Ah, you meant that it should be either > PAGE_SIZE - 1 or >= PAGE_SIZE,
which are the same. I'll fix that.
On Tuesday 27 Oct 2015 22:25:50 Andrew Perepechko wrote:
> On Tuesday 27 Oct 2015 14:46:29 Stephen Smalley wrote:
> > >> Why PAGE_SIZE-1?
> > >
> > > This is to avoid allocation of more than a single page.
> >
> > Yes, but you don't need PAGE_SIZE - 1 for that. The check can just be
> >
> > >= PAGE_SIZE, as used elsewhere in selinuxfs.c.
>
> A sequence of bytes passed to a write handler may not be 0-terminated,
> so it cannot be used directly as a scanf argument.
>
> As far as I can see, compute_av and similar handlers use transaction ops,
> which have more or less similar condition:
>
> char *simple_transaction_get(struct file *file, const char __user *buf,
> size_t size) {
> struct simple_transaction_argresp *ar;
> static DEFINE_SPINLOCK(simple_transaction_lock);
>
> if (size > SIMPLE_TRANSACTION_LIMIT - 1)
> return ERR_PTR(-EFBIG);
>
> SIMPLE_TRANSACTION_LIMIT is PAGE_SIZE subtracted by sizeof(ssize_t).
>
> Since the buffer is the result of get_zeroed_page(), it is guaranteed to
> be 0-terminated: [ssize_t][SIMPLE_TRANSACTION_LIMIT-1][\0].
>
> On the other hand, e.g. sel_write_checkreqprot() does not look correct,
> since the only reason that it won't access beyond page boundary on
> incorrect 4096-byte input is if sscanf stops parsing a number after integer
> overflow happens.
>
>
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