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Date:   Sat, 19 May 2018 13:39:14 +0200
From:   Vasily Gorbik <gor@linux.ibm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH] procfs: fix mmap() for /proc/vmcore
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On Fri, May 18, 2018 at 09:12:56PM -0700, Linus Torvalds wrote:
> On Fri, May 18, 2018 at 8:43 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> 
> > Not quite.  The things like
> >          if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
> >                  return 0;
> >          iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
> > protect most of the regular files (see mm/filemap.c).  And for devices
> (which is
> > where the majority of crap ->read()/->write() is) it's obviously not
> applicable -
> > ->s_maxbytes of *what*?
> 
> Yeah that "s_maxbytes of what" is I think the real issue. We should never
> have made s_maxbytes be super-block specific: we should have made it be
> per-inode, and then have inode_init_always() initialize it using something
> like the file_mmap_size_max() logic.
> 
> (So we'd still have a "sb_maxbytes" that filesystems would fill in, but it
> would only be used as a "fill in inode value for regular files for this
> superblock").
> 
> Then we could actually protect read/write properly, because many of the
> nasty bugs have been in character device drivers.
> 
> Oh well. It would still be a good thing to do some day, I suspect, but it's
> clearly not the case now, and so s_maxbytes actually has much less coverage
> than I was hoping for.
> 
> (And thus also the problems with /proc/vmcore - it never saw s_maxbytes
> limits before).
> 
> Oh, well. The lack of any meaningful s_maxbytes coverage for proc obviously
> means that my objections against Vasily's patch are mostly invalid. Even if
> /proc does use "generic_file_llseek()" a lot and that should limit things
> to 4G offsets, you can just use pread64/pwrite64 to see if you can screw up
> the offset.
> 
> I'd still prefer to limit the damage to just "vmcore".
> 
> Something like the below COMPLETELY UNTESTED patch? Vasily?

Would work, but file_mmap_size_max first checks
	if (S_ISREG(inode->i_mode))
		return inode->i_sb->s_maxbytes;
before
	if (file->f_mode & FMODE_UNSIGNED_OFFSET)
		return 0;
so, as it is this patch does not fix the issue.

>              Linus
>
>  fs/proc/vmcore.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index a45f0af22a60..83278c547127 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -491,7 +491,15 @@ static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>  }
>  #endif
>  
> +/* Mark vmcore as being able and willing to do 64-bit mmaps */
> +static int vmcore_open(struct inode *inode, struct file *file)
> +{
> +	file->f_mode |= FMODE_UNSIGNED_OFFSET;
> +	return 0;
> +}
> +
>  static const struct file_operations proc_vmcore_operations = {
> +	.open		= vmcore_open,
>  	.read		= read_vmcore,
>  	.llseek		= default_llseek,
>  	.mmap		= mmap_vmcore,