Re: [PATCH] sysctl: Limit the size of I/Os to PAGE_SIZE

[Josef Bacik]

On 5/13/21 12:12 PM, Josef Bacik wrote:
> On 5/13/21 12:06 PM, Matthew Wilcox (Oracle) wrote:
> > We currently allow a read or a write that is up to KMALLOC_MAX_SIZE.
> > This has caused problems when cat decides to do a 64kB read and
> > so we allocate a 64kB buffer for the sysctl handler to store into.
> > The immediate problem was fixed by switching to kvmalloc(), but it's
> > ridiculous to allocate so much memory to read what is likely to be a
> > few bytes.
> >
> > sysfs limits reads and writes to PAGE_SIZE, and I feel we should do the
> > same for sysctl.  The largest sysctl anyone's been able to come up with
> > is 433 bytes for /proc/sys/dev/cdrom/info
> >
> > This will allow simplifying the BPF sysctl code later, but I'll leave
> > that for someone who understands it better.
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > ---
> >   fs/proc/proc_sysctl.c | 15 +++++++++------
> >   1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index dea0f5ee540c..a97a8a4ff270 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -562,11 +562,14 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, 
> > struct iov_iter *iter,
> >       if (!table->proc_handler)
> >           goto out;
> > -    /* don't even try if the size is too large */
> > +    /* reads may return short values; large writes must fail now */
> > +    if (count >= PAGE_SIZE) {
> > +        if (write)
> > +            goto out;
> > +        count = PAGE_SIZE;
> > +    }
> >       error = -ENOMEM;
> > -    if (count >= KMALLOC_MAX_SIZE)
> > -        goto out;
> > -    kbuf = kvzalloc(count + 1, GFP_KERNEL);
> > +    kbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
> >       if (!kbuf)
> >           goto out;
>
> Below here we have
>
> kbuf[count] = '\0';

Nevermind I just re-read it and it's

if (write)
	kbuf[count] = '\0';

I'm stupid, you can add

Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Thanks,

Josef