From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 51EE7C71155 for ; Tue, 17 Jun 2025 06:03:35 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id A9D0E6B008A; Tue, 17 Jun 2025 02:03:34 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A4DEC6B0093; Tue, 17 Jun 2025 02:03:34 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 963926B0095; Tue, 17 Jun 2025 02:03:34 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 895446B008A for ; Tue, 17 Jun 2025 02:03:34 -0400 (EDT) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 189305D564 for ; Tue, 17 Jun 2025 06:03:34 +0000 (UTC) X-FDA: 83563850748.28.C0D4F41 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) by imf14.hostedemail.com (Postfix) with ESMTP id 18C2C10000A for ; Tue, 17 Jun 2025 06:03:31 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=BvCNgVhn; spf=pass (imf14.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.189 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1750140212; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=dm5gLE8p1RI+b2xU1al9yd130OkPO03e69aDHKUWjS8=; b=6+sSjAhFUXOB8N/DahRteWsD4uRzaVdEdMVWBKmC+Cn43g2QGMIYdw1SiV+/3pSixSuh0v sJVPQcPvztvJVO1N8PwAXFR38NrJ7zMW4iZ2MmTnIQSFTtdMcIs9+nddcDpbITC+0DCMjL qjcfehUw8tqziET0RNwZUndEnED2DyA= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=BvCNgVhn; spf=pass (imf14.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.189 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1750140212; a=rsa-sha256; cv=none; b=ae/V4EtJLao9UZ9yJtGpt0fM9x7waHSlmnMh2SpLxauU3e4JtYFFM0reMgM1TzysnzHw2n x3xCYZRRsoBTlFwSo2FimVHq5Obp8IKb/nOIu8ROvNPBllK1vwjrwgdW9elwvxKrFka39p u0ggLh+fISXuJM9gl51cructR/mlVks= Message-ID: <9533ce3a-4d87-4cff-b545-9e2f1afc607f@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1750140209; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=dm5gLE8p1RI+b2xU1al9yd130OkPO03e69aDHKUWjS8=; b=BvCNgVhnnJaBfrkxDAcwONceWTm80TxCGOZP6Sa8vhvMtQIkij3HCeciaE/5P5VdCGoaH/ gW5+NbNF7zskB9xEJsSwPkDDB3GD6yZoTCOWckPtQRiESpB0OvA9UGXUfI4sEOXNQ/xZfm suIatakHQGU1UT8PlH6WiK06kGGyElE= Date: Tue, 17 Jun 2025 14:03:24 +0800 MIME-Version: 1.0 Subject: Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk_vmas Content-Language: en-US To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, david@redhat.com, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, lorenzo.stoakes@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Lance Yang References: <20250617020544.57305-1-lance.yang@linux.dev> <6fe09fdd-ff38-42cc-b101-520204213f82@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 18C2C10000A X-Stat-Signature: ecexuco6bbijafhppge7ce5pkzpp4gby X-Rspam-User: X-HE-Tag: 1750140211-619212 X-HE-Meta: U2FsdGVkX19Od2c6H7S7sS5B/kYCleDZyOESOY/m9VrSIpdpTKg/AgneV0gYjVSvyVka/9YP/6GyrBidmhVilEovJhEPpREwPgholK79Kh4FRLAKg4B26GpT+8K3DB4tnQiO0oc+uLPYEzrN++luqCltm9/lafSt7TToWvjHtv+8XkTyNI1Kqb2pTo26fqNx8JAFqsf3pUqcIH6wBph0YazQ32J9L/du7Hy6/Fidikka6M+47JZZHoJoaEUHOZD0MVGbpxfUSNFl+clhcJapDjPtfy2uXFnP98SkPEZ3SMH/Bb2KURspXniftuEC6+bMPzeVpc0sl1pu0xDaDzNC7ZOrEjIqZ709nFApWG997Gz0gtb52ikuniNFiD12ulYSrNgYGR1T0vDVmI9Quml4K4PkHqtl5ZjJKjAnjqs5bJ3aOc1NfPaM3ZzR1tLEnD5zOyAaUFiteOxeM4E/DXudBO9usI+DK9IXqJmRX5HFXcSipMPJJ1xw+qcGJ3+yAAWVktvhapjY7KKO21ZVGtFzvSfngQoN8IFg+gavr+jHIfOAZggYt0YmuUbRFN1iE5Xk5j3zFUcuEA162gfQLFgWD11cT/2F5zP84JJjxu4GNopwOhk/gZuOPQPSQRtpiPA767QUeypxgq0/LC9TnafukCH8Fwkj3QKOCghu/H3VoYhDnj48SlPHMh/Y3GqP1TI6Ya1MA0v4jhCt6490uzIo/u2IM3Btl4zjcvAUJL6aWm7Jem3S10ETL9nnTPwcdYPPWITqZYvYleYVRCUhBTfOk17vHVBPWkCqgV0DW/iRy3cvZyjxqhhj5E9nKcuqF6bvv4tBVM0CsSY+T3mS/xlMYty8DUQXjWUYg9Iht2y4KDuUrb+OK19Uk3q4fi2uBjTxIIpWv8861Trr9bngh3ipUMMiW+2IWuwVbW1VdHYQyLW9l4iEaHIpwjgS8Ge7CkQ93OS+wJtpJq9r9igSwoc