Date: Fri, 22 Mar 2024 20:07:17 -0600 From: Alan Somers <asomers@freebsd.org> To: Shawn Webb <shawn.webb@hardenedbsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Filesystem extended attributes and Capsicum Message-ID: <CAOtMX2hiWRPQM5mbrhTifnycgroQ2Y_MeCHN8bZ9mfypQYR_GA@mail.gmail.com> In-Reply-To: <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq> References: <bjeg3z2aa5owo7uur75olwuw6le2g3whzxwfgef4ozyy6w6kog@33b426ax6inf> <CAOtMX2gC3iZ6X6HotpgxgfAfPN-8C7rg3ZCP%2Bpr5e8KddasUsQ@mail.gmail.com> <ae4isfrqtyc7l3rwqd2rz6efmuztwqxw5h67xqaqts6ra6br3l@ly347trhgdoq>
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On Fri, Mar 22, 2024 at 6:56=E2=80=AFPM 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=E2=80=AFPM Shawn Webb <shawn.webb@hardened= bsd.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)): > > > > > > =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_EXTATTR_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 nothing ha= s 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 the > > > filesystem extended attribute related syscalls is here: > > > > > > https://git.hardenedbsd.org/hardenedbsd/HardenedBSD/-/tree/hardened/c= urrent/hbsdcontrol-v2/lib/libhbsdcontrol?ref_type=3Dheads > > > > > > From the rights(4) manual page, I'm instructed all I need are to appl= y > > > 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_We= bb/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 =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 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/0= 3A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.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.
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