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Date: Wed, 24 Apr 2024 13:55:57 +0200
From: Heiko Carstens <hca@linux.ibm.com>
To: Bui Quang Minh <minhquangbui99@gmail.com>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Tony Nguyen <anthony.l.nguyen@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
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        Paolo Abeni <pabeni@redhat.com>,
        Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>,
        Rasesh Mody <rmody@marvell.com>,
        Sudarsana Kalluru <skalluru@marvell.com>, GR-Linux-NIC-Dev@marvell.com,
        Krishna Gudipati <kgudipat@brocade.com>,
        Anil Gurumurthy <anil.gurumurthy@qlogic.com>,
        Sudarsana Kalluru <sudarsana.kalluru@qlogic.com>,
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        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Fabian Frederick <fabf@skynet.be>,
        Saurav Kashyap <skashyap@marvell.com>,
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        linux-s390@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 5/5] drivers/s390/cio: ensure the copied buf is NULL
 terminated
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On Tue, Apr 23, 2024 at 09:46:35PM +0700, Bui Quang Minh wrote:
> > > -	buffer = vmemdup_user(buf, lbuf);
> > > +	buffer = vmemdup_user(buf, lbuf + 1);
> > >   	if (IS_ERR(buffer))
> > >   		return -ENOMEM;
> > > +	buffer[lbuf] = '\0';
> > 
> > This would read one byte too much from user space, and could potentially
> > fault.
> > 
> > Why isn't this simply memdup_user_nul() like all others, which would do the
> > right thing?
..
> For this case, as the original code uses vmemdup_user, which internally uses
> kvmalloc not kmalloc, so I try to keep the original behavior. And
> vmemdup_user does not have the counterpart vmemdup_user_nul. I can
> kvmalloc(lbuf + 1), then copy_to_user(lbuf) and set buffer[lbuf] = '\0' or
> do you think I should create vmemdup_user_nul?

There is no need for vmalloc() instead of kmalloc() for this particular
case. The input string is supposed to be rather short (see the sscanf()
call). So converting to memdup_user_nul() is sufficient and solves the
potential problem.