[PATCH] ptrace: it is fun to strace /sbin/init - Kernel

This is a discussion on [PATCH] ptrace: it is fun to strace /sbin/init - Kernel ; (re-send with updated changelog) Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may be useful. Introduce the kernel boot parameter to allow this, so that we can't surprise some special/secured systems. Afaics, with the recent changes ...

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Thread: [PATCH] ptrace: it is fun to strace /sbin/init

  1. [PATCH] ptrace: it is fun to strace /sbin/init

    (re-send with updated changelog)

    Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    be useful. Introduce the kernel boot parameter to allow this, so that we can't
    surprise some special/secured systems.

    Afaics, with the recent changes there is no kernel problems with ptracing init,
    it can't lose SIGNAL_UNKILLABLE flag and be killed by accident. However, admin
    should know what it does, "gdb /sbin/init 1" stops init, it can't reap zombies
    or take care of /etc/inittab until continued. It is even possible to crash init
    (and thus the whole system) if you wish, ptracer has full control.

    The "if (pid == 1)" check in ptrace_get_task_struct() is killed, ptrace_attach
    does the same check.

    Signed-off-by: Oleg Nesterov

    --- 25/kernel/ptrace.c~5_INIT_PTRACE 2008-03-16 17:22:04.000000000 +0300
    +++ 25/kernel/ptrace.c 2008-03-16 18:33:02.000000000 +0300
    @@ -160,6 +160,15 @@ int ptrace_may_attach(struct task_struct
    return !err;
    }

    +static int allow_ptrace_init;
    +
    +static int __init __allow_ptrace_init(char *str)
    +{
    + allow_ptrace_init = 1;
    + return 1;
    +}
    +__setup("init_ptrace", __allow_ptrace_init);
    +
    int ptrace_attach(struct task_struct *task)
    {
    int retval;
    @@ -168,7 +177,7 @@ int ptrace_attach(struct task_struct *ta
    audit_ptrace(task);

    retval = -EPERM;
    - if (task->pid <= 1)
    + if (unlikely(is_global_init(task)) && likely(!allow_ptrace_init))
    goto out;
    if (same_thread_group(task, current))
    goto out;
    @@ -518,12 +527,6 @@ struct task_struct *ptrace_get_task_stru
    {
    struct task_struct *child;

    - /*
    - * Tracing init is not allowed.
    - */
    - if (pid == 1)
    - return ERR_PTR(-EPERM);
    -
    read_lock(&tasklist_lock);
    child = find_task_by_vpid(pid);
    if (child)
    --- 25/Documentation/kernel-parameters.txt~5_INIT_PTRACE 2008-02-15 16:58:12.000000000 +0300
    +++ 25/Documentation/kernel-parameters.txt 2008-03-16 18:30:28.000000000 +0300
    @@ -803,6 +803,8 @@ and is between 256 and 4096 characters.
    Run specified binary instead of /sbin/init as init
    process.

    + init_ptrace [KNL] Allows to ptrace init.
    +
    initcall_debug [KNL] Trace initcalls as they are executed. Useful
    for working out where the kernel is dying during
    startup.

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  2. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    Hi!

    > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > surprise some special/secured systems.
    >
    > Afaics, with the recent changes there is no kernel problems with ptracing init,
    > it can't lose SIGNAL_UNKILLABLE flag and be killed by accident. However, admin
    > should know what it does, "gdb /sbin/init 1" stops init, it can't
    > reap zombies


    It can't be killed, but you can stop it and not ever restart it. From
    that point on, zombies will accumulate until we OOM, right?

    > @@ -803,6 +803,8 @@ and is between 256 and 4096 characters.
    > Run specified binary instead of /sbin/init as init
    > process.
    >
    > + init_ptrace [KNL] Allows to ptrace init.
    > +


    No words about it being dangerous, but I believe it is.

    If it is not, lets do the patch, but not optional.

    If it is dangerous, option does not make it better.

    --
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  3. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/24, Pavel Machek wrote:
    >
    > > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > > surprise some special/secured systems.
    > >
    > > Afaics, with the recent changes there is no kernel problems with ptracing init,
    > > it can't lose SIGNAL_UNKILLABLE flag and be killed by accident. However, admin
    > > should know what it does, "gdb /sbin/init 1" stops init, it can't
    > > reap zombies

    >
    > It can't be killed, but you can stop it and not ever restart it. From
    > that point on, zombies will accumulate until we OOM, right?


