Re: WARNING in ex_handler_uaccess

From: Rasmus Villemoes
Date: Mon Sep 21 2020 - 06:22:25 EST

On 19/09/2020 02.17, Al Viro wrote:
> On Fri, Sep 18, 2020 at 05:07:43PM -0700, Andy Lutomirski wrote:
>> On Fri, Sep 18, 2020 at 4:55 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>>> On Fri, Sep 18, 2020 at 04:31:33PM -0700, Andy Lutomirski wrote:
>>>> check_zeroed_user() looks buggy. It does:
>>>> if (!user_access_begin(from, size))
>>>> return -EFAULT;
>>>> unsafe_get_user(val, (unsigned long __user *) from, err_fault);
>>>> This is wrong if size < sizeof(unsigned long) -- you read outside the
>>>> area you verified using user_access_begin().
>>> Read the code immediately prior to that. from will be word-aligned,
>>> and size will be extended accordingly. If the area acceptable for
>>> user_access_begin() ends *NOT* on a word boundary, you have a problem
>>> and I would strongly recommend to seek professional help.
>>> All reads in that thing are word-aligned and word-sized. So I very
>>> much doubt that your analysis is correct.
>> Maybe -ETOOTIRED, but I seriously question the math in here. Suppose
>> from == (unsigned long *)1 and size == 1. Then align is 1, and we do:
>> from -= align;
>> size += align;
>> So now from = 0 and size = 2. Now we do user_access_begin(0, 2) and
>> then immediately read 4 or 8 bytes. No good.
> Could you explain what kind of insane hardware manages to do #PF-related
> checks (including SMAP, whatever) with *sub*WORD* granularity?
> If it's OK with 16bit read from word-aligned address, but barfs on 64bit
> one... I want to know what the hell had its authors been smoking.

So, not sure how the above got triggered, but I notice there might be an
edge case in check_zeroed_user():

from -= align;
size += align;

if (!user_read_access_begin(from, size))
return -EFAULT;

unsafe_get_user(val, (unsigned long __user *) from, err_fault);

Suppose size is (size_t)-3 and align is 3. What's the convention for
access_ok(whatever, 0)? Is that equivalent to access_ok(whatever, 1), or
is it always true (or $ARCH-dependent)?

But, AFAICT, no current caller of check_zeroed_user can end up passing
in a size that can overflow to 0. E.g. for the case at hand, size cannot
be more than SIZE_MAX-24.