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Message-ID: <a725e235-f4e9-84a3-a88b-dce274423a75@meta.com>
Date:   Tue, 8 Nov 2022 12:35:22 -0800
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Subject: Re: [RFC PATCH v1 1/2] maccess: fix writing offset in case of fault
 in strncpy_from_kernel_nofault()
Content-Language: en-US
To:     Francis Laniel <flaniel@linux.microsoft.com>,
        linux-kernel@vger.kernel.org
Cc:     Alban Crequy <alban.crequy@gmail.com>,
        Alban Crequy <albancrequy@microsoft.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Mykola Lysenko <mykolal@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Shuah Khan <shuah@kernel.org>, linux-mm@kvack.org,
        bpf@vger.kernel.org, linux-kselftest@vger.kernel.org
References: <20221108195211.214025-1-flaniel@linux.microsoft.com>
 <20221108195211.214025-2-flaniel@linux.microsoft.com>
From:   Yonghong Song <yhs@meta.com>
In-Reply-To: <20221108195211.214025-2-flaniel@linux.microsoft.com>
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On 11/8/22 11:52 AM, Francis Laniel wrote:
> From: Alban Crequy <albancrequy@microsoft.com>
> 
> If a page fault occurs while copying the first byte, this function resets one
> byte before dst.
> As a consequence, an address could be modified and leaded to kernel crashes if
> case the modified address was accessed later.
> 
> Signed-off-by: Alban Crequy <albancrequy@microsoft.com>
> Tested-by: Francis Laniel <flaniel@linux.microsoft.com>
> ---
>   mm/maccess.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/maccess.c b/mm/maccess.c
> index 5f4d240f67ec..074f6b086671 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -97,7 +97,7 @@ long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
>   	return src - unsafe_addr;
>   Efault:
>   	pagefault_enable();
> -	dst[-1] = '\0';
> +	dst[0] = '\0';

What if the fault is due to dst, so the above won't work, right?

The original code should work fine if the first byte copy is successful.
For the first byte copy fault, maybe just to leave it as is?

>   	return -EFAULT;
>   }
> 
> --
> 2.25.1
>