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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id t18sm16339996pfh.57.2021.04.05.10.16.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Apr 2021 10:16:36 -0700 (PDT) Date: Mon, 5 Apr 2021 10:16:35 -0700 From: Kees Cook To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Hillf Danton , Michal Hocko , Matthew Wilcox , Oleksiy Avramchenko , Steven Rostedt , Minchan Kim , huang ying , Jonathan Corbet , Russell King , Liviu Dudau , Sudeep Holla , Lorenzo Pieralisi , Andrew Lunn , Gregory Clement , Sebastian Hesselbarth , Yoshinori Sato , Brian Cain , Geert Uytterhoeven , Jonas Bonn , Stefan Kristiansson , Stafford Horne , Rich Felker , "David S. Miller" , Chris Zankel , Max Filippov , Arnd Bergmann , Greg Kroah-Hartman , Alexander Viro , Rob Herring , "Pavel Machek (CIP)" , Theodore Dubois , "Alexander A. Klimov" , Pavel Machek , Sam Ravnborg , Alexandre Belloni , Andrey Zhizhikin , Randy Dunlap , Krzysztof Kozlowski , Viresh Kumar , "Eric W. Biederman" , Thomas Gleixner , Xiaoming Ni , Robert Richter , William Cohen , Corentin Labbe , Kairui Song , Linus Torvalds , linux-doc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, uclinux-h8-devel@lists.sourceforge.jp, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, openrisc@lists.librecores.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-fsdevel@vger.kernel.org, Linux API Subject: Re: [PATCH RFC 1/3] drivers/char: remove /dev/kmem for good Message-ID: <202104051013.F432CAC4@keescook> References: <20210319143452.25948-1-david@redhat.com> <20210319143452.25948-2-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210319143452.25948-2-david@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-sh@vger.kernel.org On Fri, Mar 19, 2021 at 03:34:50PM +0100, David Hildenbrand wrote: > Exploring /dev/kmem and /dev/mem in the context of memory hot(un)plug and > memory ballooning, I started questioning the existance of /dev/kmem. > > Comparing it with the /proc/kcore implementation, it does not seem to be > able to deal with things like > a) Pages unmapped from the direct mapping (e.g., to be used by secretmem) > -> kern_addr_valid(). virt_addr_valid() is not sufficient. > b) Special cases like gart aperture memory that is not to be touched > -> mem_pfn_is_ram() > Unless I am missing something, it's at least broken in some cases and might > fault/crash the machine. > > Looks like its existance has been questioned before in 2005 and 2010 > [1], after ~11 additional years, it might make sense to revive the > discussion. > > CONFIG_DEVKMEM is only enabled in a single defconfig (on purpose or by > mistake?). All distributions I looked at disable it. > > 1) /dev/kmem was popular for rootkits [2] before it got disabled > basically everywhere. Ubuntu documents [3] "There is no modern user of > /dev/kmem any more beyond attackers using it to load kernel rootkits.". > RHEL documents in a BZ [5] "it served no practical purpose other than to > serve as a potential security problem or to enable binary module drivers > to access structures/functions they shouldn't be touching" > > 2) /proc/kcore is a decent interface to have a controlled way to read > kernel memory for debugging puposes. (will need some extensions to > deal with memory offlining/unplug, memory ballooning, and poisoned > pages, though) > > 3) It might be useful for corner case debugging [1]. KDB/KGDB might be a > better fit, especially, to write random memory; harder to shoot > yourself into the foot. > > 4) "Kernel Memory Editor" hasn't seen any updates since 2000 and seems > to be incompatible with 64bit [1]. For educational purposes, > /proc/kcore might be used to monitor value updates -- or older > kernels can be used. > > 5) It's broken on arm64, and therefore, completely disabled there. > > Looks like it's essentially unused and has been replaced by better > suited interfaces for individual tasks (/proc/kcore, KDB/KGDB). Let's > just remove it. > > [1] https://lwn.net/Articles/147901/ > [2] https://www.linuxjournal.com/article/10505 > [3] https://wiki.ubuntu.com/Security/Features#A.2Fdev.2Fkmem_disabled > [4] https://sourceforge.net/projects/kme/ > [5] https://bugzilla.redhat.com/show_bug.cgi?id=154796 > > [...] > Cc: Linux API > Signed-off-by: David Hildenbrand Yes please! As James Troup pointed out already, this was turned off in Ubuntu in 2008. I don't remember a single complaint from anyone who wasn't a rootkit author. ;) Acked-by: Kees Cook -- Kees Cook