Hello all,
Because the topic every now then comes up "which capability
should I associate with the new feature that I'm adding to
the kernel?", I propose to add the text below to the
capabilities(7) man page [1] with some recommendations
on how to go about choosing. I would be happy
to get feedback, suggestions for improvement and
so on.
Cheers,
Michael
[1] http://man7.org/linux/man-pages/man7/capabilities.7.html
Notes to kernel developers
When adding a new kernel feature that should be governed by a
capability, consider the following points.
* The goal of capabilities is divide the power of superuser into
small pieces, such that if a program that has capabilities is
compromised, its power to do damage to the system would be much
less than a similar set-user-ID-root program.
* You have the choice of either creating a new capability for
your new feature, or associating the feature with one of the
existing capabilities. Because the size of capability sets is
currently limited to 64 bits, the latter option is preferable,
unless there are compelling reasons to take the former option.
* To determine which existing capability might best be associated
with your new feature, review the list of capabilities above in
order to find a "silo" into which your new feature best fits.
* Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
vast proportion of existing capability checks are associated
with this capability, to the point where it can plausibly be
called "the new root". Don't make the problem worse. The only
new features that should be associated with CAP_SYS_ADMIN are
ones that closely match existing uses in that silo.
* If you have determined that it really is necessary to create a
new capability for your feature, avoid making (and naming) it
as a "single-use" capability. Thus, for example, the addition
of the highly specific CAP_WAKE_ALARM was probably a mistake.
Instead, try to identify and name your new capability as a
broader silo into which other related future use cases might
fit.
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
On 12/15/2016 3:40 AM, Michael Kerrisk (man-pages) wrote:
> Hello all,
>
> Because the topic every now then comes up "which capability
> should I associate with the new feature that I'm adding to
> the kernel?", I propose to add the text below to the
> capabilities(7) man page [1] with some recommendations
> on how to go about choosing. I would be happy
> to get feedback, suggestions for improvement and
> so on.
Thank you. This is long overdue.
> Cheers,
>
> Michael
>
> [1] http://man7.org/linux/man-pages/man7/capabilities.7.html
>
>
> Notes to kernel developers
> When adding a new kernel feature that should be governed by a
> capability, consider the following points.
>
> * The goal of capabilities is divide the power of superuser into
> small pieces, such that if a program that has capabilities is
I wouldn't say "small". Small implies many, and we want to
keep the number of capabilities manageable.
> compromised, its power to do damage to the system would be much
> less than a similar set-user-ID-root program.
Not "much less", just less.
Change "similar set-user-ID-root program" to "the same program
running with root privilege".
> * You have the choice of either creating a new capability for
> your new feature, or associating the feature with one of the
> existing capabilities. Because the size of capability sets is
> currently limited to 64 bits, the latter option is preferable,
The reason is not the size of the set being limited, it is
that a large set of capabilities would be unmanageable. The
fact that someone is reading this is sufficient evidence of
that.
> unless there are compelling reasons to take the former option.
>
> * To determine which existing capability might best be associated
> with your new feature, review the list of capabilities above in
> order to find a "silo" into which your new feature best fits.
One approach to take is to determine if there are other features
requiring capabilities that will always be use along with the
new feature. If the new feature is useless without these other
features, you should use the same capability as the other features.
> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
> vast proportion of existing capability checks are associated
> with this capability, to the point where it can plausibly be
> called "the new root". Don't make the problem worse. The only
> new features that should be associated with CAP_SYS_ADMIN are
> ones that closely match existing uses in that silo.
I don't agree with this advice. Use CAP_SYS_ADMIN if you are
preforming system administration functions. Odds are very good
that if a program is using one system administration feature
it will be using others.
> * If you have determined that it really is necessary to create a
> new capability for your feature, avoid making (and naming) it
> as a "single-use" capability.
Be strong. Don't say "avoid making (and naming)", say "don't make or name".
We can't allow single use capabilities. If we did that we'd have thousands
of capabilities. It's hard enough to get developers to use a coarse set of
capabilities.
> Thus, for example, the addition
> of the highly specific CAP_WAKE_ALARM was probably a mistake.
> Instead, try to identify and name your new capability as a
> broader silo into which other related future use cases might
> fit.
Need a better example. CAP_WAKE_ALARM could readily be CAP_TIME.
Hello Casey,
On 12/15/2016 05:29 PM, Casey Schaufler wrote:
> On 12/15/2016 3:40 AM, Michael Kerrisk (man-pages) wrote:
>> Hello all,
>>
>> Because the topic every now then comes up "which capability
>> should I associate with the new feature that I'm adding to
>> the kernel?", I propose to add the text below to the
>> capabilities(7) man page [1] with some recommendations
>> on how to go about choosing. I would be happy
>> to get feedback, suggestions for improvement and
>> so on.
>
> Thank you. This is long overdue.
>
>> Cheers,
>>
>> Michael
>>
>> [1] http://man7.org/linux/man-pages/man7/capabilities.7.html
>>
>>
>> Notes to kernel developers
>> When adding a new kernel feature that should be governed by a
>> capability, consider the following points.
