From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id D15F312E1DC for ; Fri, 27 Feb 2026 14:05:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772201107; cv=none; b=ZX4gNV/EV4hJ5OWxzSpERAsoXShxlodNEg29MDseqBZklMJRSu9RJxr90muJQcDioysGlCOTfJvp2buZjSvU8D+q6mBKQGB/UXyf9SxN9hTBXWxzcpVn9FWKqJl7QQE7OvUDrbwSftHNQ85WqUTicquCZww05m7zYZOvggw/9Qs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772201107; c=relaxed/simple; bh=s/z7xAAXncvwUSOTtRLOIOjAzsl8T/73uPy1pjdoxQY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZMDAdEdzAuAuGjdMCp30npouN3Ryhg9uRAkDjPtkJYO2v5+pmiz/BryVaYDszmimbLyUYP9k7YerQUtQgnl70qokg9CIlQh2yTjwFqInarhdjptpOJG6fKwFeJo6+QMpYdcDowQDYcJj5XUQHBIgDgeI/26GTjOk/31AblVzOTE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BA3EA14BF; Fri, 27 Feb 2026 06:04:58 -0800 (PST) Received: from [10.57.55.4] (unknown [10.57.55.4]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B70FC3F62B; Fri, 27 Feb 2026 06:05:03 -0800 (PST) Message-ID: Date: Fri, 27 Feb 2026 14:05:01 +0000 Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] iommu/dma: Validate page before accessing P2PDMA state To: Ashish Mhetre , Leon Romanovsky , Pranjal Shrivastava Cc: joro@8bytes.org, will@kernel.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-tegra@vger.kernel.org, linux-mm@kvack.org References: <20260224104257.1641429-1-amhetre@nvidia.com> <20260224123221.GM10607@unreal> <9d01b4e3-be5b-4c9c-8088-1d10f67f1fd8@nvidia.com> <20260225075609.GB9541@unreal> <20260226075806.GE12611@unreal> <58634d52-5d44-4ec9-b1f6-273b5c32b525@nvidia.com> From: Robin Murphy Content-Language: en-GB In-Reply-To: <58634d52-5d44-4ec9-b1f6-273b5c32b525@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2026-02-27 5:46 am, Ashish Mhetre wrote: > > > On 2/26/2026 1:28 PM, Leon Romanovsky wrote: >> External email: Use caution opening links or attachments >> >> >> On Wed, Feb 25, 2026 at 08:11:29PM +0000, Pranjal Shrivastava wrote: >>> On Wed, Feb 25, 2026 at 09:56:09AM +0200, Leon Romanovsky wrote: >>>> On Wed, Feb 25, 2026 at 10:19:41AM +0530, Ashish Mhetre wrote: >>>>> >>>>> On 2/25/2026 2:27 AM, Pranjal Shrivastava wrote: >>>>>> External email: Use caution opening links or attachments >>>>>> >>>>>> >>>>>> On Tue, Feb 24, 2026 at 02:32:21PM +0200, Leon Romanovsky wrote: >>>>>>> On Tue, Feb 24, 2026 at 10:42:57AM +0000, Ashish Mhetre wrote: >>>>>>>> When mapping scatter-gather entries that reference reserved >>>>>>>> memory regions without struct page backing (e.g., bootloader >>>>>>>> created >>>>>>>> carveouts), is_pci_p2pdma_page() dereferences the page pointer >>>>>>>> returned by sg_page() without first verifying its validity. >>>>>>> I believe this behavior started after commit 88df6ab2f34b >>>>>>> ("mm: add folio_is_pci_p2pdma()"). Prior to that change, the >>>>>>> is_zone_device_page(page) check would return false when given a >>>>>>> non‑existent page pointer. >>>>>>> >>>>> Thanks Leon for the review. This crash started after commit >>>>> 30280eee2db1 >>>>> ("iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg"). >>>>> >>>>>> Doesn't folio_is_pci_p2pdma() also check for zone device? >>>>>> I see[1] that it does: >>>>>> >>>>>> static inline bool folio_is_pci_p2pdma(const struct folio *folio) >>>>>> { >>>>>>           return IS_ENABLED(CONFIG_PCI_P2PDMA) && >>>>>>                   folio_is_zone_device(folio) && >>>>>>                   folio->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA; >>>>>> } >>>>>> >>>>>> I