linux-kernel - Re: futex local DoS on most architectures

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Date:	Mon, 11 Feb 2008 14:59:34 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Adrian Bunk <adrian.bunk@...ial.fi>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Riku Voipio <riku.voipio@...ial.fi>,
	Mikael Pettersson <mikpe@...uu.se>,
	Lennert Buytenhek <buytenh@...tstofly.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: futex local DoS on most architectures

On Mon, 11 Feb 2008, Adrian Bunk wrote:

> The issue described in [1] is still present and unfixed (and even the 
> fix there wasn't complete since it didn't cover SMP).

Damn, this slipped through my attention completely. Hotfix (which can
be easily backported) below.
 
> Thanks to Riku Voipio for noting that it is still unfixed.
> 
> cu
> Adrian
> 
> [1] http://lkml.org/lkml/2007/8/1/474

-------->

Subject: futex: disable PI/robust on archs w/o valid implementation
From: Thomas Gleixner <tglx@...utronix.de>

We have to disable the complete PI/robust functionality for those
archs, which do not implement futex_atomic_cmpxchg_inatomic(). The
code in question relies on a valid implementation and does not expect
-ENOSYS, which is returned by the stub implementation in
asm-generic/futex.h

Pointed out by: Mikael Pettersson, Riku Voipio and Adrian Bunk

This patch is intended for easy backporting and needs to be cleaned up
further for current mainline.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Ingo Molnar <mingo@...e.hu>
---
 include/asm-generic/futex.h |    2 ++
 kernel/futex.c              |   13 +++++++++++++
 2 files changed, 15 insertions(+)

Index: linux-2.6/include/asm-generic/futex.h
===================================================================
--- linux-2.6.orig/include/asm-generic/futex.h
+++ linux-2.6/include/asm-generic/futex.h
@@ -55,5 +55,7 @@ futex_atomic_cmpxchg_inatomic(int __user
 	return -ENOSYS;
 }
 
+#define FUTEX_ATOMIC_CMPXCHG_NOT_IMPLEMENTED
+
 #endif
 #endif
Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -1870,6 +1870,9 @@ asmlinkage long
 sys_set_robust_list(struct robust_list_head __user *head,
 		    size_t len)
 {
+#ifdef FUTEX_ATOMIC_CMPXCHG_NOT_IMPLEMENTED
+	return -ENOSYS;
+#else
 	/*
 	 * The kernel knows only one size for now:
 	 */
@@ -1879,6 +1882,7 @@ sys_set_robust_list(struct robust_list_h
 	current->robust_list = head;
 
 	return 0;
+#endif
 }
 
 /**
@@ -1891,6 +1895,9 @@ asmlinkage long
 sys_get_robust_list(int pid, struct robust_list_head __user * __user *head_ptr,
 		    size_t __user *len_ptr)
 {
+#ifdef FUTEX_ATOMIC_CMPXCHG_NOT_IMPLEMENTED
+	return -ENOSYS;
+#else
 	struct robust_list_head __user *head;
 	unsigned long ret;
 
@@ -1920,6 +1927,7 @@ err_unlock:
 	rcu_read_unlock();
 
 	return ret;
+#endif
 }
 
 /*
@@ -2082,6 +2090,9 @@ long do_futex(u32 __user *uaddr, int op,
 	case FUTEX_WAKE_OP:
 		ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
 		break;
+
+#ifndef FUTEX_ATOMIC_CMPXCHG_NOT_IMPLEMENTED
+
 	case FUTEX_LOCK_PI:
 		ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
 		break;
@@ -2091,6 +2102,8 @@ long do_futex(u32 __user *uaddr, int op,
 	case FUTEX_TRYLOCK_PI:
 		ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
 		break;
+#endif
+
 	default:
 		ret = -ENOSYS;
 	}
--
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