Anomalous output from getpcaps(1) for process with all capabilities
Serge E. Hallyn
serge at hallyn.com
Mon Dec 23 19:20:45 UTC 2019
Ok, thanks. So it's probably not worth worrying about this, but if we
were going to, then it might seem reasonable (though arguably so) to say
that when getcap sees all known and unknown capabilities set, then it
shows =pe. There really cannot be any other intent to the user having
set all known and unknown caps.
-serge
On Mon, Dec 23, 2019 at 10:30:24AM -0800, Andrew G. Morgan wrote:
> I think my commit message included what is going on here.
>
> On Mon, 23 Dec 2019, 10:05 am Serge E. Hallyn, <serge at hallyn.com> wrote:
>
> > Oh, hm, now that's interesting. It did fix getpcaps, but broke
> > getcap:
> >
> > serge at sl:~/src/libcap$ sudo setcap all+pe /tmp/true.1
> > old behavior:
> > serge at sl:~/src/libcap$ getcap /tmp/true.1
> > /tmp/true.1 =ep
> > serge at sl:~/src/libcap$ getpcaps 1
> > Capabilities for `1': =
> > cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,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+ep
> >
> > new behavior:
> > serge at sl:~/src/libcap$ ./progs/getcap /tmp/true
> > /tmp/true =ep
> > 38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63+ep
> > serge at sl:~/src/libcap$ ./progs/getpcaps 1
> > Capabilities for `1': =ep
> >
> > -serge
> >
> > On Sun, Dec 22, 2019 at 03:36:22PM -0800, Andrew G. Morgan wrote:
> > > New libcap behavior implemented:
> > >
> > >
> > https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=afef3ef1c62613e1cac12a2bbec6017f7d5e033e
> > >
> > > Cheers
> > >
> > > Andrew
> > >
> > > On Sun, Dec 22, 2019 at 10:00 AM Andrew G. Morgan <morgan at kernel.org>
> > wrote:
> > > >
> > > > I can take a look, but I'm not really convinced this isn't going to
> > > > cause me grief at some point in the future when the kernel adds
> > > > support for another capability and all' does not equal all. I've done
> > > > some digging and I thought it was worth writing up.
> > > >
> > > > Today, the kernel treats 'all' (as it is defined by libcap today)
> > > > differently for file and process capabilities. I suspect this is
> > > > contributing to expectations about what all means, because all as
> > > > defined today is effectively useless, and somewhere in the kernel
> > > > we've worked around it for filecaps when we shouldn't have.
> > > >
> > > > Explanation of kernel bug (no use of 'all' here):
> > > >
> > > > 1) placing, say, bit-63 (undef today) and bit-0 (cap_chown) on the
> > > > permitted (fP) vector of a file, yields a file that can execute - it
> > > > gets pP=cap_chown.
> > > >
> > > > cd libcap/progs ; make ; cp capsh pcapsh ; sudo setcap
> > > > 63,cap_setpcap,cap_chown=p pcapsh
> > > >
> > > > ./pcapsh --print
> > > >
> > > > 2) dropping cap_chown from the bounding set causes this same file to
> > > > not run (insufficient privilege) because the file has been setup to
> > > > require cap_chown, but can't have it so (by design) we deny execution
> > > > altogether -- this fails:
> > > >
> > > > ./pcapsh --drop=cap_chown --print == --print
> > > >
> > > > To be consistent, (1) should fail because bit-63 is being dropped by
> > > > the bounding set at execution time. That is, a binary that requires
> > > > some future definition for capability #63 to run successfully should
> > > > fail on a kernel that does not support it today. This but was
> > > > introduced with this kernel commit:
> > > >
> > > >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/security/commoncap.c?id=7d8b6c63751cfbbe5eef81a48c22978b3407a3ad
> > > >
> > > > Ironically, the commit message declares the fact that (4) now passes
> > > > is the fix. The real fix is to think about what we mean by 'all'.
> > > >
> > > > Cheers
> > > >
> > > > Andrew
> > > >
> > > > On Fri, Dec 20, 2019 at 8:36 PM Serge E. Hallyn <serge at hallyn.com>
> > wrote:
> > > > >
> > > > > I think the thing to do is use /proc/sys/kernel/cap_last_cap (if it
> > > > > exists) to compare *caps to ~(cap_last_cap-1) and for each bit where
> > > > > it's the same, show the full set. Been looking at the fn for a bit,
> > > > > and I might be comfortable enough to try it at this point. But, if
> > > > > you want to do it (to make sure it's done right :), please let me
> > know.
> > > > >
> > > > > -serge
> > > > >
> > > > > On Fri, Dec 20, 2019 at 07:27:46PM -0800, Andrew G. Morgan wrote:
> > > > > > /proc/1/status changed at some point to drop the undefined bits.
> > When
> > > > > > it did I changed libcap to try harder to not display unnamed
> > > > > > capabilities.
> > > > > >
> > > > > > Cheers
> > > > > >
> > > > > > Andrew
> > > > > >
> > > > > >
> > > > > > On Fri, Dec 20, 2019 at 7:08 PM Serge E. Hallyn <serge at hallyn.com>
> > wrote:
> > > > > > >
> > > > > > > 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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