>>
>> * The goal of capabilities is divide the power of superuser into
>> small pieces, such that if a program that has capabilities is
>
> I wouldn't say "small". Small implies many, and we want to
> keep the number of capabilities manageable.
Fixed.
>> compromised, its power to do damage to the system would be much
>> less than a similar set-user-ID-root program.
>
> Not "much less", just less.
Fixed.
> Change "similar set-user-ID-root program" to "the same program
> running with root privilege".
Fixed.
>> * You have the choice of either creating a new capability for
>> your new feature, or associating the feature with one of the
>> existing capabilities. Because the size of capability sets is
>> currently limited to 64 bits, the latter option is preferable,
>
> The reason is not the size of the set being limited, it is
> that a large set of capabilities would be unmanageable. The
> fact that someone is reading this is sufficient evidence of
> that.
Yep. Reworked to cover this point.
>> unless there are compelling reasons to take the former option.
>>
>> * To determine which existing capability might best be associated
>> with your new feature, review the list of capabilities above in
>> order to find a "silo" into which your new feature best fits.
>
> One approach to take is to determine if there are other features
> requiring capabilities that will always be use along with the
> new feature. If the new feature is useless without these other
> features, you should use the same capability as the other features.
Thanks, I've lifted those words exactly as you gave them into the man page.
>> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
>> vast proportion of existing capability checks are associated
>> with this capability, to the point where it can plausibly be
>> called "the new root". Don't make the problem worse. The only
>> new features that should be associated with CAP_SYS_ADMIN are
>> ones that closely match existing uses in that silo.
>
> I don't agree with this advice. Use CAP_SYS_ADMIN if you are
> preforming system administration functions. Odds are very good
> that if a program is using one system administration feature
> it will be using others.
Really? To me, the CAP_SYS_ADMIN situation is a terrible mess. Around a
third of all of the capability checks in the kernel are for that
capability. Or, to put it another way, it is so broad, that if a process
has to have that capability, it may as well be root. And because it is
so broad, the number of binaries that might need that file capability is
large. (See also https://lwn.net/Articles/486306/)
Here's an *incomplete* list of (from capabilities(7)) of what
CAP_SYS_ADMIN allows:
* Perform a range of system administration operations including:
quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
sethostname(2), and setdomainname(2);
* perform privileged syslog(2) operations (since Linux 2.6.37,
CAP_SYSLOG should be used to permit such operations);
* perform VM86_REQUEST_IRQ vm86(2) command;
* perform IPC_SET and IPC_RMID operations on arbitrary System V
IPC objects;
* override RLIMIT_NPROC resource limit;
* perform operations on trusted and security Extended Attributes
(see xattr(7));
* use lookup_dcookie(2);
* use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux
2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
* forge PID when passing socket credentials via UNIX domain
sockets;
* exceed /proc/sys/fs/file-max, the system-wide limit on the
number of open files, in system calls that open files (e.g.,
accept(2), execve(2), open(2), pipe(2));
* employ CLONE_* flags that create new namespaces with clone(2)
and unshare(2) (but, since Linux 3.8, creating user namespaces
does not require any capability);
* call perf_event_open(2);
* access privileged perf event information;
* call setns(2) (requires CAP_SYS_ADMIN in the target names‐
pace);
* call fanotify_init(2);
* call bpf(2);
* perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
operations;
* use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec‐
comp filters;
* perform madvise(2) MADV_HWPOISON operation;
* employ the TIOCSTI ioctl(2) to insert characters into the
input queue of a terminal other than the caller's controlling
terminal;
* employ the obsolete nfsservctl(2) system call;
* employ the obsolete bdflush(2) system call;
* perform various privileged block-device ioctl(2) operations;
* perform various privileged filesystem ioctl(2) operations;
* perform privileged ioctl(2) operations on the /dev/random
device (see random(4));
* install a seccomp(2) filter without first having to set the
no_new_privs thread attribute;
* modify allow/deny rules for device control groups.
* perform administrative operations on many device drivers.
I'm not sure what subset of that you want to put under "system
administration", but you'd have to be using a very broad definition
indeed to include all of that. (In fact, I think one of the very
problems with the capability is its name. Whereas most capabilities have
task-related names, this one has a role-related name. I think that's not
unconnected to the mess that this capability has become.)
So, in summary, I still think it's good advice, though it could perhaps
be more nuanced. (I had attempted to do that with the sentence "The only
new features that should be associated with CAP_SYS_ADMIN are ones that
closely match existing uses in that silo.") Still, I'm certainly open
to hearing more argument on this point.
>> * If you have determined that it really is necessary to create a
>> new capability for your feature, avoid making (and naming) it
>> as a "single-use" capability.
>
> Be strong. Don't say "avoid making (and naming)", say "don't make or name".
> We can't allow single use capabilities. If we did that we'd have thousands
> of capabilities. It's hard enough to get developers to use a coarse set of
> capabilities.
Good advice re the wording. Thanks. Fixed.
