Date: Sat, 23 Mar 2024 00:56:10 +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: <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq> In-Reply-To: <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com> References: <bjeg3z2aa5owo7uur75olwuw6le2g3whzxwfgef4ozyy6w6kog@33b426ax6inf> <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com>
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On Fri, Mar 22, 2024 at 06:20:48PM -0600, Alan Somers wrote:
> On Fri, Mar 22, 2024 at 5:38 PM Shawn Webb <shawn.webb@hardenedbsd.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)):
> >
> > ==== BEGIN CODE ====
> > static int
> > open_file(const char *path)
> > {
> > cap_rights_t rights;
> > int fd;
> >
> > fd = open(path, O_PATH | O_CLOEXEC);
> > if (fd == -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);
> > }
> > ==== END CODE ====
> >
> > Eventually, after calling cap_enter(2), the following code is called:
> >
> > ==== BEGIN CODE ====
> > #define ATTRNAME_ENABLED "hbsd.pax.aslr"
> > sz = extattr_get_fd(fd, ctx->hc_namespace, ATTRNAME_ENABLED, NULL, 0);
> > if (sz <= 0) {
> > if (errno == ENOATTR) {
> > /*
> > * This is okay, it just means that nothing has been set.
> > * No error condition here.
> > */
> > return (RES_SUCCESS);
> > }
> > return (RES_FAIL);
> > }
> > ==== END CODE ====
> >
> > For reference, the program's code is here:
> > https://git.hardenedbsd.org/shawn.webb/hbsdctrl/-/tree/main?ref_type=heads
> >
> > 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=heads
> >
> > From the rights(4) manual page, I'm instructed all I need are to apply
> > those capabilities to that file descriptor:
> >
> > ==== BEGIN PASTE ====
> > 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).
> > ==== END PASTE ====
> >
> > 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 == 93 && pid == $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/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc
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