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From:   Alexander Larkin <avlarkin82@gmail.com>
To:     torvalds@linux-foundation.org
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        dmitry.torokhov@gmail.com, linux-input@vger.kernel.org,
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Subject: Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl
Date:   Tue, 22 Jun 2021 00:30:24 +0300
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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?