Re: Inconsistent capability requirements for prctl_set_mm_exe_file()

Kirill Tkhai
2020-10-27T17:22:40Z

On 27.10.2020 15:11, Michael Kerrisk (man-pages) wrote:
> Hello Nicolas, Cyrill, and others,
>
> @Nicolas, your commit ebd6de6812387a changed the capability
> requirements for the prctl_set_mm_exe_file() operation from
>
> ns_capable(CAP_SYS_ADMIN)
>
> to
>
> ns_capable(CAP_SYS_ADMIN) || ns_capable(CAP_CHECKPOINT_RESTORE).
>
> That's fine I guess, but while looking at that change, I found
> an anomaly.
>
> The same prctl_set_mm_exe_file() functionality is also available
> via the prctl() PR_SET_MM_EXE_FILE operation, which was added
> by Cyrill's commit b32dfe377102ce668. However, there the
> prctl_set_mm_exe_file() operation is guarded by a check
>
> capable(CAP_SYS_RESOURCE).
>
> There are two things I note:
>
> * The capability requirements are different in the two cases.
> * In one case the checks are with ns_capable(), while in the
> other case the check is with capable().
>
> In both cases, the inconsistencies predate Nicolas's patch,
> and appear to have been introduced in Kirill Tkhai's commit
> 4d28df6152aa3ff.

1)Before my commit there also were different checks

!capable(CAP_SYS_RESOURCE))
and
uid_eq(cred->uid, make_kuid(ns, 0)) && gid_eq(cred->gid, make_kgid(ns, 0))

so it is not the initial reason. The commit even decreased the checks difference
and it made both the checks are about capability().

2)As I understand new PR_SET_MM_MAP interface differs in the way, that it allows to batch
a setup of prctl_mm_map parameters. So, instead of 14 prlctl calls with different arguments:
PR_SET_MM_START_CODE, PR_SET_MM_END_CODE, PR_SET_MM_START_DATA, .., PR_SET_MM_ENV_END,
we set then all at once and the performance is better.

The second advantage is that the new interface is more comfortable in case of we set all
of 14 parameters. Old interface requires special order of calls: sometimes you have to
set PR_SET_MM_START_CODE first and then PR_SET_MM_END_CODE second, some times it is vice
versa. Otherwise __prctl_check_order() in validate_prctl_map() will fail.

3)For me it looks like any combinations of parameters acceptable to be set by both interfaces
are the same (in case of we don't see on permissions checks). In case of we can assign a set of
parameters {A} using old interface, we can assign it from new interface and vice versa.
Isn't this so?! If so, we should use the same permissions check.

Kirill