>> Thus, for example, the addition
>> of the highly specific CAP_WAKE_ALARM was probably a mistake.
>> Instead, try to identify and name your new capability as a
>> broader silo into which other related future use cases might
>> fit.
>
> Need a better example.
How about CAP_PACCT then?
> CAP_WAKE_ALARM could readily be CAP_TIME.
Actually, I don't quite understand what you mean with that sentence.
Could you elaborate?
By now, the text reads:
Notes to kernel developers
When adding a new kernel feature that should be governed by a
capability, consider the following points.
* The goal of capabilities is divide the power of superuser into
pieces, such that if a program that has one or more capabili‐
ties is compromised, its power to do damage to the system would
be less than the same program running with root privilege.
* You have the choice of either creating a new capability for
your new feature, or associating the feature with one of the
existing capabilities. In order to keep the set of capabili‐
ties to a manageable size, the latter option is preferable,
unless there are compelling reasons to take the former option.
(There is also a technical limit: the size of capability sets
is currently limited to 64 bits.)
* To determine which existing capability might best be associated
with your new feature, review the list of capabilities above in
order to find a "silo" into which your new feature best fits.
One approach to take is to determine if there are other fea‐
tures requiring capabilities that will always be use along with
the new feature. If the new feature is useless without these
other features, you should use the same capability as the other
features.
* Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
vast proportion of existing capability checks are associated
with this capability, to the point where it can plausibly be
called "the new root". Don't make the problem worse. The only
new features that should be associated with CAP_SYS_ADMIN are
ones that closely match existing uses in that silo.
* If you have determined that it really is necessary to create a
new capability for your feature, don't make or name it as a
"single-use" capability. Thus, for example, the addition of
the highly specific CAP_PACCT was probably a mistake. Instead,
try to identify and name your new capability as a broader silo
into which other related future use cases might fit.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
> Hello Casey,
>
> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>> On 12/15/2016 3:40 AM, Michael Kerrisk (man-pages) wrote:
>>> Hello all,
>>>
>>> Because the topic every now then comes up "which capability
>>> should I associate with the new feature that I'm adding to
>>> the kernel?", I propose to add the text below to the
>>> capabilities(7) man page [1] with some recommendations
>>> on how to go about choosing. I would be happy
>>> to get feedback, suggestions for improvement and
>>> so on.
>> Thank you. This is long overdue.
>>
>>> Cheers,
>>>
>>> Michael
>>>
>>> [1] http://man7.org/linux/man-pages/man7/capabilities.7.html
>>>
>>>
>>> Notes to kernel developers
>>> When adding a new kernel feature that should be governed by a
>>> capability, consider the following points.
>>>
>>> * The goal of capabilities is divide the power of superuser into
>>> small pieces, such that if a program that has capabilities is
>> I wouldn't say "small". Small implies many, and we want to
>> keep the number of capabilities manageable.
> Fixed.
>
>>> compromised, its power to do damage to the system would be much
>>> less than a similar set-user-ID-root program.
>> Not "much less", just less.
> Fixed.
>
>> Change "similar set-user-ID-root program" to "the same program
>> running with root privilege".
> Fixed.
>
>>> * You have the choice of either creating a new capability for
>>> your new feature, or associating the feature with one of the
>>> existing capabilities. Because the size of capability sets is
>>> currently limited to 64 bits, the latter option is preferable,
>> The reason is not the size of the set being limited, it is
>> that a large set of capabilities would be unmanageable. The
>> fact that someone is reading this is sufficient evidence of
>> that.
> Yep. Reworked to cover this point.
>
>>> unless there are compelling reasons to take the former option.
>>>
>>> * To determine which existing capability might best be associated
>>> with your new feature, review the list of capabilities above in
>>> order to find a "silo" into which your new feature best fits.
>> One approach to take is to determine if there are other features
>> requiring capabilities that will always be use along with the
>> new feature. If the new feature is useless without these other
>> features, you should use the same capability as the other features.
> Thanks, I've lifted those words exactly as you gave them into the man page.
>
>>> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
>>> vast proportion of existing capability checks are associated
>>> with this capability, to the point where it can plausibly be
>>> called "the new root". Don't make the problem worse. The only
>>> new features that should be associated with CAP_SYS_ADMIN are
>>> ones that closely match existing uses in that silo.
>> I don't agree with this advice. Use CAP_SYS_ADMIN if you are
>> preforming system administration functions. Odds are very good
>> that if a program is using one system administration feature
>> it will be using others.
> Really? To me, the CAP_SYS_ADMIN situation is a terrible mess. Around a
> third of all of the capability checks in the kernel are for that
> capability. Or, to put it another way, it is so broad, that if a process
> has to have that capability, it may as well be root. And because it is
> so broad, the number of binaries that might need that file capability is
> large. (See also https://lwn.net/Articles/486306/)
Back in the days of the POSIX P1003.1e/2c working group
we struggled with what to do about the things that required
privilege but that were not related to the enforcement of
security policy. Everyone involved was looking to use
capabilities to meet B2* least privilege requirements in
NSA security evaluations. Because those evaluations where
of security policy, by far the easiest thing to do was to
create a single capability for all the things that didn't
show up in the security policy and declare that the people
doing the evaluation didn't have to look over there. Since
then, people have taken a more practical view that includes
security relevance in addition to security policy.
