Re: Filesystem extended attributes and Capsicum

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