Date:   Tue, 31 Jan 2023 15:49:51 +0000
From:   Will Deacon <will@kernel.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>
Subject: Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()
Message-ID: <20230131154950.GB2646@willie-the-truck>
References: <20230109052816.405335-1-anshuman.khandual@arm.com>
 <e924c1aa-5f04-fd7f-52d4-7cf22c476016@arm.com>
 <20230126133321.GB29148@willie-the-truck>
 <d454c9a2-5300-b600-a2ae-21d82d338470@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d454c9a2-5300-b600-a2ae-21d82d338470@arm.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On Fri, Jan 27, 2023 at 12:43:17PM +0000, Robin Murphy wrote:
> On 2023-01-26 13:33, Will Deacon wrote:
> > On Tue, Jan 24, 2023 at 11:11:49AM +0530, Anshuman Khandual wrote:
> > > On 1/9/23 10:58, Anshuman Khandual wrote:
> > > > Changing pfn on a user page table mapped entry, without first going through
> > > > break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> > > > to intercept such changes, via an updated pgattr_change_is_safe(). This new
> > > > check happens via __check_racy_pte_update(), which has now been renamed as
> > > > __check_safe_pte_update().
> > > > 
> > > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > > Cc: Will Deacon <will@kernel.org>
> > > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > Cc: linux-arm-kernel@lists.infradead.org
> > > > Cc: linux-kernel@vger.kernel.org
> > > > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> > > > ---
> > > > This applies on v6.2-rc3. This patch had some test time on an internal CI
> > > > system without any issues being reported.
> > > 
> > > Gentle ping, any updates on this patch ? Still any concerns ?
> > 
> > I don't think we really got to the bottom of Mark's concerns with
> > unreachable ptes on the stack, did we? I also have vague recollections
> > of somebody (Robin?) running into issues with the vmap code not honouring
> > BBM.
> 
> Doesn't ring a bell, so either it wasn't me, or it was many years ago and
> about 5 levels deep into trying to fix something else :/

Bah, sorry! Catalin reckons it may have been him talking about the vmemmap.
Catalin -- any clues?

Will