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[140.211.169.12]) by mx.google.com with ESMTPS id f23si2535162pgl.225.2019.03.06.15.13.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 06 Mar 2019 15:13:53 -0800 (PST) Received-SPF: pass (google.com: domain of akpm@linux-foundation.org designates 140.211.169.12 as permitted sender) client-ip=140.211.169.12; Authentication-Results: mx.google.com; spf=pass (google.com: domain of akpm@linux-foundation.org designates 140.211.169.12 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id 836AFB664; Wed, 6 Mar 2019 23:13:52 +0000 (UTC) Date: Wed, 6 Mar 2019 15:13:51 -0800 From: Andrew Morton To: Vlastimil Babka Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Peter Zijlstra , Greg KH , Jann Horn , Jiri Kosina , Dominique Martinet , Andy Lutomirski , Dave Chinner , Kevin Easton , Matthew Wilcox , Cyril Hrubis , Tejun Heo , "Kirill A . Shutemov" , Daniel Gruss , Jiri Kosina Subject: Re: [PATCH 1/3] mm/mincore: make mincore() more conservative Message-Id: <20190306151351.f8ae1acae51ccad1a3537284@linux-foundation.org> In-Reply-To: <20190130124420.1834-2-vbabka@suse.cz> References: <20190130124420.1834-1-vbabka@suse.cz> <20190130124420.1834-2-vbabka@suse.cz> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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, 30 Jan 2019 13:44:18 +0100 Vlastimil Babka wrote: > From: Jiri Kosina > > The semantics of what mincore() considers to be resident is not completely > clear, but Linux has always (since 2.3.52, which is when mincore() was > initially done) treated it as "page is available in page cache". > > That's potentially a problem, as that [in]directly exposes meta-information > about pagecache / memory mapping state even about memory not strictly belonging > to the process executing the syscall, opening possibilities for sidechannel > attacks. > > Change the semantics of mincore() so that it only reveals pagecache information > for non-anonymous mappings that belog to files that the calling process could > (if it tried to) successfully open for writing. "for writing" comes as a bit of a surprise. Why not for reading? Could we please explain the reasoning in the changelog and in the (presently absent) comments which describe can_do_mincore()? > @@ -189,8 +197,13 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v > vma = find_vma(current->mm, addr); > if (!vma || addr < vma->vm_start) > return -ENOMEM; > - mincore_walk.mm = vma->vm_mm; > end = min(vma->vm_end, addr + (pages << PAGE_SHIFT)); > + if (!can_do_mincore(vma)) { > + unsigned long pages = (end - addr) >> PAGE_SHIFT; I'm not sure this is correct in all cases. If addr = 4095 vma->vm_end = 4096 pages = 1000 then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it should have been 1. Please check? A mincore test suite in tools/testing/selftests would be useful, methinks. To exercise such corner cases, check for future breakage, etc. > + memset(vec, 1, pages); > + return pages; > + } > + mincore_walk.mm = vma->vm_mm; > err = walk_page_range(addr, end, &mincore_walk); > if (err < 0) > return err;