thinkpad-acpi: Avoid heap buffer overrun

thinkpad-acpi: Avoid heap buffer overrun

Avoid a heap buffer overrun triggered by an integer overflow of the
userspace controlled "count" variable.

If userspace passes in a "count" of (size_t)-1l, the kmalloc size will
overflow to ((size_t)-1l + 2) = 1, so only one byte will be allocated.
However, copy_from_user() will attempt to copy 0xFFFFFFFF (or
0xFFFFFFFFFFFFFFFF on 64bit) bytes to the buffer.

A possible testcase could look like this:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char **argv)
{
	int fd;
	char c;

	if (argc != 2) {
		printf("Usage: %s /proc/acpi/ibm/filename\n", argv[0]);
		return 1;
	}
	fd = open(argv[1], O_RDWR);
	if (fd < 0) {
		printf("Could not open proc file\n");
		return 1;
	}
	write(fd, &c, (size_t)-1l);
}

We avoid the integer overrun by putting an arbitrary limit on the count.
PAGE_SIZE sounds like a sane limit.

(note: this bug exists at least since kernel 2.6.12...)

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: stable@kernel.org