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

[Purva Yeshi]

On 10/04/25 14:24, Jarkko Sakkinen wrote:
> On Thu, Apr 10, 2025 at 09:51:09AM +0200, 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:
>
> Empty line and s/error/issue/.
>
> > > > 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?
>
> It should never happen, if the kernel is working correctly. The commit
> message implies a legit failure scenario, which would imply that the
> patch is a bug fix, which it actually is not.
>
> "Add a sanity check for boundary overflow, and zero out the value,
> if the unexpected happens" is what this patch actually does. I.e.,
> code change acceptable, commit message is all wrong.

Understood now. I’ll update the commit message to clearly state this is 
a sanity check for unexpected boundary overflows. Thanks for the 
clarification!

> > 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
>
> BR, Jarkko