linux-kernel - Re: [PATCH] x86: Lock down MSR writing in secure boot

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Date:	Fri, 8 Feb 2013 11:18:54 -0800
From:	Kees Cook <keescook@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-efi@...r.kernel.org,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] x86: Lock down MSR writing in secure boot

No. CAP_RAWIO is for reading. Writing needs a much stronger check.

-Kees

On Fri, Feb 8, 2013 at 11:17 AM, H. Peter Anvin <hpa@...or.com> wrote:
> We already have CAP_RAWIO for this in mainline; I am not sure if this should be harder than that...
>
> Kees Cook <keescook@...omium.org> wrote:
>
>>Writing to MSRs should not be allowed unless CAP_COMPROMISE_KERNEL is
>>set since it could lead to execution of arbitrary code in kernel mode.
>>
>>Signed-off-by: Kees Cook <keescook@...omium.org>
>>---
>>This would be used on top of Matthew Garrett's existing "Secure boot
>>policy support" patch series.
>>---
>> arch/x86/kernel/msr.c |    7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>>diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
>>index 4929502..adaab3d 100644
>>--- a/arch/x86/kernel/msr.c
>>+++ b/arch/x86/kernel/msr.c
>>@@ -103,6 +103,9 @@ static ssize_t msr_write(struct file *file, const
>>char __user *buf,
>>       int err = 0;
>>       ssize_t bytes = 0;
>>
>>+      if (!capable(CAP_COMPROMISE_KERNEL))
>>+              return -EPERM;
>>+
>>       if (count % 8)
>>               return -EINVAL; /* Invalid chunk size */
>>
>>@@ -150,6 +153,10 @@ static long msr_ioctl(struct file *file, unsigned
>>int ioc, unsigned long arg)
>>                       err = -EBADF;
>>                       break;
>>               }
>>+              if (!capable(CAP_COMPROMISE_KERNEL)) {
>>+                      err = -EPERM;
>>+                      break;
>>+              }
>>               if (copy_from_user(&regs, uregs, sizeof regs)) {
>>                       err = -EFAULT;
>>                       break;
>
> --
> Sent from my mobile phone. Please excuse brevity and lack of formatting.



--
Kees Cook
Chrome OS Security
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