In retrospect we should have grouped all of the attribute
changes (chmod, chown, ...) into one capability and broken
the non-policy actions into a set on 2 or three.
The way that we think of privilege has evolved. We're not
focused on policy the way we used to be. We'll never get
everyone to agree on what the *right* granularity and
grouping is, either.
-----
* The B2 least privilege requirements are amusing.
If you want more information, look up "TCSEC orange book".
> Here's an *incomplete* list of (from capabilities(7)) of what
> CAP_SYS_ADMIN allows:
>
> * Perform a range of system administration operations including:
> quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
> sethostname(2), and setdomainname(2);
> * perform privileged syslog(2) operations (since Linux 2.6.37,
> CAP_SYSLOG should be used to permit such operations);
> * perform VM86_REQUEST_IRQ vm86(2) command;
> * perform IPC_SET and IPC_RMID operations on arbitrary System V
> IPC objects;
> * override RLIMIT_NPROC resource limit;
> * perform operations on trusted and security Extended Attributes
> (see xattr(7));
> * use lookup_dcookie(2);
> * use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux
> 2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
> * forge PID when passing socket credentials via UNIX domain
> sockets;
> * exceed /proc/sys/fs/file-max, the system-wide limit on the
> number of open files, in system calls that open files (e.g.,
> accept(2), execve(2), open(2), pipe(2));
> * employ CLONE_* flags that create new namespaces with clone(2)
> and unshare(2) (but, since Linux 3.8, creating user namespaces
> does not require any capability);
> * call perf_event_open(2);
> * access privileged perf event information;
> * call setns(2) (requires CAP_SYS_ADMIN in the target names‐
> pace);
> * call fanotify_init(2);
> * call bpf(2);
> * perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
> operations;
> * use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec‐
> comp filters;
> * perform madvise(2) MADV_HWPOISON operation;
> * employ the TIOCSTI ioctl(2) to insert characters into the
> input queue of a terminal other than the caller's controlling
> terminal;
> * employ the obsolete nfsservctl(2) system call;
> * employ the obsolete bdflush(2) system call;
> * perform various privileged block-device ioctl(2) operations;
> * perform various privileged filesystem ioctl(2) operations;
> * perform privileged ioctl(2) operations on the /dev/random
> device (see random(4));
> * install a seccomp(2) filter without first having to set the
> no_new_privs thread attribute;
> * modify allow/deny rules for device control groups.
> * perform administrative operations on many device drivers.
>
> I'm not sure what subset of that you want to put under "system
> administration", but you'd have to be using a very broad definition
> indeed to include all of that. (In fact, I think one of the very
> problems with the capability is its name. Whereas most capabilities have
> task-related names, this one has a role-related name. I think that's not
> unconnected to the mess that this capability has become.)
>
> So, in summary, I still think it's good advice, though it could perhaps
> be more nuanced. (I had attempted to do that with the sentence "The only
> new features that should be associated with CAP_SYS_ADMIN are ones that
> closely match existing uses in that silo.") Still, I'm certainly open
> to hearing more argument on this point.
>
>>> * If you have determined that it really is necessary to create a
>>> new capability for your feature, avoid making (and naming) it
>>> as a "single-use" capability.
>> Be strong. Don't say "avoid making (and naming)", say "don't make or name".
>> We can't allow single use capabilities. If we did that we'd have thousands
>> of capabilities. It's hard enough to get developers to use a coarse set of
>> capabilities.
> Good advice re the wording. Thanks. Fixed.
>
>>> Thus, for example, the addition
>>> of the highly specific CAP_WAKE_ALARM was probably a mistake.
>>> Instead, try to identify and name your new capability as a
>>> broader silo into which other related future use cases might
>>> fit.
>> Need a better example.
> How about CAP_PACCT then?
>
>> CAP_WAKE_ALARM could readily be CAP_TIME.
> Actually, I don't quite understand what you mean with that sentence.
> Could you elaborate?
Should have said CAP_SYS_TIME
Setting an alarm could be considered a time management function,
depending on what it actually does.
>
> By now, the text reads:
>
> Notes to kernel developers
> When adding a new kernel feature that should be governed by a
> capability, consider the following points.
>
> * The goal of capabilities is divide the power of superuser into
> pieces, such that if a program that has one or more capabili‐
> ties is compromised, its power to do damage to the system would
> be less than the same program running with root privilege.
>
> * You have the choice of either creating a new capability for
> your new feature, or associating the feature with one of the
> existing capabilities. In order to keep the set of capabili‐
> ties to a manageable size, the latter option is preferable,
> unless there are compelling reasons to take the former option.
> (There is also a technical limit: the size of capability sets
> is currently limited to 64 bits.)