    Right (only re-parented ones and init's childs but still right),

    > > @@ -803,6 +803,8 @@ and is between 256 and 4096 characters.
    > > Run specified binary instead of /sbin/init as init
    > > process.
    > >
    > > + init_ptrace [KNL] Allows to ptrace init.
    > > +

    >
    > No words about it being dangerous, but I believe it is.


    it is, admin should know what he does,

    > If it is not, lets do the patch, but not optional.


    This will change the default historical behaviour, I can't predict
    does this matter for (say) SELinux or not. Otherwise I'd personally
    prefer to always allow to ptrace init.

    > If it is dangerous, option does not make it better.


    ptrace() is always dangerous. ptracer can crash oracle and lose data.

    /sbin/init is important, but there are other important (and sometimes
    much more important) services. Why it is so special so that we can't
    debug/strace it?

    (how many times did you try to figure out why init does _not_ work
    as expected ?)

    Oleg.

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  4. Re: [PATCH] ptrace: it is fun to strace /sbin/init


    > > > @@ -803,6 +803,8 @@ and is between 256 and 4096 characters.
    > > > Run specified binary instead of /sbin/init as init
    > > > process.
    > > >
    > > > + init_ptrace [KNL] Allows to ptrace init.
    > > > +

    > >
    > > No words about it being dangerous, but I believe it is.

    >
    > it is, admin should know what he does,
    >
    > > If it is not, lets do the patch, but not optional.

    >
    > This will change the default historical behaviour, I can't predict
    > does this matter for (say) SELinux or not. Otherwise I'd personally
    > prefer to always allow to ptrace init.
    >
    > > If it is dangerous, option does not make it better.

    >
    > ptrace() is always dangerous. ptracer can crash oracle and lose data.
    >
    > /sbin/init is important, but there are other important (and sometimes
    > much more important) services. Why it is so special so that we can't
    > debug/strace it?


    Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    optional is wrong.
    Pavel
    --
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    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pav...rses/blog.html
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  5. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On Sun, 23 Mar 2008 16:51:10 +0300
    Oleg Nesterov wrote:

    > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > surprise some special/secured systems.


    I dunno, is this really needed? If root wants to screw up his kernel then
    he is free to do so.

    And if we *really* want an extra foot-protector for this, it could be a
    runtime /proc/sys/kernel/root-can-shoot-inits-foot rather than a boot-time
    option?

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  6. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/24, Pavel Machek wrote:
    >
    > > /sbin/init is important, but there are other important (and sometimes
    > > much more important) services. Why it is so special so that we can't
    > > debug/strace it?

    >
    > Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    > optional is wrong.


    You are right, the boot parameter is silly. How about sysctl?

    Stephen, do you see any security problems if we make /sbin/init
    ptraceable by default?

    Oleg.

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  7. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On Tue 2008-03-25 02:04:58, Oleg Nesterov wrote:
    > On 03/24, Pavel Machek wrote:
    > >
    > > > /sbin/init is important, but there are other important (and sometimes
    > > > much more important) services. Why it is so special so that we can't
    > > > debug/strace it?

    > >
    > > Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    > > optional is wrong.

    >
    > You are right, the boot parameter is silly. How about sysctl?


    I'd prefer it to be hardcoded, really.

    "You can kill /sbin/init" sounds right.

    "You can kill /sbin/init on 2.6.26+" sounds... still quite ok.

    "You can kill /sbin/init on 2.6.26+ if you have /proc/sys/foo/bar ==
    1" sounds... quite wrong.

    > Stephen, do you see any security problems if we make /sbin/init
    > ptraceable by default?


    Pavel
    --
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  8. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On Tue, 25 Mar 2008 01:56:11 +0300
    Oleg Nesterov wrote:

    > On 03/24, Andrew Morton wrote:
    > >
    > > On Sun, 23 Mar 2008 16:51:10 +0300
    > > Oleg Nesterov wrote:
    > >
    > > > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > > > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > > > surprise some special/secured systems.

    > >
    > > I dunno, is this really needed?

    >
    > Well, this is the question. I think it would be very nice to have the ability
    > to debug/strace init. Especially if you try to make your own distribution /
    > your own init.
    >
    > Sometimes I see init at the top of the top's output, with this patch I have a
    > chance to see what's going on on my system.


    I agree that init should be ptraceable. I'm questioning the value of a
    knob which enables that ability.

    Why not just unconditionally enable root's abiltiy to ptrace init?
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  9. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/24, Andrew Morton wrote:
    >
    > On Sun, 23 Mar 2008 16:51:10 +0300
    > Oleg Nesterov wrote:
    >
    > > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > > surprise some special/secured systems.

