Re: [PATCH v5] x86/fsgsbase/64: Fix the base write helper functions

[Andy Lutomirski]

On Fri, Nov 16, 2018 at 3:27 PM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>
> The helper functions that purport to write the base should just write it
> only. It shouldn't have magic optimizations to change the index.
>
> Make the index explicitly changed from the caller, instead of including
> the code in the helpers.
>
> Subsequently, the task write helpers do not handle for the current task
> anymore. The range check for a base value is also factored out, to
> minimize code redundancy from the caller.
>
> v2: Fix further on the task write functions. Revert the changes on the
> task read helpers.
>
> v3: Fix putreg(). Edit the changelog.
>
> v4: Update the task write helper functions and do_arch_prctl_64(). Fix
> the comment in putreg().
>
> v5: Fix preempt_disable() calls in do_arch_prctl_64()

Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>

Ingo, Thomas: can we get this in x86/urgent, please?


> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index ffae9b9740fd..4b8ee05dd6ad 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -397,11 +397,12 @@ static int putreg(struct task_struct *child,
>                 if (value >= TASK_SIZE_MAX)
>                         return -EIO;
>                 /*
> -                * When changing the FS base, use the same
> -                * mechanism as for do_arch_prctl_64().
> +                * When changing the FS base, use do_arch_prctl_64()
> +                * to set the index to zero and to set the base
> +                * as requested.
>                  */
>                 if (child->thread.fsbase != value)
> -                       return x86_fsbase_write_task(child, value);
> +                       return do_arch_prctl_64(child, ARCH_SET_FS, value);

FWIW, this logic is and was nonsensical, but it matches historical
behavior, so I guess it's okay.  I suspect that gdb only works by
luck, since fs_base has a *higher* index than fs (and same for gs),
which means that SETREGS with a nonzero fs or gs likely only works
because the target almost always already has fs_base or gs_base == 0,
so we bypass this entire mess.

Sigh.  When you resubmit the full FSGSBASE series, I'll review the new
code extra carefully.