>
> * To determine which existing capability might best be associated
> with your new feature, review the list of capabilities above in
> order to find a "silo" into which your new feature best fits.
> One approach to take is to determine if there are other fea‐
> tures requiring capabilities that will always be use along with
> the new feature. If the new feature is useless without these
> other features, you should use the same capability as the other
> features.
>
> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
> vast proportion of existing capability checks are associated
> with this capability, to the point where it can plausibly be
> called "the new root". Don't make the problem worse. The only
> new features that should be associated with CAP_SYS_ADMIN are
> ones that closely match existing uses in that silo.
>
> * If you have determined that it really is necessary to create a
> new capability for your feature, don't make or name it as a
> "single-use" capability. Thus, for example, the addition of
> the highly specific CAP_PACCT was probably a mistake. Instead,
> try to identify and name your new capability as a broader silo
> into which other related future use cases might fit.
>
> Cheers,
>
> Michael
>
On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
<[email protected]> wrote:
> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>> Actually, I don't quite understand what you mean with that sentence.
>> Could you elaborate?
>
> Should have said CAP_SYS_TIME
>
> Setting an alarm could be considered a time management function,
> depending on what it actually does.
Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
a system from suspend, while CAP_SYS_TIME covers the ability to set
the time. One wouldn't necessarily want to give applications which
could wake a system up the capability to also set the time.
thanks
-john
On 12/15/2016 4:31 PM, John Stultz wrote:
> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
> <[email protected]> wrote:
>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>>> Actually, I don't quite understand what you mean with that sentence.
>>> Could you elaborate?
>> Should have said CAP_SYS_TIME
>>
>> Setting an alarm could be considered a time management function,
>> depending on what it actually does.
> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
> a system from suspend, while CAP_SYS_TIME covers the ability to set
> the time. One wouldn't necessarily want to give applications which
> could wake a system up the capability to also set the time.
Doesn't really matter, except that an ignorant developer
might make the mistake I did and assume that WAKE_ALARM
was somehow related to time management. If you want to use
it as an example don't let my dunderheadedness get in your
way.
> thanks
> -john
Again, thank you for taking this on. It should be a
big help.
On 12/16/2016 01:44 AM, Casey Schaufler wrote:
> On 12/15/2016 4:31 PM, John Stultz wrote:
>> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
>> <[email protected]> wrote:
>>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>>>> Actually, I don't quite understand what you mean with that sentence.
>>>> Could you elaborate?
>>> Should have said CAP_SYS_TIME
>>>
>>> Setting an alarm could be considered a time management function,
>>> depending on what it actually does.
>> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
>> a system from suspend, while CAP_SYS_TIME covers the ability to set
>> the time. One wouldn't necessarily want to give applications which
>> could wake a system up the capability to also set the time.
>
> Doesn't really matter, except that an ignorant developer
> might make the mistake I did and assume that WAKE_ALARM
> was somehow related to time management. If you want to use
> it as an example don't let my dunderheadedness get in your
> way.
Actually, I decided it wasn't such a good example anyway.
That capability could potentially be generic. (But it probably
should better have been named something like 'CAP_WAKE_SYSTEM'.)
>> thanks
>> -john
>
> Again, thank you for taking this on. It should be a
> big help.
You're welcome. And thanks for your help, Casey.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
On 12/15/2016 09:40 PM, Casey Schaufler wrote:
> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>> Hello Casey,
>>
>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>> On 12/15/2016 3:40 AM, Michael Kerrisk (man-pages) wrote:
[...]
>>>> * To determine which existing capability might best be associated
>>>> with your new feature, review the list of capabilities above in
>>>> order to find a "silo" into which your new feature best fits.
>>> One approach to take is to determine if there are other features
>>> requiring capabilities that will always be use along with the
>>> new feature. If the new feature is useless without these other
>>> features, you should use the same capability as the other features.
>> Thanks, I've lifted those words exactly as you gave them into the man page.
>>
>>>> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
>>>> vast proportion of existing capability checks are associated
>>>> with this capability, to the point where it can plausibly be
>>>> called "the new root". Don't make the problem worse. The only
>>>> new features that should be associated with CAP_SYS_ADMIN are
>>>> ones that closely match existing uses in that silo.
>>> I don't agree with this advice. Use CAP_SYS_ADMIN if you are
>>> preforming system administration functions. Odds are very good
>>> that if a program is using one system administration feature
>>> it will be using others.
>> Really? To me, the CAP_SYS_ADMIN situation is a terrible mess. Around a
>> third of all of the capability checks in the kernel are for that
>> capability. Or, to put it another way, it is so broad, that if a process
>> has to have that capability, it may as well be root. And because it is
>> so broad, the number of binaries that might need that file capability is
>> large. (See also https://lwn.net/Articles/486306/)
>
> Back in the days of the POSIX P1003.1e/2c working group
> we struggled with what to do about the things that required
> privilege but that were not related to the enforcement of
> security policy. Everyone involved was looking to use
> capabilities to meet B2* least privilege requirements in
> NSA security evaluations. Because those evaluations where
> of security policy, by far the easiest thing to do was to
> create a single capability for all the things that didn't
> show up in the security policy and declare that the people
> doing the evaluation didn't have to look over there. Since
> then, people have taken a more practical view that includes
> security relevance in addition to security policy.
