linux-kernel - Re: [PATCH] thinkpad-acpi: Avoid heap buffer overrun

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Date:	Tue, 21 Jul 2009 12:47:09 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	ibm-acpi-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thinkpad-acpi: Avoid heap buffer overrun

On Tuesday 21 July 2009 12:17:47 Michael Buesch wrote:
> On Tuesday 21 July 2009 12:16:17 Michael Buesch wrote:
> > 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);
> > }

Hm, I just realized that for it to crash the source buffer must be big enough.
&c is on the stack. Maybe that's big enough to overrun the kmalloc()acted chunk
(there may be padding beyond the 1 byte). If it's not big enough, one must
malloc()ate a big buffer and pass it to write() instead.

This is all theoretical stuff, because I can't test it, but it looks sane. :)

> > We avoid the integer overrun by putting an arbitrary limit on the count.
> > PAGE_SIZE sounds like a sane limit.
> > 
> > Signed-off-by: Michael Buesch <mb@...sch.de>
> 
> Whoops, forgot
> Cc: stable@...nel.org



-- 
Greetings, Michael.
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