linux-kernel - Re: [PATCH v2] ptrace: reintroduce usage of subjective credentials in ptrace_has

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Message-ID: <CAG48ez2LhxbnfF1Y6u_QhrqXcPKVQZyG6N9-GgSw1D6fLgsfwA@mail.gmail.com>
Date:   Thu, 16 Jan 2020 17:07:42 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     eparis@...hat.com, kernel list <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, shallyn@...co.com,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

On Wed, Jan 15, 2020 at 6:24 PM Christian Brauner
<christian.brauner@...ntu.com> wrote:
> Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> introduced the ability to opt out of audit messages for accesses to
> various proc files since they are not violations of policy.
> While doing so it somehow switched the check from ns_capable() to
> has_ns_capability{_noaudit}(). That means it switched from checking the
> subjective credentials of the task to using the objective credentials. I
> couldn't find the original lkml thread and so I don't know why this switch
> was done. But it seems wrong since ptrace_has_cap() is currently only used
> in ptrace_may_access(). And it's used to check whether the calling task
> (subject) has the CAP_SYS_PTRACE capability in the provided user namespace
> to operate on the target task (object). According to the cred.h comments
> this would mean the subjective credentials of the calling task need to be
> used.
> This switches it to use security_capable() because we only call
> ptrace_has_cap() in ptrace_may_access() and in there we already have a
> stable reference to the calling tasks creds under cred_guard_mutex so
> there's no need to go through another series of dereferences and rcu
> locking done in ns_capable{_noaudit}().

Reviewed-by: Jann Horn <jannh@...gle.com>