Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig()

Borislav Petkov
2020-02-21T17:59:06Z

On Tue, Jan 21, 2020 at 12:18:43PM -0800, Yu-cheng Yu wrote:
> When returning from a signal, there is user state in a userspace memory
> buffer and supervisor state in registers. The in-kernel buffer has neither
> valid user or supervisor state. To restore both, save supervisor fpregs

The correct formulation is "neither ... nor ... "

> first (and protect them across context switches), then restore it along

s/it/them/

> with user state.
>
> This patch introduces saving and restoring of supervisor xstates for a

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> sigreturn in the following steps:
>
> - Preserve supervisor register values by saving the whole fpu xstates.
> Saving the whole is necessary because doing XSAVES with a partial
> requested-feature bitmap (RFBM) changes xcomp_bv. Since user xstates are
> to be restored from a user buffer, saved user xstates are discarded.
>
> - Set TIF_NEED_FPU_LOAD, and do __fpu_invalidate_fpregs_state().
> This prevents a context switch from corrupting the saved xstates,
> and xstate is considered to be loaded again on return to userland.

s/considered/going to/

> - Under fpregs_lock(), restore user xstates (from the user buffer), and
> then supervisor xstates (from previously saved content).
>
> - When both parts of the restoration succeed, mark fpregs as valid.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
> ---
> arch/x86/kernel/fpu/signal.c | 26 +++++++++++++++++++++-----
> 1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index e3781a4a52a8..0d3e06a772b0 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -327,14 +327,22 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> }
>
> /*
> - * The current state of the FPU registers does not matter. By setting
> - * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate
> - * is not modified on context switch and that the xstate is considered
> + * Supervisor xstates are not modified by user space input, and
> + * need to be saved and restored. Save the whole because doing
> + * partial XSAVES changes xcomp_bv.
> + * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is
> + * not modified on context switch and that the xstate is considered

Reflow those comments to 80 cols. There's room to the right.

> * to be loaded again on return to userland (overriding last_cpu avoids
> * the optimisation).
> */
> - set_thread_flag(TIF_NEED_FPU_LOAD);
> + fpregs_lock();
> + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> + if (xfeatures_mask_supervisor())
> + copy_xregs_to_kernel(&fpu->state.xsave);
> + set_thread_flag(TIF_NEED_FPU_LOAD);

So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
this?

Why don't you simply do:

set_thread_flag(TIF_NEED_FPU_LOAD);
fpregs_lock();
if (xfeatures_mask_supervisor())
copy_xregs_to_kernel(&fpu->state.xsave);
fpregs_unlock();

> + }
> __fpu_invalidate_fpregs_state(fpu);
> + fpregs_unlock();
>
> if ((unsigned long)buf_fx % 64)
> fx_only = 1;
> @@ -360,6 +368,9 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures_user, fx_only);
> pagefault_enable();

<--- comment here why you're doing this. That function is an abomination
and needs commenting at least.

> if (!ret) {
> + if (xfeatures_mask_supervisor())
> + copy_kernel_to_xregs(&fpu->state.xsave,
> + xfeatures_mask_supervisor());
> fpregs_mark_activate();
> fpregs_unlock();
> return 0;
> @@ -389,7 +400,12 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> fpregs_lock();
> if (unlikely(init_bv))
> copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> - ret = copy_kernel_to_xregs_err(&fpu->state.xsave, xfeatures_user);
> + /*
> + * Restore previously saved supervisor xstates along with
> + * copied-in user xstates.
> + */
> + ret = copy_kernel_to_xregs_err(&fpu->state.xsave,
> + xfeatures_user | xfeatures_mask_supervisor());
>
> } else if (use_fxsr()) {
> ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size);
> --
> 2.21.0
>

--
Regards/Gruss,
Boris.