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[2003:cb:c703:7a00:852e:72cd:ed76:d72f]) by smtp.gmail.com with ESMTPSA id 11-20020a05600c228b00b003d01b84e9b2sm8092605wmf.27.2022.12.02.04.07.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 Dec 2022 04:07:03 -0800 (PST) Content-Type: multipart/mixed; boundary="------------ApHjnS14u0ZxFpWQKPOgTgNr" Message-ID: <222fc0b2-6ec0-98e7-833f-ea868b248446@redhat.com> Date: Fri, 2 Dec 2022 13:07:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Content-Language: en-US From: David Hildenbrand To: Andrew Morton Cc: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport , Nadav Amit , Andrea Arcangeli , Ives van Hoorne , Axel Rasmussen , Alistair Popple , stable@vger.kernel.org References: <20221114000447.1681003-1-peterx@redhat.com> <20221114000447.1681003-2-peterx@redhat.com> <5ddf1310-b49f-6e66-a22a-6de361602558@redhat.com> <20221130142425.6a7fdfa3e5954f3c305a77ee@linux-foundation.org> <20221201143058.80296541cc6802d1e5990033@linux-foundation.org> Organization: Red Hat Subject: Re: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is a multi-part message in MIME format. --------------ApHjnS14u0ZxFpWQKPOgTgNr Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 02.12.22 12:03, David Hildenbrand wrote: > On 01.12.22 23:30, Andrew Morton wrote: >> On Thu, 1 Dec 2022 16:42:52 +0100 David Hildenbrand wrote: >> >>> On 01.12.22 16:28, Peter Xu wrote: >>>> >>>> I didn't reply here because I have already replied with the question in >>>> previous version with a few attempts. Quotting myself: >>>> >>>> https://lore.kernel.org/all/Y3KgYeMTdTM0FN5W@x1n/ >>>> >>>> The thing is recovering the pte into its original form is the >>>> safest approach to me, so I think we need justification on why it's >>>> always safe to set the write bit. >>>> >>>> I've also got another longer email trying to explain why I think it's the >>>> other way round to be justfied, rather than justifying removal of the write >>>> bit for a read migration entry, here: >>>> >>> >>> And I disagree for this patch that is supposed to fix this hunk: >>> >>> >>> @@ -243,11 +243,15 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma, >>> entry = pte_to_swp_entry(*pvmw.pte); >>> if (is_write_migration_entry(entry)) >>> pte = maybe_mkwrite(pte, vma); >>> + else if (pte_swp_uffd_wp(*pvmw.pte)) >>> + pte = pte_mkuffd_wp(pte); >>> >>> if (unlikely(is_zone_device_page(new))) { >>> if (is_device_private_page(new)) { >>> entry = make_device_private_entry(new, pte_write(pte)); >>> pte = swp_entry_to_pte(entry); >>> + if (pte_swp_uffd_wp(*pvmw.pte)) >>> + pte = pte_mkuffd_wp(pte); >>> } >>> } >> >> David, I'm unclear on what you mean by the above. Can you please >> expand? >> >>> >>> There is really nothing to justify the other way around here. >>> If it's broken fix it independently and properly backport it