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Date:   Sun, 10 May 2020 10:49:17 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 10/10] x86/fpu/xstate: Restore supervisor states for
 signal return

On Sat, Mar 28, 2020 at 09:43:07AM -0700, Yu-cheng Yu wrote:
> v3:
> - Change copy_xregs_to_kernel() to copy_supervisor_to_kernel(), which is
>   introduced in a previous patch.
> - Update commit log.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
>  arch/x86/kernel/fpu/signal.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)

Took Sebastian and me a while to grok the whole situation so pls add the
comments below to that patch when sending it in your next revision.

Thx.

---
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 4dad5afc938d..06e88d6ebb07 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -347,7 +347,19 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 		ret = copy_user_to_fpregs_zeroing(buf_fx, user_xfeatures, fx_only);
 		pagefault_enable();
 		if (!ret) {
-			/* Restore supervisor states */
+
+			/*
+			 * Restore supervisor states: previous context switch
+			 * etc has done XSAVES and saved the supervisor states
+			 * in the kernel buffer from which they can be restored
+			 * now.
+			 *
+			 * We cannot do a single XRSTORS here - which would
+			 * be nice - because the rest of the FPU registers are
+			 * being restored from a user buffer directly. The
+			 * single XRSTORS happens below, when the user buffer
+			 * has been copied to the kernel one.
+			 */
 			if (test_thread_flag(TIF_NEED_FPU_LOAD) &&
 			    xfeatures_mask_supervisor())
 				copy_kernel_to_xregs(&fpu->state.xsave,
@@ -369,15 +381,19 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 	}
 
 	/*
-	 * Supervisor states are not modified by user space input.  Save
-	 * current supervisor states first.
 	 * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is
 	 * not modified on context switch and that the xstate is considered
 	 * to be loaded again on return to userland (overriding last_cpu avoids
 	 * the optimisation).
 	 */
 	fpregs_lock();
+
 	if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
+
+		/*
+		 * Supervisor states are not modified by user space input. Save
+		 * current supervisor states first and invalidate the FPU regs.
+		 */
 		if (xfeatures_mask_supervisor())
 			copy_supervisor_to_kernel(&fpu->state.xsave);
 		set_thread_flag(TIF_NEED_FPU_LOAD);
@@ -405,6 +421,7 @@ 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);
+
 		/*
 		 * Restore previously saved supervisor xstates along with
 		 * copied-in user xstates.


-- 
Regards/Gruss,
    Boris.

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