Re: [PATCH] tun: make sure interface usage can not overflow

[Kees Cook]

On Mon, Sep 29, 2014 at 4:48 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> On Sun, Sep 28, 2014 at 04:27:53PM -0700, Kees Cook wrote:
>> This makes the size argument a const, since it is always populated by
>> the caller. Additionally double-checks to make sure the copy_from_user
>> can never overflow, keeping CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy:
>>
>>    In function 'copy_from_user',
>>        inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1871:7:
>>        ... copy_from_user() buffer size is not provably correct
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> What exactly is the issue here?
> __tun_chr_ioctl is called with sizeof(struct compat_ifreq)
> or  sizeof (struct ifreq) as the last argument.

Correct. There is no vulnerability here; I am attempting to both make
the code more defensive to future changes, and to keep
CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy.

> So this looks like a false positive, but
> CONFIG_DEBUG_STRICT_USER_COPY_CHECKS machinery is supposed
> to avoid false positives.

The support in GCC is currently a bit faulty, and it seems that it
didn't notice the two callers were static values, so instead, adding
an explicit test keeps it happy.

> On which architecture is this?

This is on x86, but with CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
correctly enabled (gcc after 4.6 broke its ability to correctly
optimize), which I've been playing with trying to get gcc working
again. I sent the patch because it seems like it's a reasonable
defensive change to make.

If you want to look more deeply, there's some background here:
https://code.google.com/p/chromium/issues/detail?id=371036

https://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/commit/?h=gcc-bug&id=92dd7154932d8775a05dfd3de5564124c05a4150

Thanks,

-Kees

-- 
Kees Cook
Chrome OS Security
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