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

[Purva Yeshi <purvayeshi550@gmail.com>]

On 10/04/25 13:21, Stefano Garzarella wrote:
> 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
> 

Hi Jarkko, Stefano,
Thank you for the review.

I've revisited the issue and updated the implementation of 
tpm_buf_read() to zero out the *output buffer in the error paths, 
instead of initializing the return value in each caller.

static void tpm_buf_read(struct tpm_buf *buf, off_t *offset, size_t 
count, void *output)
{
	off_t next_offset;

	/* Return silently if overflow has already happened. */
	if (buf->flags & TPM_BUF_BOUNDARY_ERROR) {
		memset(output, 0, count);
		return;
	}

	next_offset = *offset + count;
	if (next_offset > buf->length) {
		WARN(1, "tpm_buf: read out of boundary\n");
		buf->flags |= TPM_BUF_BOUNDARY_ERROR;
		memset(output, 0, count);
		return;
	}

	memcpy(output, &buf->data[*offset], count);
	*offset = next_offset;
}

This approach ensures that output is always zeroed when the read fails, 
which avoids returning uninitialized stack values from the helper 
functions like tpm_buf_read_u8(), tpm_buf_read_u16(), and 
tpm_buf_read_u32().

Does this solution look acceptable for the next version of the patch?

Best regards,
Purva Yeshi