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From: Borislav Petkov <bp@alien8.de>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Rik van Riel <riel@surriel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 10/10] x86/fpu/xstate: Restore supervisor states for signal return
Date: Sun, 10 May 2020 10:49:17 +0200	[thread overview]
Message-ID: <20200510084917.GB19352@zn.tnic> (raw)
In-Reply-To: <20200328164307.17497-11-yu-cheng.yu@intel.com>

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@intel.com>
> Reviewed-by: Dave Hansen <dave.hansen@linux.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.

https://people.kernel.org/tglx/notes-about-netiquette

  reply	other threads:[~2020-05-10  8:49 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-28 16:42 [PATCH v3 00/10] Support XSAVES supervisor states Yu-cheng Yu
2020-03-28 16:42 ` [PATCH v3 01/10] x86/fpu/xstate: Rename validate_xstate_header() to validate_user_xstate_header() Yu-cheng Yu
2020-04-28 17:11   ` Borislav Petkov
2020-04-28 17:15     ` Yu-cheng Yu
2020-03-28 16:42 ` [PATCH v3 02/10] x86/fpu/xstate: Define new macros for supervisor and user xstates Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 03/10] x86/fpu/xstate: Separate user and supervisor xfeatures mask Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 04/10] x86/fpu/xstate: Introduce XSAVES supervisor states Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 05/10] x86/fpu/xstate: Define new functions for clearing fpregs and xstates Yu-cheng Yu
2020-04-29  9:27   ` Borislav Petkov
2020-04-29 16:09     ` Yu-cheng Yu
2020-04-29 16:06   ` Yu-cheng Yu
2020-04-29 16:39     ` Borislav Petkov
2020-04-29 17:02       ` Yu-cheng Yu
2020-04-29 17:32         ` Borislav Petkov
2020-04-29 17:42   ` Yu-cheng Yu
2020-04-29 19:10   ` Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 06/10] x86/fpu/xstate: Update sanitize_restored_xstate() for supervisor xstates Yu-cheng Yu
2020-05-07 13:11   ` Borislav Petkov
2020-05-07 15:55   ` Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 07/10] x86/fpu/xstate: Update copy_kernel_to_xregs_err() for XSAVES supervisor states Yu-cheng Yu
2020-05-07 13:28   ` Borislav Petkov
2020-05-07 15:58   ` Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 08/10] x86/fpu: Introduce copy_supervisor_to_kernel() Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 09/10] x86/fpu/xstate: Preserve supervisor states for slow path of __fpu__restore_sig() Yu-cheng Yu
2020-05-10  8:46   ` Borislav Petkov
2020-05-11 20:16   ` Yu-cheng Yu
2020-03-28 16:43 ` [PATCH v3 10/10] x86/fpu/xstate: Restore supervisor states for signal return Yu-cheng Yu
2020-05-10  8:49   ` Borislav Petkov [this message]
2020-05-11 20:20   ` Yu-cheng Yu
2020-05-11 20:27 ` [PATCH v3 00/10] Support XSAVES supervisor states Borislav Petkov

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