Anomalous output from getpcaps(1) for process with all capabilities
Serge E. Hallyn
serge at hallyn.com
Sat Dec 21 03:08:45 UTC 2019
I believe this is coming from the kernel. I booted an old 12.04
ubuntu server livecd (3.13 kernel) and libcap 2.22, it shows
Capabilities for `1': =ep
This was running
libcap 2.22. Same version of libcap compiled on my laptop gives me the
long output. booting an old 14.04 livecd, running 4.4 kernel,
getpcap also gives the long output.
On Mon, Dec 16, 2019 at 07:47:44PM -0800, Andrew G. Morgan wrote:
> Serge might want to comment. I seem to recall that the issue was relevant
> for Serge's namespaces and checkpointing/restarting over a kernel upgrade.
> But it has been more than a decade and I can't seem to find the email
> exchange we had back then.
>
> Cheers
>
> Andrew
>
>
> On Sun, Dec 15, 2019 at 8:52 PM Michael Kerrisk (man-pages) <
> mtk.manpages at gmail.com> wrote:
>
> > Hello Andrew,
> >
> > On 12/16/19 12:26 AM, Andrew G. Morgan wrote:
> > > [Resend with reply-all this time.]
> > >
> > > On Sun, Dec 15, 2019 at 11:17 AM Michael Kerrisk (man-pages)
> > > <mtk.manpages at gmail.com> wrote:
> > >>
> > >> Hello Andrew,
> > >>
> > >> On Sun, 15 Dec 2019 at 19:30, Andrew G. Morgan <morgan at kernel.org>
> > wrote:
> > >>>
> > >>> This changed with this commit I think:
> > >>>
> > >>>
> > https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/libcap/cap_text.c?id=3fa808f5886d08c45866217cfe6e6e9def7de04e
> > >>>
> > >>> Prior to that, we had "=ep" mean the cap set was ~0 for
> > >>> all the bitmasks in e and p. When we started to clip the
> > >>> bounding set to only defined capabilities, it meant that the
> > >>> text output started to look like
> > >>> "=ep 33,34,35,36,37,38,39,40,41,42.....63-ep" which
> > >>> was quite terrible.
> > >>
> > >> Was that really the change that caused this? That's a 2008 commit.
> > >> Certainly, I recall in 2014 or 2015 still being able to see =ep, and I
> > >> presume (but do not recall for sure) that I was using a system with a
> > >> libcap more recent than v2.11 (of which that commit was a part).
> > >
> > > The only other commit that seems relevant was this one:
> > >
> > >
> > https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/libcap/cap_text.c?id=afb17b8c007a49d93b0d30936b2d65af1bfdb039
> > >
> > > But I think this was all part of the same effort.
> > >
> > >>> So, this was seen as the least worst option.
> > >>
> > >> But surely this is fixable? Or, to put it another was, I presume
> > >> there's something that makes this difficult to fix in getpcaps, but
> > >> what is that something?
> > >
> > > I recall spending a day or more trying to avoid it, but I can't see
> > > how it is really fixable because there are too many moving parts.
> > >
> > > If the kernel adds another capability, then =ep could reasonably mean
> > > both the "old full set" or the "new full set". There are contexts
> > > where the difference matters. For example, where folk are using text
> > > representations for things like package manager shell-script setups.
> > > What they get when they say "=ep
> > > cap_setpcap,cap_sys_admin,cap_setfcap-ep" today, might suddenly be
> > > inappropriate when the new kernel adds "cap_self_destruct". At least
> > > the way it is at present, we are very explicit about what is in
> > > effect
> >
> > I can sort of see how the idea you are expressing might
> > apply when *setting* capabilities on *files*, but:
> >
> > a) I'm talking about the *display* of capabilities of a running
> > *process* using getpcaps(8).
> >
> > b) In practice, the logic that actually applies when setting
> > capabilities on files seems to run *counter* to the idea
> > that you express above (if I understand you correctly),
> > and your argument seems more relevant to files (especially
> > when *setting* file capabilities) than to processes.
> >
> > Consider the following examples:
> >
> > 1. A binary that has all but one capability is described in a
> > compact way by getcap(8):
> >
> > $ sudo setcap "=p cap_kill-p" mypog
> > $ getcap mypog
> > mypog =p cap_kill-p
> >
> > When that same binary is run, the process's capabilities
> > are described with a very different format by getpcaps(8)
> >
> > $ getpcaps $(pidof i_syscall)
> > Capabilities for `152006': =
> > cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,
> > cap_fsetid,cap_setgid,cap_setuid,cap_setpcap,
> > cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,
> > cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,
> > cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,
> > cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,
> > cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,
> > cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,
> > cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,
> > cap_block_suspend,cap_audit_read+p
> >
> > That is quite inconsistent! And also, the second notation is
> > simply very hard to read. How many capabilities are listed there?
> > Is it all of them? (When a process does have all caps in permitted,
> > the display differs only by one item.) A security-related notation
> > that is difficult to read is a risk... [*]
> >
> > 2. I just now tried the following, and it really surprised me
> > (although the reasons are quickly obvious when one considers
> > the point I made earlier in this mail thread that 'setcap =p' is
> > setting *64* bits in the file's permittted set):
> >
> > # Set "all" permitted capabilities on a file:
> >
> > $ sudo setcap =p myprog
> > $ getcap myprog
> > myprog =p
> >
> > # Set "all" 38 permitted capabilities on a file, by specifying
> > # them individually:
> >
> > $ sudo setcap 0$(for c in $(seq 1 37); do \
> > echo -n ",$c"; done)=p myprog
> > $ getcap myprog
> > cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,
> > cap_fsetid,cap_setgid,cap_setuid,cap_setpcap,
> > cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,
> > cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_kill,
> > cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,
> > cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,
> > cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,
> > cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,
> > cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,
> > cap_block_suspend,cap_audit_read+p
> >
> > I think there would be few users who would *not* be surprised
> > about the fact that two steps that seem equivalent produce
> > quite differnt output from getcap(8)!
> >
> > 3. Suppose I set all permitted capabilities on a binary:
> >
> > $ sudo setcap =p myprog
> >
> > Then actually, I have set not just the 38 existing capabilities,
> > but also 26 future capabilities, so that when "cap_self_destruct"
> > is added to the kernel, 'myprog' already has it. This seems to
> > run directly counter to your argument above (if I have understood
> > it correctly).
> >
> > My summary from the above:
> >
> > * The output notation from getpcaps(8) is (1) inconsistent (with
> > getcap(8)), and (2) difficult to read, two things that strike me
> > as risk factors in a security-related notation.
> >
> > * The fact that "setcap =p ..." sets the top 26 (currently unused)
> > bits in the permitted set is surprising, and perhaps also a
> > security risk when new capabilities are actually added to the
> > kernel, since existing binaries will automatically have those
> > capabilities.
> >
> > Thanks,
> >
> > Michael
> >
> > [*] I often joke that the cap_to_text(3) notation is one that is
> > "human-readable, but not necessarily human-comprehensible", but
> > at the same time I also note that the notation has one virtue:
> > it is compact. However, that one virtue seems to have gone out
> > the window for getpcaps(8).
> >
> > --
> > Michael Kerrisk
> > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> > Linux/UNIX System Programming Training: http://man7.org/training/
> >
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