linux-kernel - [PATCH] x86/fpu: Fix invalid FPU ptrace state after execve

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Date:   Wed, 16 Nov 2016 08:56:36 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        David Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>
Subject: [PATCH] x86/fpu: Fix invalid FPU ptrace state after execve

Robert O'Callahan reported that after an execve PTRACE_GETREGSET
NT_X86_XSTATE continues to return the pre-exec register values
until the exec'ed task modifies FPU state.  The test code is at
https://bugzilla.redhat.com/attachment.cgi?id=1164286.

What is happening is when eagerfpu is enabled, fpu__clear() did
not properly clear fpstate.  Fix it by doing just that.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Reported-by: Robert O'Callahan <robert@...llahan.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...e.de>
Cc: David Hansen <dave.hansen@...ux.intel.com>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: "Ravi V. Shankar" <ravi.v.shankar@...el.com>
---
 arch/x86/kernel/fpu/core.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 4700401..4c203c4 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -521,14 +521,14 @@ void fpu__clear(struct fpu *fpu)
 {
 	WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
 
-	if (!use_eager_fpu() || !static_cpu_has(X86_FEATURE_FPU)) {
-		/* FPU state will be reallocated lazily at the first use. */
-		fpu__drop(fpu);
-	} else {
-		if (!fpu->fpstate_active) {
-			fpu__activate_curr(fpu);
-			user_fpu_begin();
-		}
+	fpu__drop(fpu);
+
+	/*
+	 * When eagerfpu is used, make sure fpstate is cleared and initialized.
+	 */
+	if (use_eager_fpu()) {
+		fpu__activate_curr(fpu);
+		user_fpu_begin();
 		copy_init_fpstate_to_fpregs();
 	}
 }
-- 
1.9.1