linux-kernel - [tip: x86/fpu] x86/fpu: Emulate XRSTOR's behavior if the xfeatures PKRU bit is not set

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Message-ID: <166864162920.4906.12352601470149595048.tip-bot2@tip-bot2>
Date:   Wed, 16 Nov 2022 23:33:49 -0000
From:   "tip-bot2 for Kyle Huey" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Kyle Huey <me@...ehuey.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/fpu] x86/fpu: Emulate XRSTOR's behavior if the xfeatures
 PKRU bit is not set

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     d7e5aceace514a2b1b3ca3dc44f93f1704766ca7
Gitweb:        https://git.kernel.org/tip/d7e5aceace514a2b1b3ca3dc44f93f1704766ca7
Author:        Kyle Huey <me@...ehuey.com>
AuthorDate:    Tue, 15 Nov 2022 15:09:31 -08:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Wed, 16 Nov 2022 15:06:34 -08:00

x86/fpu: Emulate XRSTOR's behavior if the xfeatures PKRU bit is not set

The hardware XRSTOR instruction resets the PKRU register to its hardware
init value (namely 0) if the PKRU bit is not set in the xfeatures mask.
Emulating that here restores the pre-5.14 behavior for PTRACE_SET_REGSET
with NT_X86_XSTATE, and makes sigreturn (which still uses XRSTOR) and
ptrace behave identically. KVM has never used XRSTOR and never had this
behavior, so KVM opts-out of this emulation by passing a NULL pkru pointer
to copy_uabi_to_xstate().

Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")
Signed-off-by: Kyle Huey <me@...ehuey.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Link: https://lore.kernel.org/all/20221115230932.7126-6-khuey%40kylehuey.com
---
 arch/x86/kernel/fpu/core.c   |  8 ++++++++
 arch/x86/kernel/fpu/xstate.c | 15 ++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 46b935b..8d0f601 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -404,6 +404,14 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
 	if (ustate->xsave.header.xfeatures & ~xcr0)
 		return -EINVAL;
 
+	/*
+	 * Nullify @vpkru to preserve its current value if PKRU's bit isn't set
+	 * in the header.  KVM's odd ABI is to leave PKRU untouched in this
+	 * case (all other components are eventually re-initialized).
+	 */
+	if (!(ustate->xsave.header.xfeatures & XFEATURE_MASK_PKRU))
+		vpkru = NULL;
+
 	return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru);
 }
 EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a8cf604..714166c 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1219,8 +1219,14 @@ static int copy_from_buffer(void *dst, unsigned int offset, unsigned int size,
  *	it is harmless.
  * 2.	When called from ptrace the PKRU register will be restored from the
  *	thread_struct's pkru field. A pointer to that is passed in @pkru.
+ *	The kernel will restore it manually, so the XRSTOR behavior that resets
+ *	the PKRU register to the hardware init value (0) if the corresponding
+ *	xfeatures bit is not set is emulated here.
  * 3.	When called from KVM the PKRU register will be restored from the vcpu's
- *	pkru field. A pointer to that is passed in @pkru.
+ *	pkru field. A pointer to that is passed in @pkru. KVM hasn't used
+ *	XRSTOR and hasn't had the PKRU resetting behavior described above. To
+ *	preserve that KVM behavior, it passes NULL for @pkru if the xfeatures
+ *	bit is not set.
  */
 static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
 			       const void __user *ubuf, u32 *pkru)
@@ -1277,6 +1283,13 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
 
 		xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
 		*pkru = xpkru->pkru;
+	} else {
+		/*
+		 * KVM may pass NULL here to indicate that it does not need
+		 * PKRU updated.
+		 */
+		if (pkru)
+			*pkru = 0;
 	}
 
 	/*