believe the problem arises due to the page_folio() call in >>>>>> folio_is_pci_p2pdma(page_folio(page)); within is_pci_p2pdma_page(). >>>>>> page_folio() assumes it has a valid struct page to work with. For >>>>>> these >>>>>> carveouts, that isn't true. >>>>>> >>>>>> Potentially something like the following would stop the crash: >>>>>> >>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >>>>>> index e3c2ccf872a8..e47876021afa 100644 >>>>>> --- a/include/linux/memremap.h >>>>>> +++ b/include/linux/memremap.h >>>>>> @@ -197,7 +197,8 @@ static inline void >>>>>> folio_set_zone_device_data(struct folio *folio, void *data) >>>>>> >>>>>>    static inline bool is_pci_p2pdma_page(const struct page *page) >>>>>>    { >>>>>> -       return IS_ENABLED(CONFIG_PCI_P2PDMA) && >>>>>> +       return IS_ENABLED(CONFIG_PCI_P2PDMA) && page && >>>>>> +               pfn_valid(page_to_pfn(page)) && >>>>>>                   folio_is_pci_p2pdma(page_folio(page)); >>>>>>    } >>>>>> >>>>> Yes, this will also fix the crash. >>>>> >>>>>> But my broader question is: why are we calling a page-based API like >>>>>> is_pci_p2pdma_page() on non-struct-page memory in the first place? >>>>>> Could we instead add a helper to verify if the sg_page() return value >>>>>> is actually backed by a struct page? If it isn't, we should arguably >>>>>> skip the P2PDMA logic entirely and fall back to a dma_map_phys style >>>>>> path. Isn't handling these "pageless" physical ranges the primary >>>>>> reason >>>>>> dma_map_phys exists? >>>>> Thanks for the feedback, Pranjal. >>>>> >>>>> To clarify: are you suggesting we handle non-page-backed mappings >>>>> inside >>>>> iommu_dma_map_sg (within dma-iommu), or that callers should detect >>>>> non-page-backed memory and use dma_map_phys instead of dma_map_sg? >>>> The latter one. >>>> >>> Yup, I meant the latter. >>> >>>>> Former approach sounds better so that existing iommu_dma_map_sg >>>>> callers >>>>> don't need changes, but I'd like to confirm your preference. >>>> The bug is in callers which used wrong API, they need to be adapted. >>> Yes, the thing is, if the caller already knows that the region to be >>> mapped is NOT struct page-backed, then why does it use dma_map_sg >>> variants? >> Before dma_map_phys() was added, there was no reliable way to DMA‑map >> such memory, and using dma_map_sg() was a workaround that happened to >> work. I'm not sure whether it worked by design or by accident, but the >> correct approach now is to use dma_map_phys(). > > Thanks Leon and Pranjal for the detailed feedback. I'll update our > callers to use > dma_map_phys() for non-page-backed buffers. > > One question: would it make sense to add a check in iommu_dma_map_sg to > fail gracefully when non-page-backed buffers are passed, instead of > crashing > the kernel? No, it is the responsibility of drivers not to abuse kernel APIs inappropriately. Checking for misuse adds overhead that penalises correct users. dma_map_page/sg on non-page-backed memory has never been valid, and it would only have been system-configuration-dependent luck that it wasn't already blowing up before. I guess dma-debug could add additional checks on these APIs similarly to debug_dma_map_single(), but the fact that we've never even considered checking for made-up bogus struct page pointers only goes to show just how wrong a thing to do it is. Thanks, Robin.