Re: [Xen-devel] [PATCH v3 1/3] x86/ldt: Make modify_ldt synchronous

[Borislav Petkov <bp@xxxxxxxxx>]

On Wed, Jul 22, 2015 at 12:23:46PM -0700, Andy Lutomirski wrote:
> modify_ldt has questionable locking and does not synchronize
> threads.  Improve it: redesign the locking and synchronize all
> threads' LDTs using an IPI on all modifications.
> 
> This will dramatically slow down modify_ldt in multithreaded
> programs, but there shouldn't be any multithreaded programs that
> care about modify_ldt's performance in the first place.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>

...

> +struct ldt_struct {
> +     /*
> +      * Xen requires page-aligned LDTs with special permissions.  This is
> +      * needed to prevent us from installing evil descriptors such as
> +      * call gates.  On native, we could merge the ldt_struct and LDT
> +      * allocations, but it's not worth trying to optimize.

I don't think baremetal should care about xen and frankly, this is
getting ridiculous, slowly - baremetal has to wait with a potentially
critical security fix just because it breaks xen. Dammit, this level of
intrusiveness into x86 should've never been allowed.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--

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