linux-kernel - Re: [PATCH] char: tpm: tpm-buf: Fix uninitialized return values in read helpers

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Message-ID: <t2ri7facyvtmt6rx6xwcjos7rgtyiln7cywl2gt4effgukeejc@f3ml4apdh4zs>
Date: Thu, 10 Apr 2025 09:51:09 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Purva Yeshi <purvayeshi550@...il.com>, peterhuewe@....de, jgg@...pe.ca, 
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] char: tpm: tpm-buf: Fix uninitialized return values in
 read helpers

On Thu, Apr 10, 2025 at 09:14:58AM +0300, Jarkko Sakkinen wrote:
>On Thu, Apr 10, 2025 at 02:25:36AM +0530, Purva Yeshi wrote:
>> Fix Smatch-detected error:
>> drivers/char/tpm/tpm-buf.c:208 tpm_buf_read_u8() error:
>> uninitialized symbol 'value'.
>> drivers/char/tpm/tpm-buf.c:225 tpm_buf_read_u16() error:
>> uninitialized symbol 'value'.
>> drivers/char/tpm/tpm-buf.c:242 tpm_buf_read_u32() error:
>> uninitialized symbol 'value'.
>>
>> Call tpm_buf_read() to populate value but do not check its return
>> status. If the read fails, value remains uninitialized, causing
>> undefined behavior when returned or processed.
>>
>> Initialize value to zero to ensure a defined return even if
>> tpm_buf_read() fails, avoiding undefined behavior from using
>> an uninitialized variable.
>
>How does tpm_buf_read() fail?

If TPM_BUF_BOUNDARY_ERROR is set (or we are setting it), we are 
effectively returning random stack bytes to the caller.
Could this be a problem?

If it is, maybe instead of this patch, we could set `*output` to zero in 
the error path of tpm_buf_read(). Or return an error from tpm_buf_read() 
so callers can return 0 or whatever they want.

Thanks,
Stefano