setcap/getcap limitations

[Serge E. Hallyn]

Quoting Richard Guy Briggs (rgb at redhat.com):
> On 2017-05-11 22:37, Serge E. Hallyn wrote:
> > Quoting Richard Guy Briggs (rgb at redhat.com):
> > > Capabilities gurus,
> > 
> > I cc:d Andrew as he maintains the source package,
> > 
> > > These may arguably be useless or meaningless combinations, but I found them
> > > sufficiently puzzling as to need to document and report the behaviour.  They
> > > appear to be leftovers from a time when fE may have been a bitfield rather than
> > > a boolean.
> > > 
> > > 
> > > 1 - BUG: setcap manpage refers to non-existant cap_from_text(3) and cap_set_file(3)
> > > manpages (fedora).  (It turns out it is in libcap-devel which makes
> > > sense, but isn't very useful for users of setcap/getcap(8) rather than
> > > developers.)
> > 
> > Hm, yeah, I suppose.  In the libcap source though they all come
> > together.  Perhaps this is cause for distro bugs to make a
> > common libcap-doc package which both libcap-devel and libcap2-bin
> > depend on?  Really not sure of the best solution.
> 
> I agree it sounds like a distro-level issue, thanks.
> 
> > > 2 - BUG: setcap requires dummy capability present to set or clear effective bit
> > 
> > Getting ready for a short road trip, will look at the rest this weekend.
> 
> Any further thoughts on the three below?

Sorry!  Yes, responses inline below.

> > > While attempting to issue the setcap command to set or clear the effective bit,
> > > the latter of which is a boolean rather than a capability set, a fatal error
> > > was returned with a usage message that didn't make much sense:
> > > 	# setcap +e /tmp/test
> > > 	fatal error: Invalid argument
> > > 	usage: setcap [-q] [-v] (-r|-|<caps>) <filename> [ ... (-r|-|<capsN>) <filenameN> ]
> > >  	Note <filename> must be a regular (non-symlink) file.
> > > So in order to set it, I need to use a dummy capability, such as:
> > > 	# setcap cap_sys_admin+e /tmp/test

This I think is a bug that should be fixed in setcap.  Like you say, just
setcap -e /tmp/test should work.

> > > 
> > > 
> > > 3- BUG: getcap doesn't show effective bit set if no other fP or fI bits are set
> > > 
> > > 	# setcap cap_sys_admin+pe /tmp/test
> > > 	# getcap /tmp/test
> > > 	/tmp/test = cap_sys_admin+ep
> > > 	# getfattr --absolute-names --e hex -n security.capability /tmp/test
> > > 	security.capability=0x0100000200002000000000000000000000000000
> > > 	# setfattr -n security.capability -v 0x0100000200000000000000000000000000000000 /tmp/test
> > > 	# getcap /tmp/test 
> > > 	/tmp/test =
> > > 	# getfattr --absolute-names --e hex -n security.capability /tmp/test
> > > 	security.capability=0x0100000200000000000000000000000000000000
> > > 	# setcap cap_sys_admin-p /tmp/test
> > > 	# getfattr --absolute-names --e hex -n security.capability /tmp/test
> > > 	# file: /tmp/test
> > > 	security.capability=0x0000000200000000000000000000000000000000
> > > So, getcap lies, where getfattr is more honest but awkward.

Yeah I'd call this a bug too.  In practice, from the kernel's point of
view, it's true there is no effective bit set that requires a p or i, but
this can lead to a dangerous situation where a a tool now only added cap_net_admin
to the permitted set, but the kernel recognized it as effective.

> > > Q: How do I set one fP capability and a different fI capability?

> > > >From looking at the manpage for cap_from_text, it appears that capability sets
> > > can be specified in a similar manner to chmod(1), using "=", "+" and "-"
> > > operators, but pracitical poking doesn't seem to work that way.  "+" seems to
> > > work like "=" and "-" seems to clear all except the version field.
> > > Other than resorting to setfattr, how can I set something like
> > > cap_sys_admin+p with cap_audit_read,cap_audit_write,cap_audit_control+i ?

The manpage points to you to cap_from_text which gives you an example - you
put it in quotes :)

ubuntu at oci:~$ sudo setcap "cap_sys_admin+p cap_net_admin+i" capsh
ubuntu at oci:~$ getfattr --absolute-names --e hex -n security.capability capsh
# file: capsh
security.capability=0x0000000200002000001000000000000000000000

> > > 4 - BUG: The capabilities version doesn't get cleared when all cap bits are
> > > cleared.  This fools kernel into thinking there are fcaps set.
> > > 
> > > As can be seen above, if setcap is used with "<capability>-<p|e|i>" to unset
> > > capabilities then the version field is leftover, set.  Arguably the -r option
> > > should have been used to remove the attribute entirely, but in this case, the
> > > kernel assumes that capabilities are in use due to a valid version number
> > > present (VFS_CAP_REVISION_MASK).  This could be solved in the kernel by
> > > clearing that attribute field entirely if there are no values set other than
> > > the version number, or by having "has_cap" check for a value more than just the
> > > version number.  Is it possible to remove some capabilities from a file but not
> > > all?  Or is it intended to simply specify the new set of capabilities that are
> > > to be set and ignore or strip away the unwanted ones from the set?

Yeah I'm gonna gree this seems like a bug which could have subtle consequences.

Do you have time to address these?  If not, I may find time later in the year,
but not right now.  So I'm wondering how we should track these, since there is
no issue tracker at https://git.kernel.org/pub/scm/linux/kernel/git/morgan/libcap.git/
If I'm not stepping on toes, I could create a copy of the kernel.org tree at github
to track issues and pull requests...  But I'm pretty sure I'd be stepping on toes.

-serge
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