Date: Sat, 23 Mar 2024 03:52:00 +0000 From: Shawn Webb <shawn.webb@hardenedbsd.org> To: Alan Somers <asomers@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Filesystem extended attributes and Capsicum Message-ID: <gmrellzwibhxqazp3vt22tqk2flrsyklrcwrufr4mgnhjnj3cn@rufmfxktmop5> In-Reply-To: <CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA@mail.gmail.com> References: <bjeg3z2aa5owo7uur75olwuw6le2g3whzxwfgef4ozyy6w6kog@33b426ax6inf> <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com> <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq> <CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA@mail.gmail.com>
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On Fri, Mar 22, 2024 at 08:07:17PM -0600, Alan Somers wrote:
> On Fri, Mar 22, 2024 at 6:56=E2=80=AFPM Shawn Webb <shawn.webb@hardenedbs=
d.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@harden=
edbsd.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_GE=
T,
> > > > 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 calle=
d:
> > > >
> > > > =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_ENABL=
ED, NULL, 0);
> > > > if (sz <=3D 0) {
> > > > if (errno =3D=3D ENOATTR) {
> > > > /*
> > > > * This is okay, it just means that nothing =
has 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_typ=
e=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=
/current/hbsdcontrol-v2/lib/libhbsdcontrol?ref_type=3Dheads
> > > >
> > > > From the rights(4) manual page, I'm instructed all I need are to ap=
ply
> > > > 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_=
Webb/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 sysc=
tl
> > 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=
/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
>=20
> 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.
HardenedBSD doesn't have any modifications that would affect Capsicum
in this manner. Regardless, I reproduced the problem successfully on
FreeBSD 14.0-RELEASE without any code changes. I tried running your
DTrace script on FreeBSD 14.0-RELEASE and got this output:
=3D=3D=3D=3D BEGIN OUTPUT =3D=3D=3D=3D
$ sudo dtrace -i 'fbt:kernel::return /arg1 =3D=3D 93 && pid =3D=3D $target/=
{trace(".");}' -c "obj/hbsdctrl pax list /bin/ls"
dtrace: description 'fbt:kernel::return ' matched 31396 probes
aslr: sysdef
mprotect: sysdef
pageexec: sysdef
segvguard: sysdef
dtrace: pid 29270 has exited
CPU ID FUNCTION:NAME
3 47780 foffset_unlock_uio:return . =
=20
3 50605 foffset_lock:return . =
=20
3 47778 foffset_lock_uio:return .
=3D=3D=3D=3D END OUTPUT =3D=3D=3D=3D
But I'm still unsure what I'm missing, if anything.
Thanks,
--=20
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/03A=
4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
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