From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: linux-sgx@vger.kernel.org,
Haitao Huang <haitao.huang@linux.intel.com>,
Matthew Wilcox <willy@infradead.org>,
Jethro Beekman <jethro@fortanix.com>,
Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH v3] x86/sgx: Fix sgx_encl_may_map locking
Date: Mon, 5 Oct 2020 20:41:08 +0300 [thread overview]
Message-ID: <20201005174108.GA13602@linux.intel.com> (raw)
In-Reply-To: <20201005155519.GB11938@linux.intel.com>
On Mon, Oct 05, 2020 at 08:55:19AM -0700, Sean Christopherson wrote:
> On Mon, Oct 05, 2020 at 05:11:19PM +0300, Jarkko Sakkinen wrote:
> > @@ -317,11 +319,30 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
> > if (current->personality & READ_IMPLIES_EXEC)
> > return -EACCES;
> >
> > - xas_for_each(&xas, page, idx_end)
> > - if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > - return -EACCES;
> > + /*
> > + * No need to hold encl->lock:
> > + * 1. None of the page->* get written.
> > + * 2. page->vm_max_prot_bits is set in sgx_encl_page_alloc(). This
> > + * is before calling xa_insert(). After that it is never modified.
> > + */
>
> You forgot to cover racing with insertion, e.g. below is the snippet from my
> original patch[*], which did the lookup without protection from encl->lock.`
>
> + /*
> + * No need to take encl->lock, vm_prot_bits is set prior to
> + * insertion and never changes, and racing with adding pages is
> + * a userspace bug.
> + */
> + rcu_read_lock();
> + page = radix_tree_lookup(&encl->page_tree, idx);
> + rcu_read_unlock();
>
>
> [*]https://patchwork.kernel.org/patch/11005431/
I'm not sure why that was merged as it was but it probably was not
because of that snippet. It had encl->lock before, so it was by all
practical means covered then. I would have replaced encl->lock with that
if I ever had received a patch with just that, i.e. that particular
snippet gone to noise. That's why it was not broken before v36.
/Jarkko
next prev parent reply other threads:[~2020-10-05 19:21 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-05 14:11 [PATCH v3] x86/sgx: Fix sgx_encl_may_map locking Jarkko Sakkinen
2020-10-05 14:12 ` Matthew Wilcox
2020-10-05 17:25 ` Jarkko Sakkinen
2020-10-05 17:28 ` Jarkko Sakkinen
2020-10-05 14:28 ` Dave Hansen
2020-10-05 17:36 ` Jarkko Sakkinen
2020-10-05 15:55 ` Sean Christopherson
2020-10-05 17:41 ` Jarkko Sakkinen [this message]
2020-10-05 17:43 ` Jarkko Sakkinen
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