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Message-ID: <30278898-c4b2-4dd6-ba68-a19575f81a65@arm.com>
Date: Thu, 1 Feb 2024 14:51:39 +0530
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Subject: Re: [PATCH RFC v3 30/35] arm64: mte: ptrace: Handle pages with
 missing tag storage
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To: Alexandru Elisei <alexandru.elisei@arm.com>, catalin.marinas@arm.com,
 will@kernel.org, oliver.upton@linux.dev, maz@kernel.org,
 james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com,
 arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com,
 peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org,
 dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
 mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
 mhiramat@kernel.org, rppt@kernel.org, hughd@google.com
Cc: pcc@google.com, steven.price@arm.com, vincenzo.frascino@arm.com,
 david@redhat.com, eugenis@google.com, kcc@google.com, hyesoo.yu@samsung.com,
 linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
 kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org,
 linux-arch@vger.kernel.org, linux-mm@kvack.org,
 linux-trace-kernel@vger.kernel.org
References: <20240125164256.4147-1-alexandru.elisei@arm.com>
 <20240125164256.4147-31-alexandru.elisei@arm.com>
From: Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <20240125164256.4147-31-alexandru.elisei@arm.com>
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On 1/25/24 22:12, Alexandru Elisei wrote:
> A page can end up mapped in a MTE enabled VMA without the corresponding tag
> storage block reserved. Tag accesses made by ptrace in this case can lead
> to the wrong tags being read or memory corruption for the process that is
> using the tag storage memory as data.
> 
> Reserve tag storage by treating ptrace accesses like a fault.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
> 
> Changes since rfc v2:
> 
> * New patch, issue reported by Peter Collingbourne.
> 
>  arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index faf09da3400a..b1fa02dad4fd 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -412,10 +412,13 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
>  	while (len) {
>  		struct vm_area_struct *vma;
>  		unsigned long tags, offset;
> +		unsigned int fault_flags;
> +		struct page *page;
> +		vm_fault_t ret;
>  		void *maddr;
> -		struct page *page = get_user_page_vma_remote(mm, addr,
> -							     gup_flags, &vma);
>  
> +get_page:
> +		page = get_user_page_vma_remote(mm, addr, gup_flags, &vma);

But if there is valid page returned here in the first GUP attempt, will there
still be a subsequent handle_mm_fault() on the same vma and addr ?

>  		if (IS_ERR(page)) {
>  			err = PTR_ERR(page);
>  			break;
> @@ -433,6 +436,25 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
>  			put_page(page);
>  			break;
>  		}
> +
> +		if (tag_storage_enabled() && !page_tag_storage_reserved(page)) {

Should not '!page' be checked here as well ?

> +			fault_flags = FAULT_FLAG_DEFAULT | \
> +				      FAULT_FLAG_USER | \
> +				      FAULT_FLAG_REMOTE | \
> +				      FAULT_FLAG_ALLOW_RETRY | \
> +				      FAULT_FLAG_RETRY_NOWAIT;
> +			if (write)
> +				fault_flags |= FAULT_FLAG_WRITE;
> +
> +			put_page(page);
> +			ret = handle_mm_fault(vma, addr, fault_flags, NULL);
> +			if (ret & VM_FAULT_ERROR) {
> +				err = -EFAULT;
> +				break;
> +			}
> +			goto get_page;
> +		}
> +
>  		WARN_ON_ONCE(!page_mte_tagged(page));
>  
>  		/* limit access to the end of the page */