ACK: [L][PATCH 0/1] kernel: fix __clear_user() inline assembly constraints (LP: 2013088)

[Cory Todd]

On Tue, Mar 28, 2023 at 07:58:19PM +0200, frank.heimes at canonical.com wrote:
> BugLink: https://bugs.launchpad.net/bugs/2013088
> 
> SRU Bug Template:
> 
> [ Impact ] 
> 
>  * In case clear_user() crosses two pages and faults on the second page the
>    kernel may write lowcore contents to the first page, instead of
>    clearing it.
> 
>  * The __clear_user() inline assembly misses earlyclobber constraint
>    modifiers. Depending on compiler and compiler options this may lead to
>    incorrect code which copies kernel lowcore contents to user space instead
>    of clearing memory, in case clear_user() faults.
> 
> [ Test Plan ]
> 
>  * A little test program in C is used for testing.
> 
>  * The test will be done by IBM.
> 
> [ Where problems could occur ]
> 
>  * The modification is limited to function 'long __clear_user'.
> 
>  * And there, just to one inline assembly constraints line.
> 
>  * This is usually difficult to trace.
> 
>  * A erroneous modification may lead to a wrong behavior in
>    'long __clear_user',
> 
>  * and maybe returning a wrong size (in uaccess.c).
> 
> [ Other Info ]
>  
>  * This affects all Ubuntu releases in service, down to 18.04.
> 
>  * Since we are close to 23.04 kernel freeze, I submit a patch request for
>    23.04 right now, and will submit an SRU request for the all other Ubuntu
>    releases later.
> 
> Heiko Carstens (1):
>   s390/uaccess: add missing earlyclobber annotations to __clear_user()
> 
>  arch/s390/lib/uaccess.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
Acked-by: Cory Todd <cory.todd at canonical.com>