linux-kernel - Re: [PATCH v2] vsprintf: Check real user/group id for %pK

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Date:	Tue, 08 Oct 2013 17:49:18 -0700
From:	Joe Perches <joe@...ches.com>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, eldad@...refinery.com,
	Jiri Kosina <jkosina@...e.cz>, jgunthorpe@...idianresearch.com,
	Dan Rosenberg <dan.j.rosenberg@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	George Spelvin <linux@...izon.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] vsprintf: Check real user/group id for %pK

On Wed, 2013-10-09 at 11:15 +1100, Ryan Mallon wrote:
> Some setuid binaries will allow reading of files which have read
> permission by the real user id. This is problematic with files which
> use %pK because the file access permission is checked at open() time,
> but the kptr_restrict setting is checked at read() time. If a setuid
> binary opens a %pK file as an unprivileged user, and then elevates
> permissions before reading the file, then kernel pointer values may be
> leaked.

I think it should explicitly test 0.

Dan? Might this be any problem?

Otherwise, just style notes:

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
[]
> @@ -1312,10 +1312,26 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  				spec.field_width = default_width;
>  			return string(buf, end, "pK-error", spec);
>  		}
> -		if (!((kptr_restrict == 0) ||
> -		      (kptr_restrict == 1 &&
> -		       has_capability_noaudit(current, CAP_SYSLOG))))
> -			ptr = NULL;
> +
> +		/*
> +		 * If kptr_restrict is set to 2, then %pK always prints as
> +		 * NULL. If it is set to 1, then only print the real pointer
> +		 * value if the current proccess has CAP_SYSLOG and is running
> +		 * with the same credentials it started with. This is because
> +		 * access to files is checked at open() time, but %pK checks
> +		 * permission at read() time. We don't want to leak pointer
> +		 * values if a binary opens a file using %pK and then elevates
> +		 * privileges before reading it.
> +		 */
> +		{
> +			const struct cred *cred = current_cred();

Please add #include <linux/cred.h>

> +			if (kptr_restrict == 2 || (kptr_restrict == 1 &&
> +			     (!has_capability_noaudit(current, CAP_SYSLOG) ||
> +			      !uid_eq(cred->euid, cred->uid) ||
> +			      !gid_eq(cred->egid, cred->gid))))
> +				ptr = NULL;
> +		}
>  		break;

Also, it might be easier to read as:

		if (kptr_restrict == 0)
			break;
		else if (kptr_restrict == 1) {
			const struct cred *cred = current_cred();

			if (!has_capability_noaudit(current, CAP_SYSLOG) ||
			    !uid_eq(cred->euid, cred->uid) ||
			    !gid_eq(cred->egid, cred->gid))
				ptr = NULL;
		} else {
			ptr = NULL;
		}
		break;

>  	case 'N':
>  		switch (fmt[1]) {


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