independenty. >>> >>> But we don't know about any such broken case. >>> >>> I have no energy to spare to argue further ;) >> >> This is a silent data loss bug, which is about as bad as it gets. >> Under obscure conditions, fortunately. But please let's keep working >> it. Let's aim for something minimal for backporting purposes. We can >> revisit any cleanliness issues later. > > Okay, you activated my energy reserves. > >> >> David, do you feel that the proposed fix will at least address the bug >> without adverse side-effects? > > Usually, when I suspect something is dodgy I unconsciously push back > harder than I usually would. > > I just looked into the issue once again and realized that this patch > here (and also my alternative proposal) most likely tackles the > more-generic issue from the wrong direction. I found yet another such > bug (most probably two, just too lazy to write another reproducer). > Migration code does the right thing here -- IMHO -- and the issue should > be fixed differently. > > I'm testing an alternative patch right now and will share it later > today, along with a reproducer. > mprotect() reproducer attached. -- Thanks, David / dhildenb --------------ApHjnS14u0ZxFpWQKPOgTgNr Content-Type: text/x-csrc; charset=UTF-8; name="uffd-wp-mprotect.c" Content-Disposition: attachment; filename="uffd-wp-mprotect.c" Content-Transfer-Encoding: base64 I2RlZmluZSBfR05VX1NPVVJDRQojaW5jbHVkZSA8c3RkaW8uaD4KI2luY2x1ZGUgPHN0ZGxp Yi5oPgojaW5jbHVkZSA8c3RyaW5nLmg+CiNpbmNsdWRlIDxmY250bC5oPgojaW5jbHVkZSA8 dW5pc3RkLmg+CiNpbmNsdWRlIDxlcnJuby5oPgojaW5jbHVkZSA8cG9sbC5oPgojaW5jbHVk ZSA8cHRocmVhZC5oPgojaW5jbHVkZSA8c3lzL21tYW4uaD4KI2luY2x1ZGUgPHN5cy9zeXNj YWxsLmg+CiNpbmNsdWRlIDxzeXMvaW9jdGwuaD4KI2luY2x1ZGUgPGxpbnV4L21lbWZkLmg+ CiNpbmNsdWRlIDxsaW51eC91c2VyZmF1bHRmZC5oPgoKc2l6ZV90IHBhZ2VzaXplOwppbnQg dWZmZDsKCnN0YXRpYyB2b2lkICp1ZmZkX3RocmVhZF9mbih2b2lkICphcmcpCnsKCXN0YXRp YyBzdHJ1Y3QgdWZmZF9tc2cgbXNnOwoJc3NpemVfdCBucmVhZDsKCgl3aGlsZSAoMSkgewoJ CXN0cnVjdCBwb2xsZmQgcG9sbGZkOwoJCWludCBucmVhZHk7CgoJCXBvbGxmZC5mZCA9IHVm ZmQ7CgkJcG9sbGZkLmV2ZW50cyA9IFBPTExJTjsKCQlucmVhZHkgPSBwb2xsKCZwb2xsZmQs IDEsIC0xKTsKCQlpZiAobnJlYWR5ID09IC0xKSB7CgkJCWZwcmludGYoc3RkZXJyLCAicG9s bCgpIGZhaWxlZDogJWRcbiIsIGVycm5vKTsKCQkJZXhpdCgxKTsKCQl9CgoJCW5yZWFkID0g cmVhZCh1ZmZkLCAmbXNnLCBzaXplb2YobXNnKSk7CgkJaWYgKG5yZWFkIDw9IDApCgkJCWNv bnRpbnVlOwoKCQlpZiAobXNnLmV2ZW50ICE9IFVGRkRfRVZFTlRfUEFHRUZBVUxUIHx8CgkJ ICAgICEobXNnLmFyZy5wYWdlZmF1bHQuZmxhZ3MgJiBVRkZEX1BBR0VGQVVMVF9GTEFHX1dQ KSkgewoJCQlwcmludGYoIkZBSUw6IHdyb25nIHVmZmQtd3AgZXZlbnQgZmlyZWRcbiIpOwoJ CQlleGl0KDEpOwoJCX0KCgkJcHJpbnRmKCJQQVNTOiB1ZmZkLXdwIGZpcmVkXG4iKTsKCQll eGl0KDApOwoJfQp9CgpzdGF0aWMgaW50IHNldHVwX3VmZmQoY2hhciAqbWFwKQp7CglzdHJ1 Y3QgdWZmZGlvX2FwaSB1ZmZkaW9fYXBpOwoJc3RydWN0IHVmZmRpb19yZWdpc3RlciB1ZmZk aW9fcmVnaXN0ZXI7CglzdHJ1Y3QgdWZmZGlvX3JhbmdlIHVmZmRfcmFuZ2U7CglwdGhyZWFk