lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Thu, 15 Jun 2017 06:35:21 +0530 From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> To: Will Deacon <will.deacon@....com> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, Vineet Gupta <vgupta@...opsys.com>, Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>, Ralf Baechle <ralf@...ux-mips.org>, "David S. Miller" <davem@...emloft.net>, Heiko Carstens <heiko.carstens@...ibm.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, Andrea Arcangeli <aarcange@...hat.com>, linux-arch@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, mark.rutland@....com Subject: Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages On Wednesday 14 June 2017 10:25 PM, Will Deacon wrote: > Hi Aneesh, > > On Wed, Jun 14, 2017 at 08:55:26PM +0530, Aneesh Kumar K.V wrote: >> On Wednesday 14 June 2017 07:21 PM, Kirill A. Shutemov wrote: >>> Vlastimil noted that pmdp_invalidate() is not atomic and we can loose >>> dirty and access bits if CPU sets them after pmdp dereference, but >>> before set_pmd_at(). >>> >>> The bug doesn't lead to user-visible misbehaviour in current kernel, but >>> fixing this would be critical for future work on THP: both huge-ext4 and THP >>> swap out rely on proper dirty tracking. >>> >>> Unfortunately, there's no way to address the issue in a generic way. We need to >>> fix all architectures that support THP one-by-one. >>> >>> All architectures that have THP supported have to provide atomic >>> pmdp_invalidate(). If generic implementation of pmdp_invalidate() is used, >>> architecture needs to provide atomic pmdp_mknonpresent(). >>> >>> I've fixed the issue for x86, but I need help with the rest. >>> >>> So far THP is supported on 8 architectures. Power and S390 already provides >>> atomic pmdp_invalidate(). x86 is fixed by this patches, so 5 architectures >>> left: >>> >>> - arc; >>> - arm; >>> - arm64; >>> - mips; >>> - sparc -- it has custom pmdp_invalidate(), but it's racy too; >>> >>> Please, help me with them. >>> >>> Kirill A. Shutemov (3): >>> x86/mm: Provide pmdp_mknotpresent() helper >>> mm: Do not loose dirty and access bits in pmdp_invalidate() >>> mm, thp: Do not loose dirty bit in __split_huge_pmd_locked() >>> >> >> >> But in __split_huge_pmd_locked() we collected the dirty bit early. So even >> if we made pmdp_invalidate() atomic, if we had marked the pmd pte entry >> dirty after we collected the dirty bit, we still loose it right ? >> >> >> May be we should relook at pmd PTE udpate interface. We really need an >> interface that can update pmd entries such that we don't clear it in >> between. IMHO, we can avoid the pmdp_invalidate() completely, if we can >> switch from a pmd PTE entry to a pointer to PTE page (pgtable_t). We also >> need this interface to avoid the madvise race fixed by > > There's a good chance I'm not following your suggestion here, but it's > probably worth me pointing out that swizzling a page table entry from a > block mapping (e.g. a huge page mapped at the PMD level) to a table entry > (e.g. a pointer to a page of PTEs) can lead to all sorts of horrible > problems on ARM, including amalgamation of TLB entries and fatal aborts. > > So we really need to go via an invalid entry, with appropriate TLB > invalidation before installing the new entry. > I am not suggesting we don't do the invalidate (the need for that is documented in __split_huge_pmd_locked(). I am suggesting we need a new interface, something like Andrea suggested. old_pmd = pmdp_establish(pmd_mknotpresent()); instead of pmdp_invalidate(). We can then use this in scenarios where we want to update pmd PTE entries, where right now we go through a pmdp_clear and set_pmd path. We should really not do that for THP entries. W.r.t pmdp_invalidate() usage, I was wondering whether we can do that early in __split_huge_pmd_locked(). -aneesh
Powered by blists - more mailing lists