    >
    > I dunno, is this really needed?


    Well, this is the question. I think it would be very nice to have the ability
    to debug/strace init. Especially if you try to make your own distribution /
    your own init.

    Sometimes I see init at the top of the top's output, with this patch I have a
    chance to see what's going on on my system.

    > If root wants to screw up his kernel then
    > he is free to do so.


    I think you and Pavel are very wrong here.

    First, this has nothing to do with kernel, imho. Afaics, now there are no kernel
    problems with ptracing init.

    And. When I was admin in my previous life, I certainly was not able to patch
    the kernel and then strace/debug init.

    > And if we *really* want an extra foot-protector for this, it could be a
    > runtime /proc/sys/kernel/root-can-shoot-inits-foot rather than a boot-time
    > option?


    Even better! I agree, will re-send. I choose the boot-time paramater because
    it looks like the "most safe" option.

    Oleg.

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  10. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On Tue, 25 Mar 2008 02:18:59 +0300
    Oleg Nesterov wrote:

    > Please look at another discussion, http://marc.info/?t=120568298600007


    -EAMTOOLAZY. Whatever is at the above link should be in the changelog so we
    can understand the reasons for the patch!

    Please.
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  11. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/25, Pavel Machek wrote:
    >
    > On Tue 2008-03-25 02:04:58, Oleg Nesterov wrote:
    > > On 03/24, Pavel Machek wrote:
    > > >
    > > > > /sbin/init is important, but there are other important (and sometimes
    > > > > much more important) services. Why it is so special so that we can't
    > > > > debug/strace it?
    > > >
    > > > Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    > > > optional is wrong.

    > >
    > > You are right, the boot parameter is silly. How about sysctl?

    >
    > I'd prefer it to be hardcoded, really.


    Yes! me too.

    > "You can kill /sbin/init" sounds right.
    >
    > "You can kill /sbin/init on 2.6.26+" sounds... still quite ok.
    >
    > "You can kill /sbin/init on 2.6.26+ if you have /proc/sys/foo/bar ==
    > 1" sounds... quite wrong.


    Please look at another discussion, http://marc.info/?t=120568298600007

    When I did this simple patch, I was very sure it is "obviously good".
    But as Stephen pointed out, we have the systems that relies on the
    current behaviour, even if this behaviour is not "optimal".

    Oleg.

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  12. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/24, Andrew Morton wrote:
    >
    > On Tue, 25 Mar 2008 01:56:11 +0300
    > Oleg Nesterov wrote:
    >
    > > On 03/24, Andrew Morton wrote:
    > > >
    > > > On Sun, 23 Mar 2008 16:51:10 +0300
    > > > Oleg Nesterov wrote:
    > > >
    > > > > Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
    > > > > be useful. Introduce the kernel boot parameter to allow this, so that we can't
    > > > > surprise some special/secured systems.
    > > >
    > > > I dunno, is this really needed?

    > >
    > > Well, this is the question. I think it would be very nice to have the ability
    > > to debug/strace init. Especially if you try to make your own distribution /
    > > your own init.
    > >
    > > Sometimes I see init at the top of the top's output, with this patch I have a
    > > chance to see what's going on on my system.

    >
    > I agree that init should be ptraceable. I'm questioning the value of a
    > knob which enables that ability.
    >
    > Why not just unconditionally enable root's abiltiy to ptrace init?


    Ah, sorry, I misunderstood.

    As for me, I think it would be right to allow to ptrace init unconditionally.
    But I'd like to know what security people think, I am very much afraid there
    is something I don't know/understand (like it happened with "don't panic if
    /sbin/init exits or killed").

    Oleg.

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  13. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On 03/24, Andrew Morton wrote:
    >
    > On Tue, 25 Mar 2008 02:18:59 +0300
    > Oleg Nesterov wrote:
    >
    > > Please look at another discussion, http://marc.info/?t=120568298600007

    >
    > -EAMTOOLAZY. Whatever is at the above link should be in the changelog so we
    > can understand the reasons for the patch!


    No, no, this has nothing to do with this patch.

    This was another patch, "don't panic if /sbin/init exits or killed", which
    changed the current behaviour of /sbin/init unconditionally. And while I
    thought this change is "obviously good" it wasn't, because we have users
    which relies on the current behaviour.

    Oleg.

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  14. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    Andrew Morton writes:
    >
    > Why not just unconditionally enable root's abiltiy to ptrace init?