OfFijCAF OKXLGhmvrgQblnPtYBIqTKqADXxzlcx5tpYnhXbtK0QmR9aYUyYfxhsxOs1y8bnhO8KWxVEoqvHlbittZi61r6c1qMdXALXYA5un68AsNsRvAbD4nxTvJ9wvC361shPa8uLDhFIWVe0yzMhuWA0oncV6D3P6n74H7tgJ23WAmaYf8bxZbgxfH0kq6Oe2DloOSUK/kv/qHVyvglMdvlmmcmr2RsxY77z5oOiZrY4PYVZMuC/MBbkvukKDN2nHTl7CIFFqVHcFUGb0XTStZrsRYwSwqXTw6szF4vyTdu5NE48uZvKTfWfyhRnCC0ndbg4HV6wNq7hTx0S0UR2XlGnhrIJqPNO5U/UsjhY2s 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: List-Subscribe: List-Unsubscribe: On 2025/6/17 13:19, Barry Song wrote: > On Tue, Jun 17, 2025 at 4:57 PM Lance Yang wrote: >> >> >> >> On 2025/6/17 10:24, Barry Song wrote: >>> On Tue, Jun 17, 2025 at 2:05 PM Lance Yang wrote: >>>> >>>> From: Lance Yang >>>> >>>> The prev pointer was uninitialized, which could lead to undefined behavior >>>> where its address is taken and passed to the visit() callback without being >>>> assigned a value. >>>> >>>> Initializing it to NULL makes the code safer and prevents potential bugs >>>> if a future callback function attempts to read from it. >>> >>> Is there any read-before-write case here? I haven't found one. >> >> >> It appears that the following is a call chain showing the read-before-write >> of prev: >> >> -> madvise_vma_anon_name(..., struct vm_area_struct **prev, ...) >> Receives the address of madvise_walk_vmas's prev. >> Passes this pointer directly to madvise_update_vma. >> Note that prev is not updated before visit() is called >> if !(start > vma->vm_start) in the slow path. >> >> -> madvise_update_vma(..., struct vm_area_struct **prev, ...) >> It calls the next function with *prev. >> >> -> vma_modify_flags_name(..., *prev, ...) >> Stores the value of madvise_walk_vmas's prev in >> vmg.prev >> using the VMG_VMA_STATE macro. >> >> -> vma_modify(struct vma_merge_struct *vmg) >> Receives the vmg struct. >> Passes vmg to vma_merge_existing_range. >> >> -> vma_merge_existing_range(struct >> vma_merge_struct *vmg) >> Retrieves the value: struct >> vm_area_struct *prev = vmg->prev; >> The value is now used in a >> conditional check: >> VM_WARN_ON_VMG(prev && start <= >> prev->vm_start, vmg) >> If prev was uninitialized, this >> would cause a crash. > > Thanks! > > Do you have a reproducer? I'd like to try. Not yet ;) It was found during code review to prevent potential bugs, and the initialization itself is harmless. Thanks, Lance > >> >> Thanks, >> Lance >> >>> >>> It also looks like we're assuming that *prev == NULL implies >>> a specific condition: >>> >>> *prev = NULL; /* tell sys_madvise we drop mmap_lock */ >>> >>> *prev = NULL; /* mmap_lock has been dropped, prev is stale */ >>> >>>> >>>> Signed-off-by: Lance Yang >>>> --- >>>> mm/madvise.c | 4 ++-- >>>> 1 file changed, 2 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>> index 267d8e4adf31..c87325000303 100644 >>>> --- a/mm/madvise.c >>>> +++ b/mm/madvise.c >>>> @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start, >>>> struct vm_area_struct **prev, unsigned long start, >>>> unsigned long end, void *arg)) >>>> { >>>> + struct vm_area_struct *prev = NULL; >>>> struct vm_area_struct *vma; >>>> - struct vm_area_struct *prev; >>>> - unsigned long tmp; >>>> int unmapped_error = 0; >>>> + unsigned long tmp; >>>> int error; >>>> >>>> /* >>>> -- >>>> 2.49.0 >>>> >>> > > Thanks > Barry