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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id o4sm2006223qko.120.2020.10.07.10.36.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Oct 2020 10:36:47 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kQDMt-00110d-DV; Wed, 07 Oct 2020 14:36:47 -0300 Date: Wed, 7 Oct 2020 14:36:47 -0300 From: Jason Gunthorpe To: Daniel Vetter Cc: DRI Development , LKML , kvm@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-media@vger.kernel.org, linux-s390@vger.kernel.org, Daniel Vetter , Kees Cook , Dan Williams , Andrew Morton , John Hubbard , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Jan Kara Subject: Re: [PATCH 11/13] mm: add unsafe_follow_pfn Message-ID: <20201007173647.GW5177@ziepe.ca> References: <20201007164426.1812530-1-daniel.vetter@ffwll.ch> <20201007164426.1812530-12-daniel.vetter@ffwll.ch> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201007164426.1812530-12-daniel.vetter@ffwll.ch> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 07, 2020 at 06:44:24PM +0200, Daniel Vetter wrote: > Way back it was a reasonable assumptions that iomem mappings never > change the pfn range they point at. But this has changed: > > - gpu drivers dynamically manage their memory nowadays, invalidating > ptes with unmap_mapping_range when buffers get moved > > - contiguous dma allocations have moved from dedicated carvetouts to > cma regions. This means if we miss the unmap the pfn might contain > pagecache or anon memory (well anything allocated with GFP_MOVEABLE) > > - even /dev/mem now invalidates mappings when the kernel requests that > iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87 > ("/dev/mem: Revoke mappings when a driver claims the region") > > Accessing pfns obtained from ptes without holding all the locks is > therefore no longer a good idea. > > Unfortunately there's some users where this is not fixable (like v4l > userptr of iomem mappings) or involves a pile of work (vfio type1 > iommu). For now annotate these as unsafe and splat appropriately. > > This patch adds an unsafe_follow_pfn, which later patches will then > roll out to all appropriate places. > > Signed-off-by: Daniel Vetter > Cc: Jason Gunthorpe > Cc: Kees Cook > Cc: Dan Williams > Cc: Andrew Morton > Cc: John Hubbard > Cc: Jérôme Glisse > Cc: Jan Kara > Cc: Dan Williams > Cc: linux-mm@kvack.org > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-samsung-soc@vger.kernel.org > Cc: linux-media@vger.kernel.org > Cc: kvm@vger.kernel.org > --- > include/linux/mm.h | 2 ++ > mm/memory.c | 32 +++++++++++++++++++++++++++++++- > mm/nommu.c | 17 +++++++++++++++++ > security/Kconfig | 13 +++++++++++++ > 4 files changed, 63 insertions(+), 1 deletion(-) Makes sense to me. I wonder if we could change the original follow_pfn to require the ptep and then lockdep_assert_held() it against the page table lock? > +int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address, > + unsigned long *pfn) > +{ > +#ifdef CONFIG_STRICT_FOLLOW_PFN > + pr_info("unsafe follow_pfn usage rejected, see > CONFIG_STRICT_FOLLOW_PFN\n"); Wonder if we can print something useful here, like the current PID/process name? > diff --git a/security/Kconfig b/security/Kconfig > index 7561f6f99f1d..48945402e103 100644 > --- a/security/Kconfig > +++ b/security/Kconfig > @@ -230,6 +230,19 @@ config STATIC_USERMODEHELPER_PATH > If you wish for all usermode helper programs to be disabled, > specify an empty string here (i.e. ""). > > +config STRICT_FOLLOW_PFN > + bool "Disable unsafe use of follow_pfn" > + depends on MMU I would probably invert this CONFIG_ALLOW_UNSAFE_FOLLOW_PFN default n Jason