Re: [PATCH net v3 3/3] nfc: st21nfca: fix incorrect sizing calculations in EVT_TRANSACTION

Re: [PATCH net v3 3/3] nfc: st21nfca: fix incorrect sizing calculations in EVT_TRANSACTION

[Krzysztof Kozlowski]

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On 07/06/2022 04:57, Martin Faltesek wrote:
> The transaction buffer is allocated by using the size of the packet buf,
> and subtracting two which seem intended to remove the two tags which are
> not present in the target structure. This calculation leads to under
> counting memory because of differences between the packet contents and the
> target structure. The aid_len field is a u8 in the packet, but a u32 in
> the structure, resulting in at least 3 bytes always being under counted.
> Further, the aid data is a variable length field in the packet, but fixed
> in the structure, so if this field is less than the max, the difference is
> added to the under counting.
> 
> The last validation check for transaction->params_len is also incorrect
> since it employs the same accounting error.
> 
> To fix, perform validation checks progressively to safely reach the
> next field, to determine the size of both buffers and verify both tags.
> Once all validation checks pass, allocate the buffer and copy the data.
> This eliminates freeing memory on the error path, as those checks are
> moved ahead of memory allocation.
> 
> Fixes: 26fc6c7f02cb ("NFC: st21nfca: Add HCI transaction event support")
> Fixes: 4fbcc1a4cb20 ("nfc: st21nfca: Fix potential buffer overflows in EVT_TRANSACTION")
> Cc: stable(a)vger.kernel.org
> Signed-off-by: Martin Faltesek <mfaltesek@google.com>
> ---

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>


Best regards,
Krzysztof