linux-kernel - Re: [PATCH] initramfs: Check timestamp to prevent broken cpio archive

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Date:   Tue, 12 Oct 2021 11:02:33 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nicolas Schier <nicolas@...sle.eu>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Thomas Kühnel <thomas.kuehnel@....de>
Subject: Re: [PATCH] initramfs: Check timestamp to prevent broken cpio archive

On Fri, Oct 8, 2021 at 3:59 AM Nicolas Schier <nicolas@...sle.eu> wrote:
>
> Cpio format reserves 8 bytes for an ASCII representation of a time_t timestamp.
> While 2106-02-07 06:28:15 (time_t = 0xffffffff) is still some years in the
> future, a poorly chosen date string for KBUILD_BUILD_TIMESTAMP, converted into
> seconds since the epoch, might lead to exceeded cpio timestamp limits that
> results in a broken cpio archive.  Add timestamp checks to prevent overrun of
> the 8-byte cpio header field.

Out of curiosity, how did you figure out
"2106-02-07 06:28:15" was the overflow point?
Is it affected by leap seconds?


I got ffff816f


$ printf "%x"  $(date -d'2106-02-07 06:28:15'  +%s)
ffff816f




> My colleague Thomas Kühnel discovered the behaviour, when we accidentally fed
> SOURCE_DATE_EPOCH to KBUILD_BUILD_TIMESTAMP as is: some timestamps (e.g.
> 1607420928 = 2021-12-08 10:48:48) will be interpreted by `date` as a valid date
> specification of science fictional times (here: year 160742).  Even though this
> is bad input for KBUILD_BUILD_TIMESTAMP, it should not break the initramfs
> cpio format.
>
> Signed-off-by: Nicolas Schier <nicolas@...sle.eu>
> Cc: Thomas Kühnel <thomas.kuehnel@....de>
> ---
>  usr/gen_init_cpio.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
> index 03b21189d58b..983dcdd35925 100644
> --- a/usr/gen_init_cpio.c
> +++ b/usr/gen_init_cpio.c
> @@ -320,6 +320,12 @@ static int cpio_mkfile(const char *name, const char *location,
>                 goto error;
>         }
>
> +       if (buf.st_mtime > 0xffffffff) {
> +               fprintf(stderr, "%s: Timestamp exceeds maximum cpio timestamp, clipping.\n",
> +                       location);
> +               buf.st_mtime = 0xffffffff;
> +       }
> +
>         filebuf = malloc(buf.st_size);
>         if (!filebuf) {
>                 fprintf (stderr, "out of memory\n");
> @@ -551,6 +557,17 @@ int main (int argc, char *argv[])
>                 }
>         }
>
> +       /*
> +        * Timestamps after 2106-02-07 06:28:15 have an ascii hex time_t
> +        * representation that exceeds 8 chars and breaks the cpio header
> +        * specification.
> +        */
> +       if (default_mtime > 0xffffffff) {
> +               fprintf(stderr, "ERROR: Timestamp 0x%08x too large for cpio format\n",
> +                       default_mtime);
> +               exit(1);
> +       }
> +
>         if (argc - optind != 1) {
>                 usage(argv[0]);
>                 exit(1);
> --
> 2.30.1
>


-- 
Best Regards
Masahiro Yamada