linux-kernel - Re: [PATCH] dma-debug: Fix deadlock with netconsole or other drivers that use the DMA API

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20120419165059.c4ca4e1f.akpm@linux-foundation.org>
Date:	Thu, 19 Apr 2012 16:50:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Roland Dreier <roland@...nel.org>
Cc:	Joerg Roedel <joerg.roedel@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-debug: Fix deadlock with netconsole or other
 drivers that use the DMA API

On Thu, 19 Apr 2012 16:36:56 -0700
Roland Dreier <roland@...nel.org> wrote:

> On Thu, Apr 19, 2012 at 11:48 AM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > So *any* printk can deadlock if free_entries_lock is held and
> > global_disable==false?
> 
> apparently.
> 
> > In that case we're going to need much sterner fixes. __Any list_head
> > operation can do a printk if list_head debugging is enabled.
> > dma_debug_resize_entries() does a kfree() under free_entries_lock(!).
> >
> > Methinks we need a more general fix?
> 
> sigh... no good deed goes unpunished.
> 
> OK, will look at it.  Just to make things even more fun, all the
> err_printk() stuff can potentially deadlock on the hash bucket
> lock, although that requires enough bad luck a collision to happen.

I suppose one could do something like

static DEFINE_SPINLOCK(lock);
static struct task_struct *owner;
static unsigned depth;

/*
 * Nice comments go here
 */
static unsigned long free_entries_lock(void)
{
	unsigned long flags = 0;

	if (owner == current) {
		depth++;
	} else {
		/* Permit recursive locking */
		spin_lock_irqsave(&lock, flags);
		BUG_ON(depth != 0);
		BUG_ON(owner != NULL);
		owner = current;
	}
	return flags;
}

static void free_entries_unlock(unsigned long flags)
{
	BUG_ON(owner != current);
	if (!--depth) {
		owner = NULL;
		spin_lock_irqrestore(&lock, flags);
	}
}

After removing the bugs, I think that's safe wrt interrupts?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/