    It would be fine to allow this unconditionally if there was some mechanism
    to make sure someone else takes over reaping childs while init is ptraced.

    I like the general idea -- i used to patch kernels to allow this too,
    but it is dangerous.

    -Andi
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  15. Re: [PATCH] ptrace: it is fun to strace /sbin/init


    On Tue, 2008-03-25 at 02:04 +0300, Oleg Nesterov wrote:
    > On 03/24, Pavel Machek wrote:
    > >
    > > > /sbin/init is important, but there are other important (and sometimes
    > > > much more important) services. Why it is so special so that we can't
    > > > debug/strace it?

    > >
    > > Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    > > optional is wrong.

    >
    > You are right, the boot parameter is silly. How about sysctl?
    >
    > Stephen, do you see any security problems if we make /sbin/init
    > ptraceable by default?


    Not an issue for SELinux (we apply an orthogonal check based on security
    context, so we can already block ptrace of init independent of whether
    root/CAP_SYS_PTRACE can do it). I'm not sure though as to whether
    people using capabilities have ever relied on this special protection of
    init (e.g. custom init spawns children with lesser capabilities and
    relies on the fact that they cannot ptrace init to effectively re-gain
    those capabilities, even if they possess CAP_SYS_PTRACE).

    --
    Stephen Smalley
    National Security Agency

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  16. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    Quoting Stephen Smalley (sds@tycho.nsa.gov):
    >
    > On Tue, 2008-03-25 at 02:04 +0300, Oleg Nesterov wrote:
    > > On 03/24, Pavel Machek wrote:
    > > >
    > > > > /sbin/init is important, but there are other important (and sometimes
    > > > > much more important) services. Why it is so special so that we can't
    > > > > debug/strace it?
    > > >
    > > > Maybe. Let's kill /sbin/init protection in 2.6.26. But making it
    > > > optional is wrong.

    > >
    > > You are right, the boot parameter is silly. How about sysctl?
    > >
    > > Stephen, do you see any security problems if we make /sbin/init
    > > ptraceable by default?

    >
    > Not an issue for SELinux (we apply an orthogonal check based on security
    > context, so we can already block ptrace of init independent of whether
    > root/CAP_SYS_PTRACE can do it). I'm not sure though as to whether
    > people using capabilities have ever relied on this special protection of
    > init (e.g. custom init spawns children with lesser capabilities and
    > relies on the fact that they cannot ptrace init to effectively re-gain
    > those capabilities, even if they possess CAP_SYS_PTRACE).


    Still thinking it through, but it seems like special casing init isn't
    useful. There are likely to be other tasks with all capabilities
    set which the malicious task could just as well ptrace to do his
    mischief, right?

    thanks,
    -serge
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  17. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    Andi Kleen writes:

    > It would be fine to allow this unconditionally if there was some mechanism
    > to make sure someone else takes over reaping childs while init is
    > ptraced.


    Well, I think it isn't even necessary, unless you ptrace init for a
    long time. I would simply go with the simple 1-line patch.

    > I like the general idea -- i used to patch kernels to allow this too,
    > but it is dangerous.


    Though root-only. Root doesn't want to be limited and knows better
    anyway. I think ptracing init is useful even when no one takes care
    of the dead (especially if you want to trace waits).
    --
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  18. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    > > I like the general idea -- i used to patch kernels to allow this too,
    > > but it is dangerous.

    >
    > Though root-only. Root doesn't want to be limited and knows better
    > anyway.


    The problem is that you can deadlock if you are not very careful.

    -Andi
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  19. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    Andi Kleen writes:

    > The problem is that you can deadlock if you are not very careful.


    I can damage the system in trillion ways, one more is not a problem
    :-)

    Hopefully someone will write the details down to enable root to be
    very careful.
    --
    Krzysztof Halasa
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  20. Re: [PATCH] ptrace: it is fun to strace /sbin/init

    On Tue, Mar 25, 2008 at 03:30:29PM +0100, Krzysztof Halasa wrote:
    > Andi Kleen writes:
    >
    > > The problem is that you can deadlock if you are not very careful.

    >
    > I can damage the system in trillion ways, one more is not a problem
    > :-)


    We don't actually have that many ways for hard deadlocks even
    as root (short of doing really nasty things on /dev/kmem)

    > Hopefully someone will write the details down to enable root to be
    > very careful.


    Sure, but if the deadlocks can be avoided on the kernel level (just
    making sure the children are already reaped) then it would be even
    better.

    -Andi
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