linux-kernel - [RFC][PATCH 2/8] x86/fpu: hook up PKRU into signal user ABIs

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Message-Id: <20210622222459.9C76B034@viggo.jf.intel.com>
Date:   Tue, 22 Jun 2021 15:24:59 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, luto@...nel.org
Subject: [RFC][PATCH 2/8] x86/fpu: hook up PKRU into signal user ABIs


From: Dave Hansen <dave.hansen@...ux.intel.com>

One nice thing about having PKRU be XSAVE-managed is that it gets naturally
exposed into the XSAVE-using ABIs.  Now that XSAVE will not be used to
manage PKRU, these ABIs need to be manually enabled to deal with PKRU.

For signals (the restore_hwregs_from_user() path), it's quite
straightforward.  restore_hwregs_from_user() will update PKRU in from
the user signal buffer.  Ensure that PKRU is shuffled into the thread
storage.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org
Cc: Andy Lutomirski <luto@...nel.org>
---

 b/arch/x86/kernel/fpu/signal.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff -puN arch/x86/kernel/fpu/signal.c~fpu-sig-stash-pkru arch/x86/kernel/fpu/signal.c
--- a/arch/x86/kernel/fpu/signal.c~fpu-sig-stash-pkru	2021-06-22 14:49:07.899051760 -0700
+++ b/arch/x86/kernel/fpu/signal.c	2021-06-22 14:49:07.903051760 -0700
@@ -233,6 +233,13 @@ static int restore_hwregs_from_user(void
 
 		if (!ret && unlikely(init_bv))
 			os_xrstor(&init_fpstate.xsave, init_bv);
+
+		/*
+		 * PKRU may have been modified by XRSTOR,
+		 * save the possibly updated value:
+		 */
+		current->thread.pkru = read_pkru();
+
 		return ret;
 	} else if (use_fxsr()) {
 		return fxrstor_from_user_sigframe(buf);
_