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From: Oleg Nesterov on 23 Jun 2010 15:50
On 06/23, Kees Cook wrote:
>
> @@ -956,7 +957,15 @@ void set_task_comm(struct task_struct *tsk, char *buf)
> */
> memset(tsk->comm, 0, TASK_COMM_LEN);
> wmb();

Off-topic. I'd wish I could understand this barrier. Since the lockless
reader doesn't do rmb() I don't see how this can help. OTOH, I don't
understand why it is needed, we never change ->comm[TASK_COMM_LEN-1] == '0'.

> - strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> +
> + /* sanitize non-printable characters */
> + for (i = 0; buf[i] && i < (sizeof(tsk->comm) - 1); i++) {
> + if (!isprint(buf[i]))
> + tsk->comm[i] = '?';
> + else
> + tsk->comm[i] = buf[i];
> + }

Personally I think this makes sense.

> -extern char *get_task_comm(char *to, struct task_struct *tsk);
> +#define get_task_comm(buf, task) get_task_comm_size(buf, sizeof(buf), task)
> +extern char *get_task_comm_size(char *to, size_t len, struct task_struct *tsk);

Oh, but this means that get_task_comm(ptr, task) doesn't work?

Oleg.

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From: Alexey Dobriyan on 23 Jun 2010 16:30
On Wed, Jun 23, 2010 at 09:41:45PM +0200, Oleg Nesterov wrote:
> On 06/23, Kees Cook wrote:

> > -extern char *get_task_comm(char *to, struct task_struct *tsk);
> > +#define get_task_comm(buf, task) get_task_comm_size(buf, sizeof(buf), task)
> > +extern char *get_task_comm_size(char *to, size_t len, struct task_struct *tsk);
>
> Oh, but this means that get_task_comm(ptr, task) doesn't work?

The number of users is so small, and everyone uses TASK_COMM_LEN,
so maybe nothing should be done or "char buf[TASK_COMM_LEN]"?
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From: Kees Cook on 23 Jun 2010 17:40
On Wed, Jun 23, 2010 at 11:23:35PM +0300, Alexey Dobriyan wrote:
> On Wed, Jun 23, 2010 at 09:41:45PM +0200, Oleg Nesterov wrote:
> > On 06/23, Kees Cook wrote:
>
> > > -extern char *get_task_comm(char *to, struct task_struct *tsk);
> > > +#define get_task_comm(buf, task) get_task_comm_size(buf, sizeof(buf), task)
> > > +extern char *get_task_comm_size(char *to, size_t len, struct task_struct *tsk);
> >
> > Oh, but this means that get_task_comm(ptr, task) doesn't work?
>
> The number of users is so small, and everyone uses TASK_COMM_LEN,
> so maybe nothing should be done or "char buf[TASK_COMM_LEN]"?

I couldn't handle the pathological use of strncpy(dest, src, sizeof(src))
that is currently in get_task_comm; that's just asking for trouble.

If someone wants to use a ptr for get_task_comm, they would get to call
get_task_comm_size() instead?

-Kees

--
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Ubuntu Security Team
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From: KOSAKI Motohiro on 24 Jun 2010 20:00
> Through get_task_comm() and many direct uses of task->comm in the kernel,
> it is possible for escape codes and other non-printables to leak into
> dmesg, syslog, etc. In the worst case, these strings could be used to
> attack administrators using vulnerable terminal emulators, and at least
> cause confusion through the injection of \r characters.
>
> This patch sanitizes task->comm to only contain printable characters
> when it is set. Additionally, it redefines get_task_comm so that it is
> more obvious when misused by callers (presently nothing was incorrectly
> calling get_task_comm's unsafe use of strncpy).
>
> Signed-off-by: Kees Cook <kees.cook(a)canonical.com>

I've reviewed this patch briefly, Here is my personal concern...

On Linux, non-printable leaking is fundamental, only fixing task->comm
doesn't solve syslog exploit issue. Probably all /proc/kmsg user should
have escaping non-pritables code.

However, task->comm is one of most easy injection data of kernel, because
we have prctl(PR_SET_NAME), attacker don't need root privilege. So,
conservative assumption seems guard from crappy fault. Plus, this patch
is very small and our small TASK_COMM_LEN lead that we don't need
big performance concern.

So, I don't find demerit in this proposal. but I'm not security specialist,
it's only personal thinking.



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From: Stefani Seibold on 28 Jun 2010 13:50
Am Freitag, den 25.06.2010, 08:56 +0900 schrieb KOSAKI Motohiro:
> > Through get_task_comm() and many direct uses of task->comm in the kernel,
> > it is possible for escape codes and other non-printables to leak into
> > dmesg, syslog, etc. In the worst case, these strings could be used to
> > attack administrators using vulnerable terminal emulators, and at least
> > cause confusion through the injection of \r characters.
> >
> > This patch sanitizes task->comm to only contain printable characters
> > when it is set. Additionally, it redefines get_task_comm so that it is
> > more obvious when misused by callers (presently nothing was incorrectly
> > calling get_task_comm's unsafe use of strncpy).
> >
> > Signed-off-by: Kees Cook <kees.cook(a)canonical.com>
>
> I've reviewed this patch briefly, Here is my personal concern...
>
> On Linux, non-printable leaking is fundamental, only fixing task->comm
> doesn't solve syslog exploit issue. Probably all /proc/kmsg user should
> have escaping non-pritables code.
>
> However, task->comm is one of most easy injection data of kernel, because
> we have prctl(PR_SET_NAME), attacker don't need root privilege. So,
> conservative assumption seems guard from crappy fault. Plus, this patch
> is very small and our small TASK_COMM_LEN lead that we don't need
> big performance concern.
>
> So, I don't find demerit in this proposal. but I'm not security specialist,
> it's only personal thinking.
>
Agree. I think a escaped printk should be a more generic solution.

Stefani


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