From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Alexey Dobriyan <adobriyan@gmail.com>,
Ken Chen <kenchen@google.com>,
kernel list <linux-kernel@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Will Deacon <will.deacon@arm.com>,
Laura Abbott <labbott@redhat.com>,
Andy Lutomirski <luto@amacapital.net>,
Security Officers <security@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] proc: restrict kernel stack dumps to root
Date: Wed, 26 Sep 2018 19:03:24 -0700 [thread overview]
Message-ID: <CAGXu5jKcxDThFHsueZGFcNSvqi3JSgWA3hWrxEYPm-3YRUkbFw@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez0M90ajea8Cee32VjRV4A5CjM8FA1kpdmL6yThBrGf9Lg@mail.gmail.com>
On Wed, Sep 26, 2018 at 6:19 PM, Jann Horn <jannh@google.com> wrote:
> On Thu, Sep 13, 2018 at 4:39 PM Kees Cook <keescook@chromium.org> wrote:
>> On Thu, Sep 13, 2018 at 4:55 AM, Jann Horn <jannh@google.com> wrote:
>> > On Thu, Sep 13, 2018 at 12:28 AM Kees Cook <keescook@chromium.org> wrote:
>> >>
>> >> On Wed, Sep 12, 2018 at 8:29 AM, Jann Horn <jannh@google.com> wrote:
>> >> > +linux-api, I guess
>> >> >
>> >> > On Tue, Sep 11, 2018 at 8:39 PM Jann Horn <jannh@google.com> 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 <jannh@google.com>
>> >> >> ---
>> >> >> 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;)
>> >
>> > In my mind, this is different because it's a place where we don't have
>> > to selectively censor output while preserving parts of it, and it's a
>> > place where, as Laura said, it's useful to make lack of privileges
>> > clearly visible because that informs users that they may have to retry
>> > with more privileges.
>> >
>> > Of course, if you have an example of software that actually breaks due
>> > to this, I'll change it. But I looked at the three things in Debian
>> > codesearch that seem to use it, and from what I can tell, they all
>> > bail out cleanly when the read fails.
>>
>> I prefer -EACCESS too, but I thought I'd mention the alternative. So, I guess:
>>
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> What do I need to do to get this merged? Oh, I think I misread
> MAINTAINERS - Alexey Dobriyan is not a maintainer, just a reviewer. So
> I guess this has to go via Andrew Morton? Should I resend the patch
> with Andrew in the recipient list?
Yeah, traditionally Andrew has taken /proc patches.
-Kees
--
Kees Cook
Pixel Security
prev parent reply other threads:[~2018-09-27 8:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-11 18:39 [PATCH] proc: restrict kernel stack dumps to root Jann Horn
2018-09-12 15:29 ` Jann Horn
2018-09-12 22:27 ` Kees Cook
2018-09-12 22:47 ` Laura Abbott
2018-09-13 11:55 ` Jann Horn
2018-09-13 14:39 ` Kees Cook
2018-09-27 1:19 ` Jann Horn
2018-09-27 2:03 ` Kees Cook [this message]
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