Ahhh -- thanks for a bit of the history! Helps me to
understand a bit more why things are as they are...
> In retrospect we should have grouped all of the attribute
> changes (chmod, chown, ...) into one capability and broken
> the non-policy actions into a set on 2 or three.
>
> The way that we think of privilege has evolved. We're not
> focused on policy the way we used to be. We'll never get
> everyone to agree on what the *right* granularity and
> grouping is, either.
:-)
> -----
> * The B2 least privilege requirements are amusing.
> If you want more information, look up "TCSEC orange book".
Maybe one day...
[...]
Cheers,
Mcihael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
Quoting Michael Kerrisk (man-pages) ([email protected]):
> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
> > On 12/15/2016 4:31 PM, John Stultz wrote:
> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
> >> <[email protected]> wrote:
> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
> >>>> Actually, I don't quite understand what you mean with that sentence.
> >>>> Could you elaborate?
> >>> Should have said CAP_SYS_TIME
> >>>
> >>> Setting an alarm could be considered a time management function,
> >>> depending on what it actually does.
> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
> >> the time. One wouldn't necessarily want to give applications which
> >> could wake a system up the capability to also set the time.
> >
> > Doesn't really matter, except that an ignorant developer
> > might make the mistake I did and assume that WAKE_ALARM
> > was somehow related to time management. If you want to use
> > it as an example don't let my dunderheadedness get in your
> > way.
>
> Actually, I decided it wasn't such a good example anyway.
> That capability could potentially be generic. (But it probably
> should better have been named something like 'CAP_WAKE_SYSTEM'.)
How about:
Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
As suggested by Michael Kerrisk his is a less confusing name, and
this won't break any old userspace.
Signed-off-by: Serge Hallyn <[email protected]>
Cc: Michael Kerrisk <[email protected]>
---
include/uapi/linux/capability.h | 2 ++
1 file changed, 2 insertions(+)
diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
index fd4f87d..ba972ff 100644
--- a/include/uapi/linux/capability.h
+++ b/include/uapi/linux/capability.h
@@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
#define CAP_WAKE_ALARM 35
+#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
+
/* Allow preventing system suspends */
#define CAP_BLOCK_SUSPEND 36
--
2.7.4
On Fri, Dec 16, 2016 at 12:10 PM, Serge E. Hallyn <[email protected]> wrote:
> Quoting Michael Kerrisk (man-pages) ([email protected]):
>> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
>> > On 12/15/2016 4:31 PM, John Stultz wrote:
>> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
>> >> <[email protected]> wrote:
>> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>> >>>> Actually, I don't quite understand what you mean with that sentence.
>> >>>> Could you elaborate?
>> >>> Should have said CAP_SYS_TIME
>> >>>
>> >>> Setting an alarm could be considered a time management function,
>> >>> depending on what it actually does.
>> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
>> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
>> >> the time. One wouldn't necessarily want to give applications which
>> >> could wake a system up the capability to also set the time.
>> >
>> > Doesn't really matter, except that an ignorant developer
>> > might make the mistake I did and assume that WAKE_ALARM
>> > was somehow related to time management. If you want to use
>> > it as an example don't let my dunderheadedness get in your
>> > way.
>>
>> Actually, I decided it wasn't such a good example anyway.
>> That capability could potentially be generic. (But it probably
>> should better have been named something like 'CAP_WAKE_SYSTEM'.)
>
> How about:
>
> Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
>
> As suggested by Michael Kerrisk his is a less confusing name, and
> this won't break any old userspace.
>
> Signed-off-by: Serge Hallyn <[email protected]>
> Cc: Michael Kerrisk <[email protected]>
> ---
> include/uapi/linux/capability.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index fd4f87d..ba972ff 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
>
> #define CAP_WAKE_ALARM 35
>
> +#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
> +
I was thinking of the same thing. Although I might rename the
numerical define to WAKE_SYSTEM and put WAKE_ALARM as the alias (along
with a comment as to WAKE_ALARM being deprecated), so its more clear
which is the one that ought to be used by new code.
However, in the spirit of this thread, we might even consider
broadening the cap silo a bit further, to something like
CAP_WAKE_SUSPEND, such that it might also be able to cover broader PM
actions?
thanks
-john
Quoting John Stultz ([email protected]):
> On Fri, Dec 16, 2016 at 12:10 PM, Serge E. Hallyn <[email protected]> wrote:
> > Quoting Michael Kerrisk (man-pages) ([email protected]):
> >> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
> >> > On 12/15/2016 4:31 PM, John Stultz wrote:
> >> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
> >> >> <[email protected]> wrote:
> >> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
> >> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
> >> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
> >> >>>> Actually, I don't quite understand what you mean with that sentence.
> >> >>>> Could you elaborate?
