From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-184.mta0.migadu.com (out-184.mta0.migadu.com [91.218.175.184]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 062D019F421 for ; Tue, 17 Jun 2025 06:03:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.184 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750140215; cv=none; b=Lcaxb5OgMXVQW1JfmV/85DXxApcC9NretO+BUrBA66Z8a8tGcM+MMPnSV+uzGiN89Qt/r0BER7MxZjZiEPzb4yFfX62eDZ1rYMfpDu/1auY2igVjIEHV+w0bOWCt5Ef2bnK3U9qrMLU55ntNU/YgzNnZyLUa9H0wZ3MKADiuPMc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750140215; c=relaxed/simple; bh=R85+3xivTi16MpxfaJKqNVPZkr71tEJi5+LtItNg5gs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LqAYtvc30OF0uAVM2wsGMbkP2bBPnlxQHoo83puaNeHfk7CjZIE1/fWrTFIcOy3ti3nf0zFUE8qwNTipbfmj5vwyvPl2STyIBMM61e73jF11BEz4uh2AJmJcQBazPttOsOrT0ZnuCaBxOeFjJx38Rjh/lbI0WI5MtuoDARr9UDs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=BvCNgVhn; arc=none smtp.client-ip=91.218.175.184 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="BvCNgVhn" 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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