From: Matthew Garrett <matthew.garrett@nebula.com> To: Greg KH <gregkh@linuxfoundation.org> Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>, "keescook@chromium.org" <keescook@chromium.org>, "hpa@zytor.com" <hpa@zytor.com> Subject: Re: [PATCH V3 08/11] kexec: Disable at runtime if the kernel enforces module loading restrictions Date: Sun, 8 Sep 2013 06:44:08 +0000 [thread overview] Message-ID: <1378622648.2300.4.camel@x230> (raw) In-Reply-To: <20130908064027.GA3587@kroah.com> [-- Warning: decoded text below may be mangled, UTF-8 assumed --] [-- Attachment #1: Type: text/plain; charset="utf-8", Size: 1723 bytes --] On Sat, 2013-09-07 at 23:40 -0700, Greg KH wrote: > On Tue, Sep 03, 2013 at 07:50:15PM -0400, Matthew Garrett wrote: > > kexec permits the loading and execution of arbitrary code in ring 0, which > > is something that module signing enforcement is meant to prevent. It makes > > sense to disable kexec in this situation. > > I see no match between kexec and signed kernel modules. sig_enforce is there to prevent anyone (including root) from installing new kernel code in the running kernel. Allowing kexec to run untrusted code allows root to install new kernel code in the running kernel. At the most trivial level, grab the address of sig_enforce from kallsyms, jump to a kernel that doesn't enforce STRICT_DEVMEM, modify sig_enforce, jump back to the old kernel. > In fact, I personally _want_ signed kernel modules, and still the option > to run kexec. kexec is to run a whole new kernel/OS, not a tiny kernel > module. No, kexec is to run anything. It's expressly not limited to launching new kernels. It's easiest to demonstrate an attack using a Linux kernel, but you could launch a toy payload that did nothing other than modify one byte and then returned to the launch kernel. > If you apply this, you break everyone who is currently relying on kexec > (i.e. kdump, bootloaders, etc.), from using signed kernel modules, which > personally, seems like a very bad idea. Enforcing signed modules provides you with no additional security if you have kexec enabled. It's better to make that obvious. -- Matthew Garrett <matthew.garrett@nebula.com> ÿôèº{.nÇ+·®+%Ëÿ±éݶ\x17¥wÿº{.nÇ+·¥{±þG«éÿ{ayº\x1dÊÚë,j\a¢f£¢·hïêÿêçz_è®\x03(éÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?¨èÚ&£ø§~á¶iOæ¬z·vØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?I¥
WARNING: multiple messages have this Message-ID (diff)
From: Matthew Garrett <matthew.garrett-05XSO3Yj/JvQT0dZR+AlfA@public.gmane.org> To: Greg KH <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org> Cc: "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, "linux-efi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" <linux-efi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, "keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org" <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>, "hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org> Subject: Re: [PATCH V3 08/11] kexec: Disable at runtime if the kernel enforces module loading restrictions Date: Sun, 8 Sep 2013 06:44:08 +0000 [thread overview] Message-ID: <1378622648.2300.4.camel@x230> (raw) In-Reply-To: <20130908064027.GA3587-U8xfFu+wG4EAvxtiuMwx3w@public.gmane.org> On Sat, 2013-09-07 at 23:40 -0700, Greg KH wrote: > On Tue, Sep 03, 2013 at 07:50:15PM -0400, Matthew Garrett wrote: > > kexec permits the loading and execution of arbitrary code in ring 0, which > > is something that module signing enforcement is meant to prevent. It makes > > sense to disable kexec in this situation. > > I see no match between kexec and signed kernel modules. sig_enforce is there to prevent anyone (including root) from installing new kernel code in the running kernel. Allowing kexec to run untrusted code allows root to install new kernel code in the running kernel. At the most trivial level, grab the address of sig_enforce from kallsyms, jump to a kernel that doesn't enforce STRICT_DEVMEM, modify sig_enforce, jump back to the old kernel. > In fact, I personally _want_ signed kernel modules, and still the option > to run kexec. kexec is to run a whole new kernel/OS, not a tiny kernel > module. No, kexec is to run anything. It's expressly not limited to launching new kernels. It's easiest to demonstrate an attack using a Linux kernel, but you could launch a toy payload that did nothing other than modify one byte and then returned to the launch kernel. > If you apply this, you break everyone who is currently relying on kexec > (i.e. kdump, bootloaders, etc.), from using signed kernel modules, which > personally, seems like a very bad idea. Enforcing signed modules provides you with no additional security if you have kexec enabled. It's better to make that obvious. -- Matthew Garrett <matthew.garrett@nebula.com>
