Date: Sat, 23 Mar 2024 14:54:22 +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: <om7536duc4lo6zsu2rmwgtghya6uxouwvuxec6czxp3lp4y7ey@pov3ksihtthl> In-Reply-To: <tnbavez4e42ovbvyjrzguf6xhydrgrn3olfcwhuouxwhzh57c6@jplqitihfove> 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>
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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@hardened=
bsd.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@hard=
enedbsd.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@=
hardenedbsd.org> wrote:
> > > > > > >
> > > > > > > Hey all,
> > > > > > >
> > > > > > > I'm writing an application in which I hope to enable Capsicum=
=2E I'm
> > > > > > > experiencing an issue whereby extattr_get_fd fails with a file
> > > > > > > descriptor that has all the extended attribute capabilities e=
nabled
> > > > > > > (CAP_EXTATTR_DELETE, CAP_EXTATTR_GET, CAP_EXTATTR_LIST, and
> > > > > > > CAP_EXTATTR_SET).
> > > > > > >
> > > > > > > Looking at the kernel source (sys/kern/vfs_extattr.c) tells m=
e 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_EXTA=
TTR_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 no=
thing 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?r=
ef_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/ha=
rdened/current/hbsdcontrol-v2/lib/libhbsdcontrol?ref_type=3Dheads
> > > > > > >
> > > > > > > From the rights(4) manual page, I'm instructed all I need are=
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/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 somet=
hing
> > > > > > like this:
> > > > > >
> > > > > > dtrace -i 'fbt:kernel::return /arg1 =3D=3D 93 && pid =3D=3D $ta=
rget/
> > > > > > {trace(".");}' -c ./my_application
> > > > > >
> > > > > > That will print the name of every non-inlined kernel function t=
hat
> > > > > > returns ENOTCAPABLE during your process. But it will also prin=
t the
> > > > > > names of any other kernel functions that return an integer valu=
e 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_enotca=
p 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/Shaw=
n_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 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 $t=
arget/ {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.
ktrace output captured here:
https://hardenedbsd.org/~shawn/2024-03-23_kdump-01.txt
The relevant portion is pasted here:
=3D=3D=3D=3D BEGIN ktrace OUTPUT =3D=3D=3D=3D
41878 hbsdctrl CALL openat(AT_FDCWD,0x6894a36d4f86,0x500000<O_RDONLY|O_CL=
OEXEC|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_DELETE,=
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 capabilit=
y mode
41878 hbsdctrl PSIG SIGTRAP SIG_DFL code=3DTRAP_CAP
=3D=3D=3D=3D END ktrace OUTPUT =3D=3D=3D=3D
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.
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.
The thing that really confuses me is that the extattr_get_fd syscall
entry in syscalls.master has the CAPENABLED flag set:
=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
I think it's now time for me to file a bug report in Bugzilla.
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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