Anomalous output from getpcaps(1) for process with all capabilities

[Michael Kerrisk (man-pages)]

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/