linux-kernel - Re: [PATCH v3 2/3] s390/crash: use static swap buffer for copy_to_user

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Date:   Mon, 18 Jul 2022 17:02:06 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Alexander Egorenkov <egorenar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>, Christoph Hellwig <hch@....de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 2/3] s390/crash: use static swap buffer for
 copy_to_user_real()

On Mon, Jul 18, 2022 at 04:51:59PM +0200, Alexander Gordeev wrote:
> On Mon, Jul 18, 2022 at 02:48:21PM +0100, Matthew Wilcox wrote:
> > On Mon, Jul 18, 2022 at 03:32:40PM +0200, Alexander Gordeev wrote:
> > > +++ b/arch/s390/kernel/crash_dump.c
> > > @@ -53,6 +53,7 @@ struct save_area {
> > >  };
> > >  
> > >  static LIST_HEAD(dump_save_areas);
> > 
> > I'd suggest you need a mutex here so that simultaneous calls to
> > copy_to_user_real() don't corrupt each others data.
> 
> We stop all (but one) CPUs before calling into the capture kernel -
> one that calls these functions. Similarily to racy hsa_buf[] access
> from memcpy_hsa_iter() this should not hit.

Could you show me how that works when two processes read from
/proc/vmcore at the same time?

> As you noticed last time, it is a pre-existing race and I was
> actually going to address it in a separate fix - if the problem
> really exists.
> 
> > > +static char memcpy_real_buf[PAGE_SIZE];
> > >  
> > >  /*
> > >   * Allocate a save area
> > > @@ -179,23 +180,18 @@ int copy_oldmem_kernel(void *dst, unsigned long src, size_t count)
> > >  static int copy_to_user_real(void __user *dest, unsigned long src, unsigned long count)
> > >  {
> > >  	int offs = 0, size, rc;
> > > -	char *buf;
> > >  
> > > -	buf = (char *)__get_free_page(GFP_KERNEL);
> > > -	if (!buf)
> > > -		return -ENOMEM;
> > >  	rc = -EFAULT;
> > >  	while (offs < count) {
> > >  		size = min(PAGE_SIZE, count - offs);
> > > -		if (memcpy_real(buf, src + offs, size))
> > > +		if (memcpy_real(memcpy_real_buf, src + offs, size))
> > >  			goto out;
> > > -		if (copy_to_user(dest + offs, buf, size))
> > > +		if (copy_to_user(dest + offs, memcpy_real_buf, size))
> > >  			goto out;
> > >  		offs += size;
> > >  	}
> > >  	rc = 0;
> > >  out:
> > > -	free_page((unsigned long)buf);
> > >  	return rc;
> > >  }
> 
> Thanks!