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[209.85.219.177]) by smtp.gmail.com with ESMTPSA id 203-v6sm872891ywv.34.2018.09.12.15.27.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 12 Sep 2018 15:27:55 -0700 (PDT) Received: by mail-yb1-f177.google.com with SMTP id t71-v6so2460586ybi.7 for ; Wed, 12 Sep 2018 15:27:54 -0700 (PDT) X-Received: by 2002:a25:19c3:: with SMTP id 186-v6mr2111427ybz.410.1536791274293; Wed, 12 Sep 2018 15:27:54 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:a25:5f04:0:0:0:0:0 with HTTP; Wed, 12 Sep 2018 15:27:53 -0700 (PDT) In-Reply-To: References: <20180911183909.233413-1-jannh@google.com> From: Kees Cook Date: Wed, 12 Sep 2018 15:27:53 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] proc: restrict kernel stack dumps to root To: Jann Horn Cc: Alexey Dobriyan , Ken Chen , kernel list , "linux-fsdevel@vger.kernel.org" , Will Deacon , Laura Abbott , Andy Lutomirski , Security Officers , Catalin Marinas , Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Linux API Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 12, 2018 at 8:29 AM, Jann Horn wrote: > +linux-api, I guess > > On Tue, Sep 11, 2018 at 8:39 PM Jann Horn wrote: >> >> Restrict the ability to inspect kernel stacks of arbitrary tasks to root >> in order to prevent a local attacker from exploiting racy stack unwinding >> to leak kernel task stack contents. >> See the added comment for a longer rationale. >> >> There don't seem to be any users of this userspace API that can't >> gracefully bail out if reading from the file fails. Therefore, I believe >> that this change is unlikely to break things. >> In the case that this patch does end up needing a revert, the next-best >> solution might be to fake a single-entry stack based on wchan. >> >> Fixes: 2ec220e27f50 ("proc: add /proc/*/stack") >> Cc: stable@vger.kernel.org >> Signed-off-by: Jann Horn >> --- >> fs/proc/base.c | 14 ++++++++++++++ >> 1 file changed, 14 insertions(+) >> >> diff --git a/fs/proc/base.c b/fs/proc/base.c >> index ccf86f16d9f0..7e9f07bf260d 100644 >> --- a/fs/proc/base.c >> +++ b/fs/proc/base.c >> @@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns, >> unsigned long *entries; >> int err; >> >> + /* >> + * The ability to racily run the kernel stack unwinder on a running task >> + * and then observe the unwinder output is scary; while it is useful for >> + * debugging kernel issues, it can also allow an attacker to leak kernel >> + * stack contents. >> + * Doing this in a manner that is at least safe from races would require >> + * some work to ensure that the remote task can not be scheduled; and >> + * even then, this would still expose the unwinder as local attack >> + * surface. >> + * Therefore, this interface is restricted to root. >> + */ >> + if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN)) >> + return -EACCES; In the past, we've avoided hard errors like this in favor of just censoring the output. Do we want to be more cautious here? (i.e. return 0 or a fuller seq_printf(m, "[<0>] privileged\n"); return 0;) >> + >> entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries), >> GFP_KERNEL); >> if (!entries) >> -- >> 2.19.0.rc2.392.g5ba43deb5a-goog >> -Kees -- Kees Cook Pixel Security