linux-kernel - Re: [PATCH] module: fix signature check failures when using in-kernel decompression

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Message-ID: <CAD=FV=UQdT==0KH+VsWcmHikTpwdDMSTkBa_GXRhXPacwrkR5Q@mail.gmail.com>
Date:   Fri, 14 Jan 2022 08:12:41 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] module: fix signature check failures when using in-kernel decompression

Hi,

On Thu, Jan 13, 2022 at 4:52 PM Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
>
> The new flag MODULE_INIT_COMPRESSED_FILE unintentionally trips check in
> module_sig_check(). The check was supposed to catch case when version
> info or magic was removed from a signed module, making signature
> invalid, but it was coded too broadly and was catching this new flag as
> well.
>
> Change the check to only test the 2 particular flags affecting signature
> validity.
>
> Fixes: b1ae6dc41eaa ("module: add in-kernel support for decompressing")
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>  kernel/module.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 201398d58079..24dab046e16c 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2883,12 +2883,13 @@ static int module_sig_check(struct load_info *info, int flags)
>         const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
>         const char *reason;
>         const void *mod = info->hdr;
> -
> +       bool mangled_module = flags & (MODULE_INIT_IGNORE_MODVERSIONS |
> +                                      MODULE_INIT_IGNORE_VERMAGIC);
>         /*
> -        * Require flags == 0, as a module with version information
> -        * removed is no longer the module that was signed
> +        * Do not allow mangled modules as a module with version information
> +        * removed is no longer the module that was signed.
>          */
> -       if (flags == 0 &&
> +       if (!mangled_module &&

Seems reasonable to me. I guess the only question in my mind is
whether this is the best way to handle "unknown" new flags. If someone
introduces a new flag to "uapi/linux/module.h" should we consider it
as "mangled" or not? Given that I can't predict the future and the
comments seem to indicate that we're only trying to detect
version-related issues, your choice seems OK to me.

Reviewed-by: Douglas Anderson <dianders@...omium.org>