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

[Tim Gardner]

On 3/28/23 11:58 AM, 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: Tim Gardner <tim.gardner at canonical.com>
-- 
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Tim Gardner
Canonical, Inc