[PATCH 6.6 302/396] x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

Greg Kroah-Hartman
2024-04-01T16:53:19Z

From: Adamos Ttofari <attofari@amazon.de>

[ Upstream commit 10e4b5166df9ff7a2d5316138ca668b42d004422 ]

Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and
commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a
per CPU variable xfd_state to keep the MSR_IA32_XFD value cached, in
order to avoid unnecessary writes to the MSR.

On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which
wipes out any stale state. But the per CPU cached xfd value is not
reset, which brings them out of sync.

As a consequence a subsequent xfd_update_state() might fail to update
the MSR which in turn can result in XRSTOR raising a #NM in kernel
space, which crashes the kernel.

To fix this, introduce xfd_set_state() to write xfd_state together
with MSR_IA32_XFD, and use it in all places that set MSR_IA32_XFD.

Fixes: 672365477ae8 ("x86/fpu: Update XFD state where required")
Signed-off-by: Adamos Ttofari <attofari@amazon.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240322230439.456571-1-chang.seok.bae@intel.com

Closes: https://lore.kernel.org/lkml/20230511152818.13839-1-attofari@amazon.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kernel/fpu/xstate.c | 5 +++--
arch/x86/kernel/fpu/xstate.h | 14 ++++++++++----
2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index ef6906107c541..571a43b3105df 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -178,10 +178,11 @@ void fpu__init_cpu_xstate(void)
* Must happen after CR4 setup and before xsetbv() to allow KVM
* lazy passthrough. Write independent of the dynamic state static
* key as that does not work on the boot CPU. This also ensures
- * that any stale state is wiped out from XFD.
+ * that any stale state is wiped out from XFD. Reset the per CPU
+ * xfd cache too.
*/
if (cpu_feature_enabled(X86_FEATURE_XFD))
- wrmsrl(MSR_IA32_XFD, init_fpstate.xfd);
+ xfd_set_state(init_fpstate.xfd);

/*
* XCR_XFEATURE_ENABLED_MASK (aka. XCR0) sets user features
diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 3518fb26d06b0..19ca623ffa2ac 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -148,20 +148,26 @@ static inline void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rs
#endif

#ifdef CONFIG_X86_64
+static inline void xfd_set_state(u64 xfd)
+{
+ wrmsrl(MSR_IA32_XFD, xfd);
+ __this_cpu_write(xfd_state, xfd);
+}
+
static inline void xfd_update_state(struct fpstate *fpstate)
{
if (fpu_state_size_dynamic()) {
u64 xfd = fpstate->xfd;

- if (__this_cpu_read(xfd_state) != xfd) {
- wrmsrl(MSR_IA32_XFD, xfd);
- __this_cpu_write(xfd_state, xfd);
- }
+ if (__this_cpu_read(xfd_state) != xfd)
+ xfd_set_state(xfd);
}
}

extern int __xfd_enable_feature(u64 which, struct fpu_guest *guest_fpu);
#else
+static inline void xfd_set_state(u64 xfd) { }
+
static inline void xfd_update_state(struct fpstate *fpstate) { }

static inline int __xfd_enable_feature(u64 which, struct fpu_guest *guest_fpu) {
--
2.43.0