X3QgdGhyZWFkOwoKCXVmZmQgPSBzeXNjYWxsKF9fTlJfdXNlcmZhdWx0ZmQsCgkJICAgICAg IE9fQ0xPRVhFQyB8IE9fTk9OQkxPQ0sgfCBVRkZEX1VTRVJfTU9ERV9PTkxZKTsKCWlmICh1 ZmZkIDwgMCkgewoJCWZwcmludGYoc3RkZXJyLCAic3lzY2FsbCgpIGZhaWxlZDogJWRcbiIs IGVycm5vKTsKCQlyZXR1cm4gLWVycm5vOwoJfQoKCXVmZmRpb19hcGkuYXBpID0gVUZGRF9B UEk7Cgl1ZmZkaW9fYXBpLmZlYXR1cmVzID0gVUZGRF9GRUFUVVJFX1BBR0VGQVVMVF9GTEFH X1dQOwoJaWYgKGlvY3RsKHVmZmQsIFVGRkRJT19BUEksICZ1ZmZkaW9fYXBpKSA8IDApIHsK CQlmcHJpbnRmKHN0ZGVyciwgIlVGRkRJT19BUEkgZmFpbGVkOiAlZFxuIiwgZXJybm8pOwoJ CXJldHVybiAtZXJybm87Cgl9CgoJaWYgKCEodWZmZGlvX2FwaS5mZWF0dXJlcyAmIFVGRkRf RkVBVFVSRV9QQUdFRkFVTFRfRkxBR19XUCkpIHsKCQlmcHJpbnRmKHN0ZGVyciwgIlVGRkRf RkVBVFVSRV9XUklURVBST1RFQ1QgbWlzc2luZ1xuIik7CgkJcmV0dXJuIC1FTk9TWVM7Cgl9 CgoJdWZmZGlvX3JlZ2lzdGVyLnJhbmdlLnN0YXJ0ID0gKHVuc2lnbmVkIGxvbmcpIG1hcDsK CXVmZmRpb19yZWdpc3Rlci5yYW5nZS5sZW4gPSBwYWdlc2l6ZTsKCXVmZmRpb19yZWdpc3Rl ci5tb2RlID0gVUZGRElPX1JFR0lTVEVSX01PREVfV1A7CglpZiAoaW9jdGwodWZmZCwgVUZG RElPX1JFR0lTVEVSLCAmdWZmZGlvX3JlZ2lzdGVyKSA8IDApIHsKCQlmcHJpbnRmKHN0ZGVy ciwgIlVGRkRJT19SRUdJU1RFUiBmYWlsZWQ6ICVkXG4iLCBlcnJubyk7CgkJcmV0dXJuIC1l cnJubzsKCX0KCglwdGhyZWFkX2NyZWF0ZSgmdGhyZWFkLCBOVUxMLCB1ZmZkX3RocmVhZF9m biwgTlVMTCk7CgoJcmV0dXJuIDA7Cn0KCmludCBtYWluKGludCBhcmdjLCBjaGFyICoqYXJn dikKewoJc3RydWN0IHVmZmRpb193cml0ZXByb3RlY3QgdWZmZF93cml0ZXByb3RlY3Q7Cglj aGFyICptYXA7CglpbnQgZmQ7CgoJcGFnZXNpemUgPSBnZXRwYWdlc2l6ZSgpOwoJZmQgPSBt ZW1mZF9jcmVhdGUoInRlc3QiLCAwKTsKCWlmIChmZCA8IDApIHsKCQlmcHJpbnRmKHN0ZGVy ciwgIm1lbWZkX2NyZWF0ZSgpIGZhaWxlZFxuIik7CgkJcmV0dXJuIC1lcnJubzsKCX0KCWlm IChmdHJ1bmNhdGUoZmQsIHBhZ2VzaXplKSkgewoJCWZwcmludGYoc3RkZXJyLCAiZnRydW5j YXRlKCkgZmFpbGVkXG4iKTsKCQlyZXR1cm4gLWVycm5vOwoJfQoKCS8qIFN0YXJ0IG91dCB3 aXRob3V0IHdyaXRlIHByb3RlY3Rpb24uICovCgltYXAgPSBtbWFwKE5VTEwsIHBhZ2VzaXpl LCBQUk9UX1JFQUR8UFJPVF9XUklURSwgTUFQX1NIQVJFRCwgZmQsIDApOwoJaWYgKG1hcCA9 PSBNQVBfRkFJTEVEKSB7CgkJZnByaW50ZihzdGRlcnIsICJtbWFwKCkgZmFpbGVkXG4iKTsK CQlyZXR1cm4gLWVycm5vOwoJfQoKCWlmIChzZXR1cF91ZmZkKG1hcCkpCgkJcmV0dXJuIDE7 CgoJLyogUG9wdWxhdGUgYSBwYWdlIC4uLiAqLwoJbWVtc2V0KG1hcCwgMCwgcGFnZXNpemUp OwoKCS8qIC4uLiBhbmQgd3JpdGUtcHJvdGVjdCBpdCB1c2luZyB1ZmZkLXdwLiAqLwoJdWZm ZF93cml0ZXByb3RlY3QucmFuZ2Uuc3RhcnQgPSAodW5zaWduZWQgbG9uZykgbWFwOwoJdWZm ZF93cml0ZXByb3RlY3QucmFuZ2UubGVuID0gcGFnZXNpemU7Cgl1ZmZkX3dyaXRlcHJvdGVj dC5tb2RlID0gVUZGRElPX1dSSVRFUFJPVEVDVF9NT0RFX1dQOwoJaWYgKGlvY3RsKHVmZmQs IFVGRkRJT19XUklURVBST1RFQ1QsICZ1ZmZkX3dyaXRlcHJvdGVjdCkpIHsKCQlmcHJpbnRm KHN0ZGVyciwgIlVGRkRJT19XUklURVBST1RFQ1QgZmFpbGVkOiAlZFxuIiwgZXJybm8pOwoJ CXJldHVybiAtZXJybm87Cgl9CgoJLyogV3JpdGUtcHJvdGVjdCB0aGUgd2hvbGUgbWFwcGlu ZyB0ZW1wb3JhcmlseS4gKi8KCW1wcm90ZWN0KG1hcCwgcGFnZXNpemUsIFBST1RfUkVBRCk7 CgltcHJvdGVjdChtYXAsIHBhZ2VzaXplLCBQUk9UX1JFQUR8UFJPVF9XUklURSk7CgoJLyog VGVzdCBpZiB1ZmZkLXdwIGZpcmVzLiAqLwoJbWVtc2V0KG1hcCwgMSwgcGFnZXNpemUpOwoK CXByaW50ZigiRkFJTDogdWZmZC13cCBkaWQgbm90IGZpcmVcbiIpOwoJcmV0dXJuIDE7Cn0K --------------ApHjnS14u0ZxFpWQKPOgTgNr--