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Date: Tue, 27 May 2008 20:33:12 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Andrew G. Morgan" <morgan@...nel.org>
Cc: Chris Wright <chrisw@...s-sol.org>,
Dave Jones <davej@...emonkey.org.uk>,
Linux Kernel <linux-kernel@...r.kernel.org>,
bojan@...ursive.com, "Serge E. Hallyn" <serue@...ibm.com>,
Linux Security Modules List
<linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] security: was "Re: capget() overflows buffers."
On Mon, 26 May 2008 18:17:15 -0700 "Andrew G. Morgan" <morgan@...nel.org> wrote:
> Source code out there hard-codes a notion of what the
> _LINUX_CAPABILITY_VERSION #define means in terms of the semantics of the
> raw capability system calls capget() and capset(). Its unfortunate, but
> true.
>
> Since the confusing header file has been in a released kernel, there is
> software that is erroneously using 64-bit capabilities with the semantics
> of 32-bit compatibilities. These recently compiled programs may suffer
> corruption of their memory when sys_getcap() overwrites more memory than
> they are coded to expect, and the raising of added capabilities when using
> sys_capset().
>
> As such, this patch does a number of things to clean up the situation
> for all. It
>
> 1. forces the _LINUX_CAPABILITY_VERSION define to always retain its
> legacy value.
>
> 2. adopts a new #define strategy for the kernel's internal
> implementation of the preferred magic.
>
> 3. depreciates v2 capability magic in favor of a new (v3) magic
> number. The functionality of v3 is entirely equivalent to v2,
> the only difference being that the v2 magic causes the kernel
> to log a "depreciated" warning so the admin can find applications
> that may be using v2 inappropriately.
>
> [User space code continues to be encouraged to use the libcap API which
> protects the application from details like this. libcap-2.10 is the first
> to support v3 capabilities.]
>
> ...
>
> + * Version 2 capabilities worked fine, but the linux/capability.h file
> + * that accompanied their introduction encouraged their use without
> + * the necessary user-space source code changes. As such, we have
> + * created a version 3 with equivalent functionality to version 2, but
> + * with a header change to protect legacy source code from using
> + * version 2 when it wanted to use version 1. If your system has code
> + * that trips the following warning, it is using version 2 specific
> + * capabilities and may be doing so insecurely.
> + *
> + * The remedy is to either upgrade your version of libcap (to 2.10+,
> + * if the application is linked against it), or recompile your
> + * application with modern kernel headers and this warning will go
> + * away.
> + */
> +
> +static void warn_depreciated_v2(void)
"deprecated", please. "depreciated" is where you don't want your
investments to be.
--- a/include/linux/capability.h~capabilities-remain-source-compatible-with-32-bit-raw-legacy-capability-support-fix
+++ a/include/linux/capability.h
@@ -31,7 +31,7 @@ struct task_struct;
#define _LINUX_CAPABILITY_VERSION_1 0x19980330
#define _LINUX_CAPABILITY_U32S_1 1
-#define _LINUX_CAPABILITY_VERSION_2 0x20071026 /* depreciated - use v3 */
+#define _LINUX_CAPABILITY_VERSION_2 0x20071026 /* deprecated - use v3 */
#define _LINUX_CAPABILITY_U32S_2 2
#define _LINUX_CAPABILITY_VERSION_3 0x20080522
--- a/kernel/capability.c~capabilities-remain-source-compatible-with-32-bit-raw-legacy-capability-support-fix
+++ a/kernel/capability.c
@@ -68,13 +68,13 @@ static void warn_legacy_capability_use(v
* away.
*/
-static void warn_depreciated_v2(void)
+static void warn_deprecated_v2(void)
{
static int warned = 0;
if (!warned) {
char name[sizeof(current->comm)];
- printk(KERN_INFO "warning: `%s' uses depreciated v2"
+ printk(KERN_INFO "warning: `%s' uses deprecated v2"
" capabilities in a way that may be insecure.\n",
get_task_comm(name, current));
warned = 1;
@@ -98,7 +98,7 @@ static int cap_validate_magic(cap_user_h
*tocopy = _LINUX_CAPABILITY_U32S_1;
break;
case _LINUX_CAPABILITY_VERSION_2:
- warn_depreciated_v2();
+ warn_deprecated_v2();
/*
* fall through - v3 is otherwise equivalent to v2.
*/
_
> +{
> + static int warned = 0;
> + if (!warned) {
We were going to have a ONCE() macro to do this but that seems to have
not happened.
> + char name[sizeof(current->comm)];
> +
> + printk(KERN_INFO "warning: `%s' uses depreciated v2"
> + " capabilities in a way that may be insecure.\n",
> + get_task_comm(name, current));
> + warned = 1;
> + }
> +}
> +
> +/*
> + * Version check. Return the number of u32s in each capability flag
> + * array, or a negative value on error.
> + */
> +static int cap_validate_magic(cap_user_header_t header, unsigned *tocopy)
> +{
> + __u32 version;
> +
> + if (get_user(version, &header->version))
> + return -EFAULT;
> +
> + switch (version) {
> + case _LINUX_CAPABILITY_VERSION_1:
> + warn_legacy_capability_use();
> + *tocopy = _LINUX_CAPABILITY_U32S_1;
> + break;
> + case _LINUX_CAPABILITY_VERSION_2:
> + warn_depreciated_v2();
> + /*
> + * fall through - v3 is otherwise equivalent to v2.
> + */
> + case _LINUX_CAPABILITY_VERSION_3:
> + *tocopy = _LINUX_CAPABILITY_U32S_3;
> + break;
> + default:
> + if (put_user(_KERNEL_CAPABILITY_VERSION, &header->version))
This is a bit fragile. It is dependent upon the compiler's view of
sizeof(_KERNEL_CAPABILITY_VERSION).
And it'll work OK for now:
#define _LINUX_CAPABILITY_VERSION_3 0x20080522
...
#define _KERNEL_CAPABILITY_VERSION _LINUX_CAPABILITY_VERSION_3
but if, for example, someone comes up and accidentally puts an "L" at
the end of that 0x20080522 then whoops, we just broke the ABI. I think
it would be good to do something a bit more explicit there.
Perhaps:
--- a/kernel/capability.c~capabilities-remain-source-compatible-with-32-bit-raw-legacy-capability-support-fix-fix
+++ a/kernel/capability.c
@@ -106,7 +106,7 @@ static int cap_validate_magic(cap_user_h
*tocopy = _LINUX_CAPABILITY_U32S_3;
break;
default:
- if (put_user(_KERNEL_CAPABILITY_VERSION, &header->version))
+ if (put_user((u32)_KERNEL_CAPABILITY_VERSION, &header->version))
return -EFAULT;
return -EINVAL;
}
_
?
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