a8e049cd3abd342936b6@syzkaller.appspotmail.com,
        stable@vger.kernel.org
Message-ID: <20220823160810.181275-1-code@siddh.me>
Subject: [PATCH v2] loop: Check for overflow while configuring loop
Date:   Tue, 23 Aug 2022 21:38:10 +0530
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The userspace can configure a loop using an ioctl call, wherein
a configuration of type loop_config is passed (see lo_ioctl()'s
case on line 1550 of drivers/block/loop.c). This proceeds to call
loop_configure() which in turn calls loop_set_status_from_info()
(see line 1050 of loop.c), passing &config->info which is of type
loop_info64*. This function then sets the appropriate values, like
the offset.

loop_device has lo_offset of type loff_t (see line 52 of loop.c),
which is typdef-chained to long long, whereas loop_info64 has
lo_offset of type __u64 (see line 56 of include/uapi/linux/loop.h).

The function directly copies offset from info to the device as
follows (See line 980 of loop.c):
=09lo->lo_offset =3D info->lo_offset;

This results in an overflow, which triggers a warning in iomap_iter()
due to a call to iomap_iter_done() which has:
=09WARN_ON_ONCE(iter->iomap.offset > iter->pos);

Thus, check for negative value during loop_set_status_from_info().

Bug report: https://syzkaller.appspot.com/bug?id=3Dc620fe14aac810396d3c3edc=
9ad73848bf69a29e
Reported-and-tested-by: syzbot+a8e049cd3abd342936b6@syzkaller.appspotmail.c=
om
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes since v1:
- Do not break userspace API, so check loop_device for overflow.
- Use EOVERFLOW instead of EINVAL.

 drivers/block/loop.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index e3c0ba93c1a3..ad92192c7d61 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -979,6 +979,11 @@ loop_set_status_from_info(struct loop_device *lo,
=20
 =09lo->lo_offset =3D info->lo_offset;
 =09lo->lo_sizelimit =3D info->lo_sizelimit;
+
+=09/* loff_t vars have been assigned __u64 */
+=09if (lo->lo_offset < 0 || lo->lo_sizelimit < 0)
+=09=09return -EOVERFLOW;
+
 =09memcpy(lo->lo_file_name, info->lo_file_name, LO_NAME_SIZE);
 =09lo->lo_file_name[LO_NAME_SIZE-1] =3D 0;
 =09lo->lo_flags =3D info->lo_flags;
--=20
2.35.1