Date: Sat, 23 Mar 2024 16:00:42 +0000 From: Shawn Webb <shawn.webb@hardenedbsd.org> To: alan somers <asomers@gmail.com> Cc: Alan Somers <asomers@freebsd.org>, freebsd-hackers@freebsd.org Subject: Re: Filesystem extended attributes and Capsicum Message-ID: <56n3xzefvyd3s3b6p3ipggf5hshawrts4nezkw2oud3haihdhj@besj4txt7yar> In-Reply-To: <om7536duc4lo6zsu2rmwgtghya6uxouwvuxec6czxp3lp4y7ey@pov3ksihtthl> References: <bjeg3z2aa5owo7uur75olwuw6le2g3whzxwfgef4ozyy6w6kog@33b426ax6inf> <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com> <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq> <CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA@mail.gmail.com> <gmrellzwibhxqazp3vt22tqk2flrsyklrcwrufr4mgnhjnj3cn@rufmfxktmop5> <CAOtMX2hoXr6-6rJHRcmistroZLqyZZLeCByU_amPNPVikOnG_A@mail.gmail.com> <tnbavez4e42ovbvyjrzguf6xhydrgrn3olfcwhuouxwhzh57c6@jplqitihfove> <om7536duc4lo6zsu2rmwgtghya6uxouwvuxec6czxp3lp4y7ey@pov3ksihtthl>
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On Sat, Mar 23, 2024 at 02:54:22PM +0000, Shawn Webb wrote:
> On Sat, Mar 23, 2024 at 02:25:26PM +0000, Shawn Webb wrote:
> > On Sat, Mar 23, 2024 at 07:58:37AM -0600, alan somers wrote:
> > > On Fri, Mar 22, 2024 at 9:52=E2=80=AFPM Shawn Webb <shawn.webb@harden=
edbsd.org> wrote:
> > > >
> > > > 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@ha=
rdenedbsd.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.web=
b@hardenedbsd.org> wrote:
> > > > > > > >
> > > > > > > > Hey all,
> > > > > > > >
> > > > > > > > I'm writing an application in which I hope to enable Capsic=
um. I'm
> > > > > > > > experiencing an issue whereby extattr_get_fd fails with a f=
ile
> > > > > > > > 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 exten=
ded
> > > > > > > > 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_EX=
TATTR_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, ATTRNA=
ME_ENABLED, 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_type=3Dheads
> > > > > > > >
> > > > > > > > The library code, which is what's responsible for calling t=
he
> > > > > > > > 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 a=
re to apply
> > > > > > > > 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/maste=
r/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 som=
ething
> > > > > > > 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 pr=
int the
> > > > > > > names of any other kernel functions that return an integer va=
lue of
> > > > > > > 93. From there, guess which function is the real source of t=
he error.
> > > > > > > Then you can do
> > > > > >
> > > > > > DTrace is unavailable on this particular system.
> > > > > >
> > > > > > It does indeed fail with ENOTCAPABLE. I have the kern.trap_enot=
cap 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/Sh=
awn_Webb/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
> > > > >
> > > > > Without dtrace, you've got your work cut out for you. I suggest
> > > > > simply adding all capabilities, verifying that extattr_get_fd wor=
ks,
> > > > > and then removing capabilities until it fails. Or, run your prog=
ram
> > > > > on vanilla FreeBSD with dtrace.
> > > >
> > > > HardenedBSD doesn't have any modifications that would affect Capsic=
um
> > > > 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 .
> > > > 3 50605 foffset_lock:return .
> > > > 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.
> > >=20
> > > That's red herring. Those functions return void, but dtrace doesn't
> > > know it. So the "93" is just register garbage. I also notice that
> > > kern_extattr_get_fd isn't listed. Are you sure that your program is
> > > really failing with ENOTCAPABLE? You can also try running it with
> > > ktrace. kdump will show you exactly what capabilities you limited the
> > > file descriptor to. That can help you verify if you applied the
> > > limits correctly.
> >=20
> > I'm pretty sure it's failing with ENOTCAPABLE, since the
> > kern.trap_enotcap sysctl logic is being hit at the point of the
> > extattr_get_fd syscall. I'll see what I can do
> > with ktrace.
>=20
> ktrace output captured here:
> https://hardenedbsd.org/~shawn/2024-03-23_kdump-01.txt
>=20
> The relevant portion is pasted here:
>=20
> =3D=3D=3D=3D BEGIN ktrace OUTPUT =3D=3D=3D=3D
> 41878 hbsdctrl CALL openat(AT_FDCWD,0x6894a36d4f86,0x500000<O_RDONLY|O_=
CLOEXEC|O_PATH>)
> 41878 hbsdctrl NAMI "/scratch/tmp/ls"
> 41878 hbsdctrl RET openat 3
> 41878 hbsdctrl CALL cap_rights_limit(0x3,0x6894a36d4bc0)
> 41878 hbsdctrl STRU cap_rights_t CAP_SEEK,CAP_FSTATAT,CAP_EXTATTR_DELET=
E,CAP_EXTATTR_GET,CAP_EXTATTR_LIST,CAP_EXTATTR_SET,CAP_ACL_GET
> 41878 hbsdctrl RET cap_rights_limit 0
> 41878 hbsdctrl CALL cap_enter
> 41878 hbsdctrl RET cap_enter 0
> 41878 hbsdctrl CALL extattr_get_fd(0x3,0x2,0x33ff63afbaf,0,0)
> 41878 hbsdctrl NAMI "freebsd:system:hbsd.pax.segvguard"
> 41878 hbsdctrl CAP restricted VFS lookup
> 41878 hbsdctrl RET extattr_get_fd -1 errno 94 Not permitted in capabil=
ity mode
> 41878 hbsdctrl PSIG SIGTRAP SIG_DFL code=3DTRAP_CAP
> =3D=3D=3D=3D END ktrace OUTPUT =3D=3D=3D=3D
>=20
> As you can see, I'm adding a bunch more capabilities to the file
> descriptor. The point of failure is still extattr_get_fd returning
> ENOTCAPABLE.
>=20
> I'm beginning to suspect that while extattr_get_fd is documented to be
> allowed with a file descriptor with the CAP_EXTATTR_GET capability,
> this functionality is broken in FreeBSD.
>=20
> The thing that really confuses me is that the extattr_get_fd syscall
> entry in syscalls.master has the CAPENABLED flag set:
>=20
> =3D=3D=3D=3D BEGIN syscalls.master =3D=3D=3D=3D
> 372 AUE_EXTATTR_GET_FD STD|CAPENABLED {
> ssize_t extattr_get_fd(
> int fd,
> int attrnamespace,
> _In_z_ const char *attrname,
> _Out_writes_bytes_(nbytes) void *data,
> size_t nbytes
> );
> }
> =3D=3D=3D=3D END syscalls.master =3D=3D=3D=3D
>=20
> I think it's now time for me to file a bug report in Bugzilla.
Bug report submitted:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D277908
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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