linux-kernel - Re: [PATCH] futex: futex_find_get

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Date:	Thu, 8 Jul 2010 11:28:19 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhltc@...ibm.com>
Subject: Re: [PATCH] futex: futex_find_get_task remove credentails check

On Wed 30-06-10 09:43:27, Darren Hart wrote:
> On 06/30/2010 02:55 AM, Michal Hocko wrote:
> >On Wed 30-06-10 09:01:15, Michal Hocko wrote:
> >>On Tue 29-06-10 09:41:02, Linus Torvalds wrote:
> >>>On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko<mhocko@...e.cz>  wrote:
> >>>>
> >>>>futex_find_get_task is currently used (through lookup_pi_state) from two
> >>>>contexts, futex_requeue and futex_lock_pi_atomic. While credentials check
> >>>>makes sense in the first code path, the second one is more problematic
> >>>>because this check requires that the PI lock holder (pid parameter) has
> >>>>the same uid and euid as the process's euid which is trying to lock the
> >>>>same futex (current).
> >>>
> >>>So exactly why does it make sense to check the credentials in the
> >>>first code path then?
> >>
> >>I though that requeue needs this for security reasons (don't let requeue
> >>process for other user), but when I thought about that again you are
> >>right and the only what matters should be accessibility of the shared
> >>memory.
> >
> >And here is the patch which does the thing.
> >
> >--
> >
> > From 082c5ad2c482a8e78b61b17e213e750b006176aa Mon Sep 17 00:00:00 2001
> >From: Michal Hocko<mhocko@...e.cz>
> >Date: Wed, 30 Jun 2010 09:51:19 +0200
> >Subject: [PATCH] futex: futex_find_get_task remove credentails check
> >
> >futex_find_get_task is currently used (through lookup_pi_state) from two
> >contexts, futex_requeue and futex_lock_pi_atomic. None of the paths
> >looks it needs the credentials check, though. Different (e)uids
> >shouldn't matter at all because the only thing that is important for
> >shared futex is the accessibility of the shared memory.
> >
> >The credentail check results in glibc assert failure or process hang (if
> >glibc is compiled without assert support) for shared robust pthread
> >mutex with priority inheritance if a process tries to lock already held
> >lock owned by a process with a different euid:
> >
> >pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.
> >
> >The problem is that futex_lock_pi_atomic which is called when we try to
> >lock already held lock checks the current holder (tid is stored in the
> >futex value) to get the PI state. It uses lookup_pi_state which in turn
> >gets task struct from futex_find_get_task. ESRCH is returned either when
> >the task is not found or if credentials check fails.
> >futex_lock_pi_atomic simply returns if it gets ESRCH. glibc code,
> >however,  doesn't expect that robust lock returns with ESRCH because it
> >should get either success or owner died.
> >
> >Signed-off-by: Michal Hocko<mhocko@...e.cz>
> 
> Without hearing back from Ingo on the original intent of the
> credentials check, this looks right to me.

Could you comment on that Ingo, please?

> 
> Acked-by: Darren Hart <dvhltc@...ibm.com>
> 
> 
> >---
> >  kernel/futex.c |   17 ++++-------------
> >  1 files changed, 4 insertions(+), 13 deletions(-)
> >
> >diff --git a/kernel/futex.c b/kernel/futex.c
> >index e7a35f1..6a3a5fa 100644
> >--- a/kernel/futex.c
> >+++ b/kernel/futex.c
> >@@ -429,20 +429,11 @@ static void free_pi_state(struct futex_pi_state *pi_state)
> >  static struct task_struct * futex_find_get_task(pid_t pid)
> >  {
> >  	struct task_struct *p;
> >-	const struct cred *cred = current_cred(), *pcred;
> >
> >  	rcu_read_lock();
> >  	p = find_task_by_vpid(pid);
> >-	if (!p) {
> >-		p = ERR_PTR(-ESRCH);
> >-	} else {
> >-		pcred = __task_cred(p);
> >-		if (cred->euid != pcred->euid&&
> >-		    cred->euid != pcred->uid)
> >-			p = ERR_PTR(-ESRCH);
> >-		else
> >-			get_task_struct(p);
> >-	}
> >+	if (p)
> >+		get_task_struct(p);
> >
> >  	rcu_read_unlock();
> >
> >@@ -564,8 +555,8 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
> >  	if (!pid)
> >  		return -ESRCH;
> >  	p = futex_find_get_task(pid);
> >-	if (IS_ERR(p))
> >-		return PTR_ERR(p);
> >+	if (!p)
> >+		return -ESRCH;
> >
> >  	/*
> >  	 * We need to look at the task state flags to figure out,
> 
> 
> -- 
> Darren Hart
> IBM Linux Technology Center
> Real-Time Linux Team

-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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