Re: [PATCH 5/5] drivers/s390/cio: ensure the copied buf is NULL terminated

[Bui Quang Minh]

On 4/23/24 13:50, Heiko Carstens wrote:
> On Mon, Apr 22, 2024 at 11:41:40PM +0700, Bui Quang Minh wrote:
> > Currently, we allocate a lbuf-sized kernel buffer and copy lbuf from
> > userspace to that buffer. Later, we use scanf on this buffer but we don't
> > ensure that the string is terminated inside the buffer, this can lead to
> > OOB read when using scanf. Fix this issue by allocating 1 more byte to at
> > the end of buffer and write NULL terminator to the end of buffer after
> > userspace copying.
> >
> > Fixes: a4f17cc72671 ("s390/cio: add CRW inject functionality")
> > Signed-off-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx>
> > ---
> >   drivers/s390/cio/cio_inject.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/s390/cio/cio_inject.c b/drivers/s390/cio/cio_inject.c
> > index 8613fa937237..9b69fbf49f60 100644
> > --- a/drivers/s390/cio/cio_inject.c
> > +++ b/drivers/s390/cio/cio_inject.c
> > @@ -95,10 +95,11 @@ static ssize_t crw_inject_write(struct file *file, const char __user *buf,
> >   		return -EINVAL;
> >   	}
> >   
> > -	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?

Thanks for your review. It's my mistake, I blindly follow the pattern in 
rvu_debugfs

static ssize_t rvu_dbg_qsize_write(struct file *filp,
				   const char __user *buffer, size_t count,
				   loff_t *ppos, int blktype)
{
	cmd_buf = memdup_user(buffer, count + 1);
	if (IS_ERR(cmd_buf))
		return -ENOMEM;

	cmd_buf[count] = '\0';
}

I will send a patch to fix this too.

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?

Thanks,
Quang Minh.