linux-kernel - Re: [RFC PATCH 1/6] set_user: Perform RLIMIT_NPROC capability check against new user credentials

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Message-ID: <87h795xhxs.fsf@email.froward.int.ebiederm.org>
Date:   Fri, 11 Feb 2022 14:32:47 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Solar Designer <solar@...nwall.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Alexey Gladkov <legion@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 1/6] set_user: Perform RLIMIT_NPROC capability check
 against new user credentials

Solar Designer <solar@...nwall.com> writes:

> Hi Michal,
>
> On Mon, Feb 07, 2022 at 01:17:55PM +0100, Michal Koutný wrote:
>> The check is currently against the current->cred but since those are
>> going to change and we want to check RLIMIT_NPROC condition after the
>> switch, supply the capability check with the new cred.
>> But since we're checking new_user being INIT_USER any new cred's
>> capability-based allowance may be redundant when the check fails and the
>> alternative solution would be revert of the commit 2863643fb8b9
>> ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")
>> 
>> Fixes: 2863643fb8b9 ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")
>> 
>> Cc: Solar Designer <solar@...nwall.com>
>> Cc: Christian Brauner <christian.brauner@...ntu.com>
>> Signed-off-by: Michal Koutný <mkoutny@...e.com>
>> ---
>>  kernel/sys.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index 8ea20912103a..48c90dcceff3 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -481,7 +481,8 @@ static int set_user(struct cred *new)
>>  	 */
>>  	if (ucounts_limit_cmp(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) >= 0 &&
>>  			new_user != INIT_USER &&
>> -			!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>> +			!security_capable(new, &init_user_ns, CAP_SYS_RESOURCE, CAP_OPT_NONE) &&
>> +			!security_capable(new, &init_user_ns, CAP_SYS_ADMIN, CAP_OPT_NONE))
>>  		current->flags |= PF_NPROC_EXCEEDED;
>>  	else
>>  		current->flags &= ~PF_NPROC_EXCEEDED;
>
> Thank you for working on this and CC'ing me on it.  This is related to
> the discussion Christian and I had in September:
>
> https://lore.kernel.org/all/20210913100140.bxqlg47pushoqa3r@wittgenstein/
>
> Christian was going to revert 2863643fb8b9, but apparently that never
> happened.  Back then, I also suggested:
>
> "Alternatively, we could postpone the set_user() calls until we're
> running with the new user's capabilities, but that's an invasive change
> that's likely to create its own issues."

Back then you mentioned that apache suexec was broken.  Do you have
any more details?

I would like to make certain the apache suexec issue is fixed but
without a few details I can't do that.  I tried looking but I can't
find an public report about apache suexec being broken.

My goal is to come up with a very careful and conservative set of
patches that fix all of the known issues with RLIMIT_NPROC.

Eric