On Thu, Mar 10, 2016 at 01:33:07AM +0100, Michael Kerrisk (man-pages) wrote:
Hello Krzysztof
On 03/09/2016 09:15 PM, Krzysztof Adamski wrote:
On Wed, Mar 09, 2016 at 01:49:41AM +0100, Michael Kerrisk (man-pages) wrote:
Hello Krzysztof
Sorry for the delayed follow up.
On 10/12/2015 09:45 PM, Krzysztof Adamski wrote:
An EPERM error can be returned when using filesystem capabilities and
capabilities to be added are not in permitted set.
This error return values was introduced by this patch:
5459c16 security: protect legacy applications from executing with
insufficient privilege
Can you explain in more detail the scenario where EPERM can be produced.
I can't see/produce it. Also, the code in the commit that you mention,
which was part of Linux 2.6.27, was thoroughly changed in Linux 2.6.29.
Hi Michael,
If you're interested in details, I explained it quite extensively here:
http://k.japko.eu/systemd-nspawn-ping-debug.html
The summary is that I used nspawn from systemd which drops some
capabilites (CAP_NET_ADMIN is amoung them) when spawning a container.
Now, since I used Fedora in container, my ping binary had filesystem
capabilites set:
# getcap /bin/ping
/bin/ping = cap_net_admin,cap_net_raw+ep
So when executing this application, kernel tried to give me
CAP_NET_ADMIN capabilities but they where not on permitted set so I've
got EPERM.
It was found in kernel 4.1 back then and I just retested this on kernel
4.3. Unfortunately I don't have time now to verify this for the latest
kernel but I would be surprised if this feature was removed.
Would you like me to extend/change the description in the patch somehow?
Okay -- I got it at last. I think the explanation requires quite
some detail. I added this to execve(2):
EPERM A "capability-dumb" applications would not obtain the
full set of permitted capabilities granted by the exe‐
cutable file. See capabilities(7).
And this to capabilities(7):
Safety checking for capability-dumb binaries
A capability-dumb binary is a legacy application that has been
marked to have file capabilities, but has not been converted to
use the libcap(3) API to manipulate its capabilities. (In
other words, this is a traditional set-user-ID-root program
that has been switched to use file capabilities, but whose code
has not been modified to understand capabilities.) For such
applications, the effective capability bit is set on the file,
so that the file permitted capabilities are automatically
enabled in the process effective set when executing the file.
The kernel recognizes a file which has the effective capability
bit set as capability-dumb for the purpose of the check
described here.
When executing a capability-dumb binary, the kernel checks if
the process obtained all permitted capabilities that were spec‐
ified in the file permitted set, after the capability transfor‐
mations described above have been performed. (The typical rea‐
son why this might not occur is that the capability bounding
set masked out some of the capabilities in the file permitted
set.) If the process did not obtain the full set of file per‐
mitted capabilities, then execve(2) fails with the error EPERM.
This prevents possible security risks that could arise when a
capability-dumb application is executed with less privilege
that it needs. Note that, by definition, the application could
not itself recognize this problem, since it does not employ the
libcap(3) API.
Let me know if you see anything amiss.
This looks good to me. It is however a little confusing (in Linux) to
say that this is legacy mode since you could use libcap API long before
filesystem capabilities were actually implemented in the kernel :)
Best regards,
Krzysztof Adamski
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