> >> >>> Should have said CAP_SYS_TIME
> >> >>>
> >> >>> Setting an alarm could be considered a time management function,
> >> >>> depending on what it actually does.
> >> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
> >> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
> >> >> the time. One wouldn't necessarily want to give applications which
> >> >> could wake a system up the capability to also set the time.
> >> >
> >> > Doesn't really matter, except that an ignorant developer
> >> > might make the mistake I did and assume that WAKE_ALARM
> >> > was somehow related to time management. If you want to use
> >> > it as an example don't let my dunderheadedness get in your
> >> > way.
> >>
> >> Actually, I decided it wasn't such a good example anyway.
> >> That capability could potentially be generic. (But it probably
> >> should better have been named something like 'CAP_WAKE_SYSTEM'.)
> >
> > How about:
> >
> > Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
> >
> > As suggested by Michael Kerrisk his is a less confusing name, and
> > this won't break any old userspace.
> >
> > Signed-off-by: Serge Hallyn <[email protected]>
> > Cc: Michael Kerrisk <[email protected]>
> > ---
> > include/uapi/linux/capability.h | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> > index fd4f87d..ba972ff 100644
> > --- a/include/uapi/linux/capability.h
> > +++ b/include/uapi/linux/capability.h
> > @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
> >
> > #define CAP_WAKE_ALARM 35
> >
> > +#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
> > +
>
> I was thinking of the same thing. Although I might rename the
> numerical define to WAKE_SYSTEM and put WAKE_ALARM as the alias (along
> with a comment as to WAKE_ALARM being deprecated), so its more clear
> which is the one that ought to be used by new code.
>
> However, in the spirit of this thread, we might even consider
> broadening the cap silo a bit further, to something like
> CAP_WAKE_SUSPEND, such that it might also be able to cover broader PM
> actions?
Or just CAP_UNSUSPEND?
On Fri, Dec 16, 2016 at 1:05 PM, Serge E. Hallyn <[email protected]> wrote:
> Quoting John Stultz ([email protected]):
>> On Fri, Dec 16, 2016 at 12:10 PM, Serge E. Hallyn <[email protected]> wrote:
>> > Quoting Michael Kerrisk (man-pages) ([email protected]):
>> >> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
>> >> > On 12/15/2016 4:31 PM, John Stultz wrote:
>> >> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
>> >> >> <[email protected]> wrote:
>> >> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>> >> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>> >> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>> >> >>>> Actually, I don't quite understand what you mean with that sentence.
>> >> >>>> Could you elaborate?
>> >> >>> Should have said CAP_SYS_TIME
>> >> >>>
>> >> >>> Setting an alarm could be considered a time management function,
>> >> >>> depending on what it actually does.
>> >> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
>> >> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
>> >> >> the time. One wouldn't necessarily want to give applications which
>> >> >> could wake a system up the capability to also set the time.
>> >> >
>> >> > Doesn't really matter, except that an ignorant developer
>> >> > might make the mistake I did and assume that WAKE_ALARM
>> >> > was somehow related to time management. If you want to use
>> >> > it as an example don't let my dunderheadedness get in your
>> >> > way.
>> >>
>> >> Actually, I decided it wasn't such a good example anyway.
>> >> That capability could potentially be generic. (But it probably
>> >> should better have been named something like 'CAP_WAKE_SYSTEM'.)
>> >
>> > How about:
>> >
>> > Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
>> >
>> > As suggested by Michael Kerrisk his is a less confusing name, and
>> > this won't break any old userspace.
>> >
>> > Signed-off-by: Serge Hallyn <[email protected]>
>> > Cc: Michael Kerrisk <[email protected]>
>> > ---
>> > include/uapi/linux/capability.h | 2 ++
>> > 1 file changed, 2 insertions(+)
>> >
>> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
>> > index fd4f87d..ba972ff 100644
>> > --- a/include/uapi/linux/capability.h
>> > +++ b/include/uapi/linux/capability.h
>> > @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
>> >
>> > #define CAP_WAKE_ALARM 35
>> >
>> > +#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
>> > +
>>
>> I was thinking of the same thing. Although I might rename the
>> numerical define to WAKE_SYSTEM and put WAKE_ALARM as the alias (along
>> with a comment as to WAKE_ALARM being deprecated), so its more clear
>> which is the one that ought to be used by new code.
>>
>> However, in the spirit of this thread, we might even consider
>> broadening the cap silo a bit further, to something like
>> CAP_WAKE_SUSPEND, such that it might also be able to cover broader PM
>> actions?
>
> Or just CAP_UNSUSPEND?
I guess I was trying to capture it could be use for actions like both
waking and suspending the system.
thanks
-john
On 12/16/2016 09:10 PM, Serge E. Hallyn wrote:
> Quoting Michael Kerrisk (man-pages) ([email protected]):
>> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
>>> On 12/15/2016 4:31 PM, John Stultz wrote:
>>>> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
>>>> <[email protected]> wrote:
>>>>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>>>>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>>>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
>>>>>> Actually, I don't quite understand what you mean with that sentence.
