Date: Fri, 22 Mar 2024 20:07:17 -0600 From: Alan Somers <asomers@freebsd.org> To: Shawn Webb <shawn.webb@hardenedbsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Filesystem extended attributes and Capsicum Message-ID: <CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA@mail.gmail.com> In-Reply-To: <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq> References: <bjeg3z2aa5owo7uur75olwuw6le2g3whzxwfgef4ozyy6w6kog@33b426ax6inf> <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com> <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, Mar 22, 2024 at 6:56=E2=80=AFPM Shawn Webb <shawn.webb@hardenedbsd.=
org> wrote:
>
> On Fri, Mar 22, 2024 at 06:20:48PM -0600, Alan Somers wrote:
> > On Fri, Mar 22, 2024 at 5:38=E2=80=AFPM Shawn Webb <shawn.webb@hardened=
bsd.org> wrote:
> > >
> > > Hey all,
> > >
> > > I'm writing an application in which I hope to enable Capsicum. I'm
> > > experiencing an issue whereby extattr_get_fd fails with a file
> > > descriptor that has all the extended attribute capabilities enabled
> > > (CAP_EXTATTR_DELETE, CAP_EXTATTR_GET, CAP_EXTATTR_LIST, and
> > > CAP_EXTATTR_SET).
> > >
> > > Looking at the kernel source (sys/kern/vfs_extattr.c) tells me that
> > > kern_extattr_get_fd only requires CAP_EXTATTR_GET.
> > >
> > > So I'm a bit puzzled as to why my call to extattr_get_fd(2) is
> > > failing. Am I doing something wrong or are filesystem extended
> > > attributes not supported in a Capabilities-enabled process?
> > >
> > > Here's how I'm creating the file descriptor (before calling
> > > cap_enter(2)):
> > >
> > > =3D=3D=3D=3D BEGIN CODE =3D=3D=3D=3D
> > > static int
> > > open_file(const char *path)
> > > {
> > > cap_rights_t rights;
> > > int fd;
> > >
> > > fd =3D open(path, O_PATH | O_CLOEXEC);
> > > if (fd =3D=3D -1) {
> > > return (-1);
> > > }
> > >
> > > memset(&rights, 0, sizeof(rights));
> > > cap_rights_init(&rights, CAP_EXTATTR_DELETE, CAP_EXTATTR_GET,
> > > CAP_EXTATTR_LIST, CAP_EXTATTR_SET);
> > > cap_rights_limit(fd, &rights);
> > >
> > > return (fd);
> > > }
> > > =3D=3D=3D=3D END CODE =3D=3D=3D=3D
> > >
> > > Eventually, after calling cap_enter(2), the following code is called:
> > >
> > > =3D=3D=3D=3D BEGIN CODE =3D=3D=3D=3D
> > > #define ATTRNAME_ENABLED "hbsd.pax.aslr"
> > > sz =3D extattr_get_fd(fd, ctx->hc_namespace, ATTRNAME_ENABLED=
, NULL, 0);
> > > if (sz <=3D 0) {
> > > if (errno =3D=3D ENOATTR) {
> > > /*
> > > * This is okay, it just means that nothing ha=
s been set.
> > > * No error condition here.
> > > */
> > > return (RES_SUCCESS);
> > > }
> > > return (RES_FAIL);
> > > }
> > > =3D=3D=3D=3D END CODE =3D=3D=3D=3D
> > >
> > > For reference, the program's code is here:
> > > https://git.hardenedbsd.org/shawn.webb/hbsdctrl/-/tree/main?ref_type=
=3Dheads
> > >
> > > The library code, which is what's responsible for calling the
> > > filesystem extended attribute related syscalls is here:
> > >
> > > https://git.hardenedbsd.org/hardenedbsd/HardenedBSD/-/tree/hardened/c=
urrent/hbsdcontrol-v2/lib/libhbsdcontrol?ref_type=3Dheads
> > >
> > > From the rights(4) manual page, I'm instructed all I need are to appl=
y
> > > those capabilities to that file descriptor:
> > >
> > > =3D=3D=3D=3D BEGIN PASTE =3D=3D=3D=3D
> > > CAP_EXTATTR_DELETE Permit extattr_delete_fd(2).
> > >
> > > CAP_EXTATTR_GET Permit extattr_get_fd(2).
> > >
> > > CAP_EXTATTR_LIST Permit extattr_list_fd(2).
> > >
> > > CAP_EXTATTR_SET Permit extattr_set_fd(2).
> > > =3D=3D=3D=3D END PASTE =3D=3D=3D=3D
> > >
> > > So I'm a bit unsure if I'm doing something wrong.
> > >
> > > Thanks,
> > >
> > > --
> > > Shawn Webb
> > > Cofounder / Security Engineer
> > > HardenedBSD
> > >
> > > Tor-ified Signal: +1 303-901-1600 / shawn_webb_opsec.50
> > > https://git.hardenedbsd.org/hardenedbsd/pubkeys/-/raw/master/Shawn_We=
bb/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
> >
> > What error code does it fail with? If it's ENOTCAPABLE, then I
> > suggest using dtrace to find the reason why it fails. Do something
> > like this:
> >
> > dtrace -i 'fbt:kernel::return /arg1 =3D=3D 93 && pid =3D=3D $target/
> > {trace(".");}' -c ./my_application
> >
> > That will print the name of every non-inlined kernel function that
> > returns ENOTCAPABLE during your process. But it will also print the
> > names of any other kernel functions that return an integer value of
> > 93. From there, guess which function is the real source of the error.
> > Then you can do
>
> DTrace is unavailable on this particular system.
>
> It does indeed fail with ENOTCAPABLE. I have the kern.trap_enotcap sysctl
> set to 1 so that I can know at exactly what point we're failing, and
> it's indeed at extattr_get_fd.
>
> Thanks,
>
> --
> Shawn Webb
> Cofounder / Security Engineer
> HardenedBSD
>
> Tor-ified Signal: +1 303-901-1600 / shawn_webb_opsec.50
> https://git.hardenedbsd.org/hardenedbsd/pubkeys/-/raw/master/Shawn_Webb/0=
3A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
Without dtrace, you've got your work cut out for you. I suggest
simply adding all capabilities, verifying that extattr_get_fd works,
and then removing capabilities until it fails. Or, run your program
on vanilla FreeBSD with dtrace.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA>