linux-kernel - Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl

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Message-Id: <20210621213024.1698133-1-avlarkin82@gmail.com>
Date:   Tue, 22 Jun 2021 00:30:24 +0300
From:   Alexander Larkin <avlarkin82@...il.com>
To:     torvalds@...ux-foundation.org
Cc:     avlarkin82@...il.com, dan.carpenter@...cle.com,
        dmitry.torokhov@...il.com, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org, murray.mcallister@...il.com,
        security@...nel.org
Subject: Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl

Also I did userspace test (that shows how kernel overwrites (out of array bound) the userspace):
1. The buttons is "__u16 buttons[5]" in userspace,
2. buttons[5] = 1;
3. ioctl(fd, JSIOCGBTNMAP, buttons)
4. printf("new %i\n", buttons[5]),
and the output is "new 0", so the value is being overwritten by kernel (so 1024 bytes copied
to 10 bytes buffer).

It looks like I don't understand what is the expected value of the _IOC_SIZE(cmd),
why not 10 for this read ioctl example? Is it possible to make this ioctl safe, so
it doesn't copy more data than user can handle?