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

[Masahiro Yamada]

On Tue, Oct 12, 2021 at 5:29 PM Nicolas Schier <nicolas@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > To: Nicolas Schier <nicolas@xxxxxxxxx>
> > Cc: Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Linux Kbuild
> >  mailing list <linux-kbuild@xxxxxxxxxxxxxxx>, Thomas Kühnel
> >  <thomas.kuehnel@xxxxxx>
> > Subject: Re: [PATCH] initramfs: Check timestamp to prevent broken cpio
> >  archive
> > Message-ID: <CAK7LNASDu7RK0vLtx1991abx880DtQHK+U2FK3qKbH5Kcz3ipw@xxxxxxxxxxxxxx>
> >
> > On Fri, Oct 8, 2021 at 3:59 AM Nicolas Schier <nicolas@xxxxxxxxx> 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.
>


Ah, time zone!

Right, my time zone is JST (+09:00).

And, thanks for the update.





>
>
> > > 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@xxxxxxxxx>
> > > Cc: Thomas Kühnel <thomas.kuehnel@xxxxxx>
> > > ---
> > >  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
>
> --
> epost|xmpp: nicolas@xxxxxxxxx          irc://oftc.net/nsc
> ↳ gpg: 18ed 52db e34f 860e e9fb  c82b 7d97 0932 55a0 ce7f
>      -- frykten for herren er opphav til kunnskap --



-- 
Best Regards
Masahiro Yamada