linux-kernel - [PATCH] x86/fpu: Deacticate FPU state after failure during state load

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Date:   Fri, 20 Dec 2019 20:59:06 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.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>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] x86/fpu: Deacticate FPU state after failure during state load

In __fpu__restore_sig() we need to reset `fpu_fpregs_owner_ctx' if the
FPU state was not fully restored. Otherwise the following may happen
(on the same CPU):

Task A                     Task B               fpu_fpregs_owner_ctx
*active*                                        A.fpu
__fpu__restore_sig()
                           ctx switch           load B.fpu
                           *active*             B.fpu
fpregs_lock()
copy_user_to_fpregs_zeroing()
  copy_kernel_to_xregs() *modify*
  copy_user_to_xregs() *fails*
fpregs_unlock()
                          ctx switch            skip loading B.fpu,
                          *active*              B.fpu

In the succeess case we set `fpu_fpregs_owner_ctx' to the current task.
In the failure case we *might* have modified the FPU state because we
loaded the init state. In this case we need to reset
`fpu_fpregs_owner_ctx' to ensure that the FPU state of the following
task is loaded from saved state (and not skip because it was the
previous state).

Reset `fpu_fpregs_owner_ctx' after a failure during restore occured to
ensure that the FPU state for the next task is always loaded.

Fixes: 5f409e20b7945 ("x86/fpu: Defer FPU state load until return to userspace")
Reported-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Debugged-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/kernel/fpu/signal.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 0071b794ed193..400a05e1c1c51 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -352,6 +352,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 			fpregs_unlock();
 			return 0;
 		}
+		fpregs_deactivate(fpu);
 		fpregs_unlock();
 	}
 
@@ -403,6 +404,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 	}
 	if (!ret)
 		fpregs_mark_activate();
+	else
+		fpregs_deactivate(fpu);
 	fpregs_unlock();
 
 err_out:
-- 
2.24.1