From: "Tobin C. Harding" <me@tobin.cc>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org, kernel-hardening@lists.openwall.com,
linux-kernel@vger.kernel.org,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [RFC] Warn the user when they could overflow mapcount
Date: Thu, 8 Feb 2018 14:18:04 +1100 [thread overview]
Message-ID: <20180208031804.GD3304@eros> (raw)
In-Reply-To: <20180208021112.GB14918@bombadil.infradead.org>
On Wed, Feb 07, 2018 at 06:11:12PM -0800, Matthew Wilcox wrote:
>
> Kirill and I were talking about trying to overflow page->_mapcount
> the other day and realised that the default settings of pid_max and
> max_map_count prevent it [1]. But there isn't even documentation to
> warn a sysadmin that they've just opened themselves up to the possibility
> that they've opened their system up to a sufficiently-determined attacker.
>
> I'm not sufficiently wise in the ways of the MM to understand exactly
> what goes wrong if we do wrap mapcount. Kirill says:
>
> rmap depends on mapcount to decide when the page is not longer mapped.
> If it sees page_mapcount() == 0 due to 32-bit wrap we are screwed;
> data corruption, etc.
>
> That seems pretty bad. So here's a patch which adds documentation to the
> two sysctls that a sysadmin could use to shoot themselves in the foot,
> and adds a warning if they change either of them to a dangerous value.
> It's possible to get into a dangerous situation without triggering this
> warning (already have the file mapped a lot of times, then lower pid_max,
> then raise max_map_count, then map the file a lot more times), but it's
> unlikely to happen.
>
> Comments?
>
> [1] map_count counts the number of times that a page is mapped to
> userspace; max_map_count restricts the number of times a process can
> map a page and pid_max restricts the number of processes that can exist.
> So map_count can never be larger than pid_max * max_map_count.
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 412314eebda6..ec90cd633e99 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -718,6 +718,8 @@ pid_max:
> PID allocation wrap value. When the kernel's next PID value
> reaches this value, it wraps back to a minimum PID value.
> PIDs of value pid_max or larger are not allocated.
> +Increasing this value without decreasing vm.max_map_count may
> +allow a hostile user to corrupt kernel memory
>
> ==============================================================
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index ff234d229cbb..0ab306ea8f80 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -379,7 +379,8 @@ While most applications need less than a thousand maps, certain
> programs, particularly malloc debuggers, may consume lots of them,
> e.g., up to one or two maps per allocation.
>
> -The default value is 65536.
> +The default value is 65530. Increasing this value without decreasing
> +pid_max may allow a hostile user to corrupt kernel memory.
Just checking - did you mean the final '0' on this value?
Tobin
next prev parent reply other threads:[~2018-02-08 3:18 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-08 2:11 [RFC] Warn the user when they could overflow mapcount Matthew Wilcox
2018-02-08 2:56 ` Jann Horn
2018-02-08 4:04 ` Matthew Wilcox
2018-02-08 17:58 ` valdis.kletnieks
2018-02-08 18:05 ` Daniel Micay
2018-02-08 18:56 ` Matthew Wilcox
2018-02-08 19:33 ` Daniel Micay
2018-02-08 19:42 ` Matthew Wilcox
2018-02-08 19:48 ` Daniel Micay
2018-02-08 20:21 ` Matthew Wilcox
2018-02-08 21:37 ` [RFC] Limit mappings to ten per page per process Matthew Wilcox
2018-02-09 4:26 ` Kirill A. Shutemov
2018-02-14 13:51 ` Matthew Wilcox
2018-02-14 14:05 ` Kirill A. Shutemov
2018-02-09 1:47 ` [RFC] Warn the user when they could overflow mapcount Daniel Micay
2018-02-08 3:18 ` Tobin C. Harding [this message]
2018-02-08 4:06 ` Matthew Wilcox
2018-03-02 21:26 ` [RFC] Handle mapcount overflows Matthew Wilcox
2018-03-02 22:03 ` Matthew Wilcox
2019-05-01 14:41 ` Jann Horn
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180208031804.GD3304@eros \
--to=me@tobin.cc \
--cc=kernel-hardening@lists.openwall.com \
--cc=kirill.shutemov@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).