Re: [PATCH 4.20 282/352] fs/proc/base.c: use ns_capable instead of capable for timerslack_ns

[Eric W. Biederman]

Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:

> 4.20-stable review patch.  If anyone has any objections, please let me
> know.

No objection.  But I think of this as a feature addition rather than a
fix for something.  As a feature that we now allow something we
previously did not does this qualify for a backport to stable?

It is probably no more harmful in this instance than adding PCI IDs to a
driver.  So I am not worried.  I am curious the current guidelines
are.

In most cases a small relaxation of permissions like this requires a lot
of bug fixing as typically code protected by capable(CAP_XXX) has been
written and tested assuming a trusted root user.  Those bug fixes are
many times too large for a stable backport.

Eric


> ------------------
>
> [ Upstream commit 8da0b4f692c6d90b09c91f271517db746a22ff67 ]
>
> Access to timerslack_ns is controlled by a process having CAP_SYS_NICE
> in its effective capability set, but the current check looks in the root
> namespace instead of the process' user namespace.  Since a process is
> allowed to do other activities controlled by CAP_SYS_NICE inside a
> namespace, it should also be able to adjust timerslack_ns.
>
> Link: http://lkml.kernel.org/r/20181030180012.232896-1-bmgordon@xxxxxxxxxx
> Signed-off-by: Benjamin Gordon <bmgordon@xxxxxxxxxx>
> Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Cc: Oren Laadan <orenl@xxxxxxxxxxx>
> Cc: Ruchi Kandoi <kandoiruchi@xxxxxxxxxx>
> Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Nick Kralevich <nnk@xxxxxxxxxx>
> Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
> Cc: Elliott Hughes <enh@xxxxxxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  fs/proc/base.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ce3465479447..98525af0953e 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2356,10 +2356,13 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>  		return -ESRCH;
>  
>  	if (p != current) {
> -		if (!capable(CAP_SYS_NICE)) {
> +		rcu_read_lock();
> +		if (!ns_capable(__task_cred(p)->user_ns, CAP_SYS_NICE)) {
> +			rcu_read_unlock();
>  			count = -EPERM;
>  			goto out;
>  		}
> +		rcu_read_unlock();
>  
>  		err = security_task_setscheduler(p);
>  		if (err) {
> @@ -2392,11 +2395,14 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>  		return -ESRCH;
>  
>  	if (p != current) {
> -
> -		if (!capable(CAP_SYS_NICE)) {
> +		rcu_read_lock();
> +		if (!ns_capable(__task_cred(p)->user_ns, CAP_SYS_NICE)) {
> +			rcu_read_unlock();
>  			err = -EPERM;
>  			goto out;
>  		}
> +		rcu_read_unlock();
> +
>  		err = security_task_getscheduler(p);
>  		if (err)
>  			goto out;