[PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache

["Xin Li (Intel)" <xin@zytor.com>]

The FRED RSP0 MSR (pointing to the top of the kernel stack for user
level event delivery) and its per CPU cache should be kept in sync to
avoid redundant writes in the exit to user space path, as a result,
a write to the FRED RSP0 MSR is paired with a write to its per CPU
cache as fred_update_rsp0() does.

However as the FRED RSP0 MSR is set to 0 in cpu_init_fred_exceptions(),
it gets out of sync with its per CPU cache during a CPU offline/online
cycle, which causes #DF exceptions if no context switch happens after
a CPU offline/online cycle and before exit to user space.

Fix the bug through resynchronizing the FRED RSP0 MSR with its per CPU
cache value in cpu_init_fred_exceptions().

Fixes: fe85ee391966 ("x86/entry: Set FRED RSP0 on return to userspace instead of context switch")
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Cc: stable@vger.kernel.org
---
 arch/x86/kernel/fred.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fred.c b/arch/x86/kernel/fred.c
index 8d32c3f48abc..9524ace96bfa 100644
--- a/arch/x86/kernel/fred.c
+++ b/arch/x86/kernel/fred.c
@@ -50,7 +50,18 @@ void cpu_init_fred_exceptions(void)
 	       FRED_CONFIG_ENTRYPOINT(asm_fred_entrypoint_user));
 
 	wrmsrl(MSR_IA32_FRED_STKLVLS, 0);
-	wrmsrl(MSR_IA32_FRED_RSP0, 0);
+
+	/*
+	 * Resynchronize the FRED RSP0 MSR with its per CPU cache value.
+	 *
+	 * Another option is to leave the FRED RSP0 MSR as-is, because the RESET
+	 * state of FRED MSRs is zero and INIT does not change the value of the
+	 * FRED MSRs in a CPU offline/online cycle.  But it doesn't seem safe to
+	 * depend on the properties of INIT as that's way too many things that
+	 * could cause bugs.
+	 */
+	wrmsrl(MSR_IA32_FRED_RSP0, __this_cpu_read(fred_rsp0));
+
 	wrmsrl(MSR_IA32_FRED_RSP1, 0);
 	wrmsrl(MSR_IA32_FRED_RSP2, 0);
 	wrmsrl(MSR_IA32_FRED_RSP3, 0);

base-commit: cf6b067860f69fe01b08ebae9138fe0f89854398
-- 
2.47.1