linux-kernel - Re: [PATCH] ksmbd: Annotate struct copychunk_ioctl_req with __counted_by

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Message-ID: <d6341fc4-1d9d-46af-b809-f30430b30455@talpey.com>
Date: Tue, 24 Sep 2024 14:05:18 -0400
From: Tom Talpey <tom@...pey.com>
To: Thorsten Blum <thorsten.blum@...ux.dev>,
 Namjae Jeon <linkinjeon@...nel.org>, Steve French <sfrench@...ba.org>,
 Sergey Senozhatsky <senozhatsky@...omium.org>, Kees Cook <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-cifs@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] ksmbd: Annotate struct copychunk_ioctl_req with
 __counted_by_le()

On 9/24/2024 6:22 AM, Thorsten Blum wrote:
> Add the __counted_by_le compiler attribute to the flexible array member
> Chunks to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> CONFIG_FORTIFY_SOURCE.
> 
> Read Chunks[0] after checking that ChunkCount is not 0.
> 
> Compile-tested only.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@...ux.dev>
> ---
>   fs/smb/server/smb2pdu.c | 2 +-
>   fs/smb/server/smb2pdu.h | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
> index 461c4fc682ac..0670bdf3e167 100644
> --- a/fs/smb/server/smb2pdu.c
> +++ b/fs/smb/server/smb2pdu.c
> @@ -7565,7 +7565,6 @@ static int fsctl_copychunk(struct ksmbd_work *work,
>   	ci_rsp->TotalBytesWritten =
>   		cpu_to_le32(ksmbd_server_side_copy_max_total_size());
>   
> -	chunks = (struct srv_copychunk *)&ci_req->Chunks[0];
>   	chunk_count = le32_to_cpu(ci_req->ChunkCount);
>   	if (chunk_count == 0)
>   		goto out;
> @@ -7579,6 +7578,7 @@ static int fsctl_copychunk(struct ksmbd_work *work,
>   		return -EINVAL;
>   	}
>   
> +	chunks = (struct srv_copychunk *)&ci_req->Chunks[0];
>   	for (i = 0; i < chunk_count; i++) {
>   		if (le32_to_cpu(chunks[i].Length) == 0 ||
>   		    le32_to_cpu(chunks[i].Length) > ksmbd_server_side_copy_max_chunk_size())
> diff --git a/fs/smb/server/smb2pdu.h b/fs/smb/server/smb2pdu.h
> index 73aff20e22d0..f01121dbf358 100644
> --- a/fs/smb/server/smb2pdu.h
> +++ b/fs/smb/server/smb2pdu.h
> @@ -194,7 +194,7 @@ struct copychunk_ioctl_req {
>   	__le64 ResumeKey[3];
>   	__le32 ChunkCount;
>   	__le32 Reserved;
> -	__u8 Chunks[]; /* array of srv_copychunk */
> +	__u8 Chunks[] __counted_by_le(ChunkCount); /* array of srv_copychunk */
>   } __packed;
>   

This isn't correct. The u8 is just a raw buffer, copychunk structs are
marshaled into it, and they're 24 bytes each.

The better fix would be to type the buffer contents properly, but that
will have protocol implications and should be thoroughly tested.

NAK on this change for now.

Tom.

>   struct srv_copychunk {