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

[Nicolas Schier <nicolas@fjasle.eu>]

On Tue, Oct 12, 2021 at 11:02:33AM +0900, Masahiro Yamada wrote:
> Date: Tue, 12 Oct 2021 11:02:33 +0900
> From: Masahiro Yamada <masahiroy@kernel.org>
> To: Nicolas Schier <nicolas@fjasle.eu>
> Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Linux Kbuild
>  mailing list <linux-kbuild@vger.kernel.org>, Thomas Kühnel
>  <thomas.kuehnel@avm.de>
> Subject: Re: [PATCH] initramfs: Check timestamp to prevent broken cpio
>  archive
> Message-ID: <CAK7LNASDu7RK0vLtx1991abx880DtQHK+U2FK3qKbH5Kcz3ipw@mail.gmail.com>
> 
> On Fri, Oct 8, 2021 at 3:59 AM Nicolas Schier <nicolas@fjasle.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

You have a local time zone offset of -9h?
I just did

$ TZ=UTC date -d@$((0xffffffff))
Sun Feb  7 06:28:15 UTC 2106

I should have mentioned the time zone info in the commit message.



> > 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@fjasle.eu>
> > Cc: Thomas Kühnel <thomas.kuehnel@avm.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

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