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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id g138-20020a815290000000b0054629ed8300sm4571040ywb.80.2023.05.10.20.40.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 May 2023 20:40:45 -0700 (PDT) Date: Wed, 10 May 2023 20:40:41 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Matthew Wilcox cc: Hugh Dickins , Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Greg Ungerer , Michal Simek , Thomas Bogendoerfer , Helge Deller , John David Anglin , "Aneesh Kumar K.V" , Michael Ellerman , Alexandre Ghiti , Palmer Dabbelt , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda , John Paul Adrian Glaubitz , "David S. Miller" , Chris Zankel , Max Filippov , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 01/23] arm: allow pte_offset_map[_lock]() to fail In-Reply-To: Message-ID: References: <77a5d8c-406b-7068-4f17-23b7ac53bc83@google.com> <5011977-d876-6a24-a3fc-c7e6a02877b8@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Queue-Id: B9DBF100003 X-Rspam-User: X-Rspamd-Server: rspam06 X-Stat-Signature: f6u8m7tjb1oq4grw9czkx4dqs6kkh8d5 X-HE-Tag: 1683776446-126744 X-HE-Meta: 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 eDqgWH0U U4N9PiNZh7Mn+jUr9vOjh1/v5ShbMCKNP2ztdrJGLXOnRhFEWLsMmUeGE6/cPhwS1EIxfjw7spSZrr+yKWTGEypXBOx8pBXcnqetQv4dZ6gayba66OoJCVOAKBPEcxU/oSsxKGxrTqaOVRtxqKd7tAHgFTGRpZi/oxliTgY5EwIbh/YXlzNb3dk/bjKVaxhjKL6RQAPJy+Q371ciegHNaA3yZUee8/4oEQOxOme2GvgbcTYO9riAaSBV6BQcbRsCuEoLSZ/Sa7VyzoONS/JdvmykuDd/y79+wGBXP7+jyhepGpnYn+wk2ZdkMY133nLUv8d+ryiSwL/IHg4AqVvxLSIDMBF3WxKGCQA8wetUWmFRTHxL5xpD/7v6GxYC/5nTBmlpeAwNn/ciLjofN2Tp7H9kUQhp3PwNMPvY/3EFyrREkcw4xbjl1/NaFH+YxubMpwBMGWQ9zJqgIQW6pWfvGp84Amw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 10 May 2023, Matthew Wilcox wrote: > On Tue, May 09, 2023 at 09:42:44PM -0700, Hugh Dickins wrote: > > diff --git a/arch/arm/lib/uaccess_with_memcpy.c b/arch/arm/lib/uaccess_with_memcpy.c > > index e4c2677cc1e9..2f6163f05e93 100644 > > --- a/arch/arm/lib/uaccess_with_memcpy.c > > +++ b/arch/arm/lib/uaccess_with_memcpy.c > > @@ -74,6 +74,9 @@ pin_page_for_write(const void __user *_addr, pte_t **ptep, spinlock_t **ptlp) > > return 0; > > > > pte = pte_offset_map_lock(current->mm, pmd, addr, &ptl); > > + if (unlikely(!pte)) > > + return 0; > > Failing seems like the wrong thig to do if we transitioned from a PTE > to PMD here? Looks to me like we should goto a new label right after > the 'pmd = pmd_offset(pud, addr);', no? I'm pretty sure it's right as is; but probably more by luck than care - I do not think I studied this code as closely as you have now made me do; and it's clear that this is a piece of code where rare transient issues could come up, and must be handled correctly. Thank you for making me look again. The key is in the callers of pin_page_for_write(): __copy_to_user_memcpy() and __clear_user_memset(). They're doing "while (!pin_page_for_write())" loops - they hope for the fast path of getting pte_lock or pmd_lock on the page, and doing a __memcpy() or __memset() to the user address; but if anything goes "wrong", a __put_user() to fault in the page (or fail) then pin_page_for_write() again. "if (unlikely(!pte)) return 0" says that the expected fast path did not succeed, so please __put_user() and have another go. It is somewhere I could have done a "goto again", but that would be superfluous when it's already designed that way at the outer level. Hugh