Re: [RFC] [PATCH -mm] oom_kill: remove uid==0 checks

[Andrew Morgan <morgan@kernel.org>]

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Serge E. Hallyn wrote:
> Andrew, I've cc:d you here bc in doing this patch I noticed that your
> 64-bit capabilities patch switched this code from an explicit check
> of cap_t(p->cap_effective) to using __capable().  That means that
> now being glossed over by the oom killer means PF_SUPERPRIV will
> be set.  Is that intentional?

Yes, I switched the check because the old one didn't work with the new
capability representation.

However, I had not thought this aspect of this replacement through. At
the time, it seemed obvious but in this case it actually depends on
whether you think using privilege (PF_SUPERPRIV) means "benefited from
privilege", or "successfully completed a privileged operation".

I suspect, in this case, the correct thing to do is add the equivalent of:

#define CAPABLE_PROBE_ONLY(a,b)   (!security_capable(a,b))

and use that in the code in question. That is, return to the old
behavior in a way that will not break if we ever need to add more bits.

Thanks for finding this.

Cheers

Andrew

> 
> Signed-off-by: Serge Hallyn <serue@us.ibm.com>
> ---
>  mm/oom_kill.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 016127e..9fd8d5d 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -128,7 +128,7 @@ unsigned long badness(struct task_struct *p, unsigned long uptime,
>  	 * Superuser processes are usually more important, so we make it
>  	 * less likely that we kill those.
>  	 */
> -	if (__capable(p, CAP_SYS_ADMIN) || p->uid == 0 || p->euid == 0)
> +	if (__capable(p, CAP_SYS_ADMIN) || __capable(p, CAP_SYS_RESOURCE))
>  		points /= 4;
>  
>  	/*

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