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

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Message-ID: <e3dfcd3e-5f1f-4477-ace3-a97913a947cc@talpey.com>
Date: Tue, 24 Sep 2024 17:54:36 -0400
From: Tom Talpey <tom@...pey.com>
To: Thorsten Blum <thorsten.blum@...ux.dev>
Cc: 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>, 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 4:56 PM, Thorsten Blum wrote:
> On 24. Sep 2024, at 21:33, Thorsten Blum <thorsten.blum@...ux.dev> wrote:
>>> On 24. Sep 2024, at 20:05, Tom Talpey <tom@...pey.com> wrote:
>>> 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.
>>
>> Hm, I see.
>>
>> How does this for-loop work then? It iterates over ci_req->ChunkCount
>> and expects a srv_copychunk at each ci_req->Chunks[i]?
>>
>> 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())
>> break;
>> total_size_written += le32_to_cpu(chunks[i].Length);
>> }
> 
> Never mind, I just realized that the pointer arithmetic takes the array
> offset into account.
> 
> A srv_copychunk takes up 24 bytes and therefore 24 __u8[] slots. The
> __counted_by annotation is essentially off by a factor of 24.

Right, and the fix is probably obvious, but the copychunk code has
yielded a few surprises in the past, so I'd really like to see any
change tested... Maybe Steve has a ready-to-go section in his set.

Tom.