Re: Filesystem extended attributes and Capsicum
- Reply: Shawn Webb : "Re: Filesystem extended attributes and Capsicum"
- In reply to: alan somers : "Re: Filesystem extended attributes and Capsicum"
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Date: Sat, 23 Mar 2024 14:25:26 UTC
On Sat, Mar 23, 2024 at 07:58:37AM -0600, alan somers wrote:
> On Fri, Mar 22, 2024 at 9:52 PM Shawn Webb <shawn.webb@hardenedbsd.org> wrote:
> >
> > On Fri, Mar 22, 2024 at 08:07:17PM -0600, Alan Somers wrote:
> > > On Fri, Mar 22, 2024 at 6:56 PM Shawn Webb <shawn.webb@hardenedbsd.org> wrote:
> > > >
> > > > 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
> > >
> > > 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:
> >
> > ==== BEGIN OUTPUT ====
> > $ sudo dtrace -i 'fbt:kernel::return /arg1 == 93 && pid == $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 .
> > ==== END OUTPUT ====
> >
> > But I'm still unsure what I'm missing, if anything.
>
> 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.
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.
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