next prev parent reply other threads:[~2013-09-08 6:44 UTC|newest] Thread overview: 108+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-09-03 23:50 Matthew Garrett 2013-09-03 23:50 ` (unknown), Matthew Garrett 2013-09-03 23:50 ` [PATCH V3 01/11] Add secure_modules() call Matthew Garrett 2013-09-04 0:45 ` James Morris 2013-09-04 0:45 ` James Morris 2013-09-05 2:14 ` joeyli 2013-09-05 2:14 ` joeyli 2013-09-03 23:50 ` [PATCH V3 02/11] PCI: Lock down BAR access when module security is enabled Matthew Garrett 2013-09-04 0:45 ` James Morris 2013-09-04 0:45 ` James Morris 2013-09-04 16:57 ` David Woodhouse 2013-09-04 16:57 ` David Woodhouse 2013-09-04 17:04 ` Matthew Garrett 2013-09-04 17:04 ` Matthew Garrett 2013-09-04 18:58 ` David Woodhouse 2013-09-04 19:01 ` Matthew Garrett 2013-09-04 19:01 ` Matthew Garrett 2013-09-04 19:31 ` David Woodhouse 2013-09-04 19:31 ` David Woodhouse 2013-09-03 23:50 ` [PATCH V3 03/11] x86: Lock down IO port " Matthew Garrett 2013-09-04 0:45 ` James Morris 2013-09-05 3:52 ` H. Peter Anvin 2013-09-05 3:52 ` H. Peter Anvin 2013-09-05 3:58 ` Matthew Garrett 2013-09-05 3:58 ` Matthew Garrett 2013-09-05 15:36 ` H. Peter Anvin 2013-09-05 15:36 ` H. Peter Anvin 2013-09-03 23:50 ` [PATCH V3 04/11] ACPI: Limit access to custom_method Matthew Garrett 2013-09-04 0:46 ` James Morris 2013-09-04 0:46 ` James Morris 2013-09-03 23:50 ` [PATCH V3 05/11] asus-wmi: Restrict debugfs interface when module loading is restricted Matthew Garrett 2013-09-04 0:46 ` James Morris 2013-09-04 0:46 ` James Morris 2013-09-03 23:50 ` [PATCH V3 06/11] Restrict /dev/mem and /dev/kmem " Matthew Garrett 2013-09-04 0:47 ` James Morris 2013-09-04 0:47 ` James Morris 2013-09-03 23:50 ` [PATCH V3 07/11] acpi: Ignore acpi_rsdp kernel parameter " Matthew Garrett 2013-09-03 23:50 ` Matthew Garrett 2013-09-03 23:50 ` [PATCH V3 08/11] kexec: Disable at runtime if the kernel enforces module loading restrictions Matthew Garrett 2013-09-04 0:48 ` James Morris 2013-09-04 0:48 ` James Morris 2013-09-04 20:09 ` jerry.hoemann 2013-09-04 20:09 ` jerry.hoemann-VXdhtT5mjnY 2013-09-04 20:12 ` Matthew Garrett 2013-09-04 20:12 ` Matthew Garrett 2013-09-04 20:12 ` Matthew Garrett 2013-09-04 20:14 ` Josh Boyer 2013-09-04 20:14 ` Josh Boyer 2013-09-04 20:14 ` Josh Boyer 2013-09-08 6:40 ` Greg KH 2013-09-08 6:40 ` Greg KH 2013-09-08 6:44 ` Matthew Garrett [this message] 2013-09-08 6:44 ` Matthew Garrett 2013-09-08 7:24 ` Greg KH 2013-09-08 7:24 ` Greg KH 2013-09-08 14:40 ` Matthew Garrett 2013-09-08 14:40 ` Matthew Garrett 2013-09-08 15:51 ` Kees Cook 2013-09-08 15:51 ` Kees Cook 2013-09-08 16:18 ` Greg KH 2013-09-08 16:18 ` Greg KH 2013-09-08 16:24 ` Matthew Garrett 2013-09-08 16:24 ` Matthew Garrett 2013-09-08 16:39 ` Greg KH 2013-09-08 16:39 ` Greg KH 2013-09-08 16:59 ` Matthew Garrett 2013-09-08 16:59 ` Matthew Garrett 2013-09-08 17:22 ` Greg KH 2013-09-08 17:22 ` Greg KH 2013-09-08 17:25 ` Matthew Garrett 2013-09-08 17:25 ` Matthew Garrett 2013-09-08 17:11 ` James Bottomley 2013-09-08 17:11 ` James Bottomley 2013-09-08 17:15 ` Matthew Garrett 2013-09-08 17:15 ` Matthew Garrett 2013-09-08 17:22 ` James Bottomley 2013-09-08 17:22 ` James Bottomley 2013-09-08 17:27 ` Matthew Garrett 2013-09-08 17:27 ` Matthew Garrett 2013-09-08 17:32 ` James Bottomley 2013-09-08 17:32 ` James Bottomley 2013-09-08 17:38 ` Matthew Garrett 2013-09-08 17:38 ` Matthew Garrett 2013-09-03 23:50 ` [PATCH V3 09/11] uswsusp: Disable when module loading is restricted Matthew Garrett 2013-09-04 0:48 ` James Morris 2013-09-04 0:48 ` James Morris 2013-09-05 3:20 ` joeyli 2013-09-05 3:20 ` joeyli 2013-09-03 23:50 ` [PATCH V3 10/11] x86: Restrict MSR access " Matthew Garrett 2013-09-04 0:49 ` James Morris 2013-09-04 0:49 ` James Morris 2013-09-03 23:50 ` [PATCH V3 11/11] Add option to automatically enforce module signatures when in Secure Boot mode Matthew Garrett 2013-09-03 23:50 ` Matthew Garrett 2013-09-04 1:42 ` James Morris 2013-09-04 1:42 ` James Morris 2013-09-04 1:42 ` Matthew Garrett 2013-09-04 1:42 ` Matthew Garrett 2013-09-05 3:13 ` joeyli 2013-09-05 8:24 ` joeyli 2013-09-05 8:24 ` joeyli 2013-09-05 10:16 ` Matt Fleming 2013-09-05 10:16 ` Matt Fleming 2013-09-05 12:54 ` Matthew Garrett 2013-09-05 12:54 ` Matthew Garrett 2013-09-04 15:53 ` Kees Cook 2013-09-04 15:53 ` Re: Kees Cook 2013-09-04 16:05 ` Re: Josh Boyer 2013-09-04 16:05 ` Re: Josh Boyer
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