From patchwork Wed Mar 25 11:13:46 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 11457553 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 1D553139A for ; Wed, 25 Mar 2020 11:14:00 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EFD3720714 for ; Wed, 25 Mar 2020 11:13:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="gYTlkku6"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="YKnz9Hec" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EFD3720714 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:Message-Id:Date:Subject:To :From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References: List-Owner; bh=38afgLrpbg3SejKGmpmlCG2gJNNc0DJem0UDzbjrWpQ=; b=gYTlkku6/Tox26 PlAHOGqHuhQrV+b5+vp5tTjvaEmJLRZtARhyb2IRPr2qDmv1sAg7lrGitJax4aFIHKpmAO63NBY5H jQTrnx42b75YfchTBjsl+x+aiOSP0LHLcrdrp1vZ4fKZRD5TUojd4fcVkkAsZ8IC4wOhMfSi54ZvU Ii2SmYdK/gyw2J0zTx4wOHsvDeCfb4ZHACzjEBgGqMfPy+cyrjNoiOEntqNsHpkoL3dXzmkV+9oOJ e/vQZJENLPBONK2mHFkDDzpoQ1KntdwTkPQ+IJIchgIW3dMigPS9yDGRVahWD+c8yCiX0/etdlc+l 9pyMGoNjwSg2mifWfOMA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jH3yu-0006YD-VS; Wed, 25 Mar 2020 11:13:56 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jH3yr-0006XQ-V7 for linux-arm-kernel@lists.infradead.org; Wed, 25 Mar 2020 11:13:55 +0000 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DEA2B20714; Wed, 25 Mar 2020 11:13:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585134832; bh=BzCuAlwuxAGX3F005NMoJ1SQbjE8q7I0TQauHBZ+31o=; h=From:To:Cc:Subject:Date:From; b=YKnz9Hecl5fxuD2C7GFv4OgVyH5qObz8ssS4YRbT+X2qm4uFTkze2Xy/WOTKAIx4p MbacL9p1KQ5APanJBD1/IcPOu7Ub8XAddFVr6IVWbskaNV8/HkmR6cCGJGgDkaVzFe G5G4M+ftNeePFy0vNPDu5KMBY8SVoDgdZvQcZT4c= From: Will Deacon To: linux-kernel@vger.kernel.org Subject: [PATCH] mm/mremap: Add comment explaining the untagging behaviour of mremap() Date: Wed, 25 Mar 2020 11:13:46 +0000 Message-Id: <20200325111347.32553-1-will@kernel.org> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200325_041354_024391_55847D43 X-CRM114-Status: GOOD ( 13.85 ) X-Spam-Score: -5.2 (-----) X-Spam-Report: SpamAssassin version 3.4.4 on bombadil.infradead.org summary: Content analysis details: (-5.2 points) pts rule name description ---- ---------------------- -------------------------------------------------- -5.0 RCVD_IN_DNSWL_HI RBL: Sender listed at https://www.dnswl.org/, high trust [198.145.29.99 listed in list.dnswl.org] -0.0 SPF_PASS SPF: sender matches SPF record 0.0 SPF_HELO_NONE SPF: HELO does not publish an SPF Record -0.1 DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature -0.0 DKIMWL_WL_HIGH DKIMwl.org - Whitelisted High sender X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Will Deacon , Linus Torvalds , kernel-team@android.com, linux-arm-kernel@lists.infradead.org, Catalin Marinas Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org Commit dcde237319e6 ("mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()") changed mremap() so that only the 'old' address is untagged, leaving the 'new' address in the form it was passed from userspace. This prevents the unexpected creation of aliasing virtual mappings in userspace, but looks a bit odd when you read the code. Add a comment justifying the untagging behaviour in mremap(). Cc: Catalin Marinas Reported-by: Linus Torvalds Signed-off-by: Will Deacon Reviewed-by: Catalin Marinas --- mm/mremap.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index af363063ea23..d28f08a36b96 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -606,6 +606,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len, LIST_HEAD(uf_unmap_early); LIST_HEAD(uf_unmap); + /* + * There is a deliberate asymmetry here: we strip the pointer tag + * from the old address but leave the new address alone. This is + * for consistency with mmap(), where we prevent the creation of + * aliasing mappings in userspace by leaving the tag bits of the + * mapping address intact. A non-zero tag will cause the subsequent + * range checks to reject the address as invalid. + * + * See Documentation/arm64/tagged-address-abi.rst for more information. + */ addr = untagged_addr(addr); if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))