linux-kernel - Re: drop SECURITY_FILE

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Date:	Wed, 18 Nov 2009 14:33:30 -0500
From:	Steve Grubb <sgrubb@...hat.com>
To:	"Andrew G. Morgan" <morgan@...nel.org>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-security-module@...r.kernel.org,
	Kees Cook <kees.cook@...onical.com>,
	Andreas Gruenbacher <agruen@...e.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	George Wilson <gcwilson@...ibm.com>,
	KaiGai Kohei <kaigai@...gai.gr.jp>
Subject: Re: drop SECURITY_FILE_CAPABILITIES?

On Wednesday 18 November 2009 01:36:20 pm Andrew G. Morgan wrote:
> On Wed, Nov 18, 2009 at 9:49 AM, Steve Grubb <sgrubb@...hat.com> wrote:
> > On Wednesday 18 November 2009 11:40:13 am Andrew G. Morgan wrote:
> >> >> But back to detecting the capability version number...if I pass 0 as
> >> >> the version in the header, why can't the kernel just say oh you want
> >> >> the preferred version number, stuff it in the header, and return the
> >> >> syscall with success and not EINVAL?
> >>
> >> This is so a library can understand that it doesn't understand the
> >> current ABI.
> >
> > If user space is passing a NULL for the cap_user_data_t argument, user
> > space has a pretty good idea that its not expecting actual capabilities
> > to be filled in. My basic point is that there is no way to "correctly"
> > use the capabilities API to determine what the preferred version is.
> 
> But older kernels didn't do that.

True, but now we have the problem.


> >> The intention is for it to fail safe and not blunder on doing
> >> "security" related operations with an imperfect idea of the current
> >> kernel interface.
> >>
> >> This is how libcap figures out it can work with the hosting kernel:
> >
> > capget(0x20080522, 0, NULL)             = -1 EFAULT (Bad address)
> 
> I'm not sure what this is supposed to do. This system call takes two
> arguments and none of them work as your above snippet suggests.

This is from running "strace /usr/sbin/getcap libcap.h". I think strace is 
splitting arg 1 into its 2 elements within the structure for display purposes. 
You can strace it yourself and see. :)


> SYSCALL_DEFINE2(capget, cap_user_header_t, header, cap_user_data_t,
>  dataptr) 165 {
>  166         int ret = 0;
>  167         pid_t pid;
>  168         unsigned tocopy;
>  169         kernel_cap_t pE, pI, pP;
>  170
>  171         ret = cap_validate_magic(header, &tocopy);
>  172         if (ret != 0)
>  173                 return ret;
> 
> ie., two arguments, both of which are pointers. dataptr is not touched
> if you supply incorrect magic... The return at line 173 is taken if
> header is explored and does not contain the correct magic (ie.
> Invalid) - which it overwrites with the kernel-preferred value in the
> header, and returns EINVAL...

OK, this is the right place to make a fix. Something along the lines of:

@@ -169,8 +169,11 @@ SYSCALL_DEFINE2(capget, cap_user_header_
 	kernel_cap_t pE, pI, pP;
 
 	ret = cap_validate_magic(header, &tocopy);
-	if (ret != 0)
+	if (ret != 0) {
+		if (ret == -EINVAL && dataptr == NULL)
+			return 0;
 		return ret;
+	}
 
 	if (get_user(pid, &header->pid))
 		return -EFAULT;


> I don't see an EFAULT problem here.

It comes when get_user fails above.
 
-Steve
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