Re: [PATCH] hw/acpi/erst.c: Fix memset argument order

[Christian A. Ehrhardt]

Hi Markus,

On Thu, Oct 20, 2022 at 08:14:32AM +0200, Markus Armbruster wrote:
> "Christian A. Ehrhardt" <lk@c--e.de> writes:
> 
> > Fix memset argument order: The second argument is
> > the value, the length goes last.
> 
> Impact of the bug?

Well, this is a memory error, i.e. the potential impact is
anything from silent data corruption to arbitrary code execution.
Phillipe described this accurately as "Ouch".

> > Cc: Eric DeVolder <eric.devolder@oracle.com>
> > Cc: qemu-stable@nongnu.org
> > Fixes: f7e26ffa590 ("ACPI ERST: support for ACPI ERST feature")
> > Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
> >          /* Write the record into the slot */

[ ... ]

> This first copies @record_length bytes into the exchange buffer.
> 
> > -        memset(nvram + record_length, exchange_length - record_length, 
> > 0xFF);
> > +        memset(nvram + record_length, 0xFF, exchange_length - 
> > record_length);
> 
> The new code pads it to the full exchange buffer size.
> 
> The old code writes 0xFF bytes.
> 
> If 0xFF < exchange_length - record_length, the padding doesn't extend to
> the end of the buffer.  Impact?

Incorrect and insufficient data is written.

> If 0xFF > exchange_length - record_length, we write beyond the end of
> the buffer.  Impact?

Buffer overrun with well known potentially catastrophic consequences.
Additionally, incorrect data is used for the padding.

      regards    Christian