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From:   Ke Wu <mikewu@google.com>
Date:   Fri, 31 May 2019 11:03:17 -0700
Message-ID: <CANRnR9Q4AY1nyTebWgkVM-wUfZv7kKNLnsrchWPBH4HPbB-X6Q@mail.gmail.com>
Subject: Re: security/loadpin: Allow to exclude specific file types
To:     Colin Ian King <colin.king@canonical.com>
Cc:     Kees Cook <keescook@chromium.org>,
        James Morris <jamorris@linux.microsoft.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        linux-security-module@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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I think Coverity is correct. Note that it's the size of
kernel_read_file_str (rather than exclude_read_files) doesn't equal to
ignore_read_file_id.

This is because READING_MAX_ID is also an element in
kernel_read_file_str, which makes the size of kernel_read_file_str to
be READING_MAX_ID+1. I will send a new patch to fix the issue. Thanks
for the  analysis!


On Fri, May 31, 2019 at 7:49 AM Colin Ian King <colin.king@canonical.com> wrote:
>
> On 31/05/2019 15:44, Kees Cook wrote:
> > On Fri, May 31, 2019 at 11:46:29AM +0100, Colin Ian King wrote:
> >> Hi,
> >>
> >> Static analysis with Coverity on linux-next has found a potential issue
> >> with the following commit:
> >>
> >> commit 1633a4f04cc171fc638deb5c95af96032d3c591b
> >> Author: Ke Wu <mikewu@google.com>
> >> Date:   Thu May 30 12:22:08 2019 -0700
> >>
> >>     security/loadpin: Allow to exclude specific file types
> >>
> >>
> >> 209                for (j = 0; j < ARRAY_SIZE(kernel_read_file_str); j++) {
> >> 210                        if (strcmp(cur, kernel_read_file_str[j]) == 0) {
> >> 211                                pr_info("excluding: %s\n",
> >> 212                                        kernel_read_file_str[j]);
> >>
> >> CID 81977 (#1 of 1): Out-of-bounds write
> >> overrun-local: Overrunning array ignore_read_file_id of 8 4-byte
> >> elements at element index 8 (byte offset 35) using index j (which
> >> evaluates to 8).
> >>
> >> 213                                ignore_read_file_id[j] = 1;
> >>
> >> According to Coverity ignore_read_file_id is an array of 8 integers.
> >> However, ARRAY_SIZE(kernel_read_file_str) is 9, so we have an out of
> >> bounds write on ignore_read_file[j] when j is 8.
> >
> > What am I missing? This doesn't fail the build:
> >
> > +       BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
> > +                    ARRAY_SIZE(ignore_read_file_id));
> >
> > They have the same number of elements.
> >
>
> Yep, that's very true. I'll discuss this with Coverity as this seems
> like a weird false positive.
>
> Apologies for the noise.
>
> Colin


-- 
Ke Wu | Software Engineer | mikewu@google.com | Google Inc.