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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id w36sm100489qtc.48.2020.09.10.15.11.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2020 15:11:17 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kGUmi-004RcY-HM; Thu, 10 Sep 2020 19:11:16 -0300 Date: Thu, 10 Sep 2020 19:11:16 -0300 From: Jason Gunthorpe To: John Hubbard Cc: Linus Torvalds , Alexander Gordeev , Gerald Schaefer , Dave Hansen , LKML , linux-mm , linux-arch , Andrew Morton , Russell King , Mike Rapoport , Catalin Marinas , Will Deacon , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Jeff Dike , Richard Weinberger , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Arnd Bergmann , Andrey Ryabinin , linux-x86 , linux-arm , linux-power , linux-sparc , linux-um , linux-s390 , Vasily Gorbik , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding Message-ID: <20200910221116.GQ87483@ziepe.ca> References: <20200907180058.64880-2-gerald.schaefer@linux.ibm.com> <0dbc6ec8-45ea-0853-4856-2bc1e661a5a5@intel.com> <20200909142904.00b72921@thinkpad> <20200909192534.442f8984@thinkpad> <20200909180324.GI87483@ziepe.ca> <20200910093925.GB29166@oc3871087118.ibm.com> <20200910181319.GO87483@ziepe.ca> <0c9bcb54-914b-e582-dd6d-3861267b6c94@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0c9bcb54-914b-e582-dd6d-3861267b6c94@nvidia.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 10, 2020 at 02:22:37PM -0700, John Hubbard wrote: > Or am I way off here, and it really is possible (aside from the current > s390 situation) to observe something that "is no longer a page table"? Yes, that is the issue. Remember there is no locking for GUP fast. While a page table cannot be freed there is nothing preventing the page table entry from being concurrently modified. Without the stack variable it looks like this: pud_t pud = READ_ONCE(*pudp); if (!pud_present(pud)) return pmd_offset(pudp, address); And pmd_offset() expands to return (pmd_t *)pud_page_vaddr(*pud) + pmd_index(address); Between the READ_ONCE(*pudp) and (*pud) inside pmd_offset() the value of *pud can change, eg to !pud_present. Then pud_page_vaddr(*pud) will crash. It is not use after free, it is using data that has not been validated. Jason