Re: drop SECURITY_FILE_CAPABILITIES?

From: Steve Grubb
Date: Wed Nov 18 2009 - 14:35:14 EST


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@xxxxxxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/