>>>>>> Could you elaborate?
>>>>> Should have said CAP_SYS_TIME
>>>>>
>>>>> Setting an alarm could be considered a time management function,
>>>>> depending on what it actually does.
>>>> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
>>>> a system from suspend, while CAP_SYS_TIME covers the ability to set
>>>> the time. One wouldn't necessarily want to give applications which
>>>> could wake a system up the capability to also set the time.
>>>
>>> Doesn't really matter, except that an ignorant developer
>>> might make the mistake I did and assume that WAKE_ALARM
>>> was somehow related to time management. If you want to use
>>> it as an example don't let my dunderheadedness get in your
>>> way.
>>
>> Actually, I decided it wasn't such a good example anyway.
>> That capability could potentially be generic. (But it probably
>> should better have been named something like 'CAP_WAKE_SYSTEM'.)
>
> How about:
>
> Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
>
> As suggested by Michael Kerrisk his is a less confusing name, and
> this won't break any old userspace.
Yes, but. The names are used in user-space: should cap_to_text() or
cap_from_text() be reworked to understand these new names? For the former,
it seems like a bad idea, and for the latter it's dubious I would say.
So, I'm concerned that renaming/adding a synonym at the API level
creates some inconsistency with user-space. Maybe I worry too much?
What certainly would be good is to more clearly document the (broader)
purpose of the (misnamed) capability in capabilities(7). Something along
the lines of John's suggestion.
Cheers,
Michael
> Signed-off-by: Serge Hallyn <[email protected]>
> Cc: Michael Kerrisk <[email protected]>
> ---
> include/uapi/linux/capability.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index fd4f87d..ba972ff 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
>
> #define CAP_WAKE_ALARM 35
>
> +#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
> +
> /* Allow preventing system suspends */
>
> #define CAP_BLOCK_SUSPEND 36
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
On Friday, December 16, 2016 01:16:15 PM John Stultz wrote:
> On Fri, Dec 16, 2016 at 1:05 PM, Serge E. Hallyn <[email protected]> wrote:
> > Quoting John Stultz ([email protected]):
> >> On Fri, Dec 16, 2016 at 12:10 PM, Serge E. Hallyn <[email protected]> wrote:
> >> > Quoting Michael Kerrisk (man-pages) ([email protected]):
> >> >> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
> >> >> > On 12/15/2016 4:31 PM, John Stultz wrote:
> >> >> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
> >> >> >> <[email protected]> wrote:
> >> >> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
> >> >> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
> >> >> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
> >> >> >>>> Actually, I don't quite understand what you mean with that sentence.
> >> >> >>>> Could you elaborate?
> >> >> >>> Should have said CAP_SYS_TIME
> >> >> >>>
> >> >> >>> Setting an alarm could be considered a time management function,
> >> >> >>> depending on what it actually does.
> >> >> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
> >> >> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
> >> >> >> the time. One wouldn't necessarily want to give applications which
> >> >> >> could wake a system up the capability to also set the time.
> >> >> >
> >> >> > Doesn't really matter, except that an ignorant developer
> >> >> > might make the mistake I did and assume that WAKE_ALARM
> >> >> > was somehow related to time management. If you want to use
> >> >> > it as an example don't let my dunderheadedness get in your
> >> >> > way.
> >> >>
> >> >> Actually, I decided it wasn't such a good example anyway.
> >> >> That capability could potentially be generic. (But it probably
> >> >> should better have been named something like 'CAP_WAKE_SYSTEM'.)
> >> >
> >> > How about:
> >> >
> >> > Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
> >> >
> >> > As suggested by Michael Kerrisk his is a less confusing name, and
> >> > this won't break any old userspace.
> >> >
> >> > Signed-off-by: Serge Hallyn <[email protected]>
> >> > Cc: Michael Kerrisk <[email protected]>
> >> > ---
> >> > include/uapi/linux/capability.h | 2 ++
> >> > 1 file changed, 2 insertions(+)
> >> >
> >> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> >> > index fd4f87d..ba972ff 100644
> >> > --- a/include/uapi/linux/capability.h
> >> > +++ b/include/uapi/linux/capability.h
> >> > @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
> >> >
> >> > #define CAP_WAKE_ALARM 35
> >> >
> >> > +#define CAP_WAKE_SYSTEM CAP_WAKE_ALARM
> >> > +
> >>
> >> I was thinking of the same thing. Although I might rename the
> >> numerical define to WAKE_SYSTEM and put WAKE_ALARM as the alias (along
> >> with a comment as to WAKE_ALARM being deprecated), so its more clear
> >> which is the one that ought to be used by new code.
> >>
> >> However, in the spirit of this thread, we might even consider
> >> broadening the cap silo a bit further, to something like
> >> CAP_WAKE_SUSPEND, such that it might also be able to cover broader PM
> >> actions?
> >
> > Or just CAP_UNSUSPEND?
>
> I guess I was trying to capture it could be use for actions like both
> waking and suspending the system.
Well, CAP_SYS_PM comes to mind then.
Thanks,
Rafael