arm64: sdei: Run the event handler on the kernel vectors

With KPTI, VBAR_EL1 holds the trampoline vectors from the return to EL0
until the next exception from EL0 maps the kernel and installs the
kernel vectors. The trampoline vectors only handle exceptions from EL0;
their current-EL half is zero-filled.

An SDEI event is dispatched regardless of PSTATE.DAIF, so it lands there
whenever the CPU is in user space. The SDEI entry trampoline maps the
kernel but leaves VBAR_EL1 alone. An exception taken by the handler then
executes the zeros, takes an undefined instruction exception to the same
vector, and loops there forever. The event never completes and the CPU
is lost; RCU stall reports are the first sign of it.

RAS notifications land in user space whenever a task consumes the
error, and NMI backtraces land wherever the CPU happens to be. The GHES
handler takes no exception on its own, but any exception in the handler
path does it: a WARN, a kprobe hit, or a fault fixed up by a _nofault()
accessor.

Install the kernel vectors in __sdei_handler() when VBAR_EL1 holds the
trampoline vectors, and restore it before returning to firmware. The
switch covers irqentry_nmi_enter() and irqentry_nmi_exit() too, as Usama
Arif pointed out: their tracepoints and context tracking WARNs can take
an exception as well.

The IRQ-like resume into user space must still go through the
trampoline to map the kernel. do_sdei_event() takes the interrupted
VBAR_EL1 and keeps basing the resume address on it.

Fixes: 79e9aa59dc ("arm64: sdei: Add trampoline code for remapping the kernel")
Cc: stable@vger.kernel.org
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Breno Leitao <leitao@debian.org>
[catalin.marinas@arm.com: use raw_cpu_read() for this_cpu_vector]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/sdei.h

@@ -44,7 +44,8 @@ asmlinkage unsigned long __sdei_handler(struct pt_regs *regs,
 					struct sdei_registered_event *arg);
 
 unsigned long do_sdei_event(struct pt_regs *regs,
-			    struct sdei_registered_event *arg);
+			    struct sdei_registered_event *arg,
+			    unsigned long vbar);
 
 unsigned long sdei_arch_get_entry_point(int conduit);
 #define sdei_arch_get_entry_point(x)	sdei_arch_get_entry_point(x)

arch/arm64/kernel/entry-common.c

@@ -31,6 +31,7 @@
 #include <asm/stacktrace.h>
 #include <asm/sysreg.h>
 #include <asm/system_misc.h>
+#include <asm/vectors.h>
 
 /*
  * Where the context being returned to had an active percpu GPR critical
@@ -1040,6 +1041,7 @@ asmlinkage void noinstr __noreturn handle_bad_stack(struct pt_regs *regs)
 asmlinkage noinstr unsigned long
 __sdei_handler(struct pt_regs *regs, struct sdei_registered_event *arg)
 {
+	unsigned long vbar;
 	irqentry_state_t state;
 	unsigned long ret;
 
@@ -1065,10 +1067,27 @@ __sdei_handler(struct pt_regs *regs, struct sdei_registered_event *arg)
 	else if (cpu_has_pan())
 		set_pstate_pan(0);
 
+	/*
+	 * With KPTI, VBAR_EL1 holds the trampoline vectors from the return to
+	 * EL0 until the next exception from EL0 maps the kernel. They only
+	 * handle exceptions from EL0: an exception taken here would run their
+	 * empty current-EL half and never come back. Switch to the kernel
+	 * vectors. The IRQ-like resume keeps using the interrupted VBAR, so it
+	 * still goes through the trampoline.
+	 */
+	vbar = read_sysreg(vbar_el1);
+	if (arm64_kernel_unmapped_at_el0() &&
+	    vbar == (unsigned long)raw_cpu_read(this_cpu_vector)) {
+		write_sysreg((unsigned long)vectors, vbar_el1);
+		isb();
+	}
+
 	state = irqentry_nmi_enter(regs);
-	ret = do_sdei_event(regs, arg);
+	ret = do_sdei_event(regs, arg, vbar);
 	irqentry_nmi_exit(regs, state);
 
+	write_sysreg(vbar, vbar_el1);
+
 	return ret;
 }
 #endif /* CONFIG_ARM_SDE_INTERFACE */

arch/arm64/kernel/sdei.c

@@ -200,16 +200,18 @@ unsigned long sdei_arch_get_entry_point(int conduit)
  *  SDEI_EV_HANDLED -  success, return to the interrupted context.
  *  SDEI_EV_FAILED  -  failure, return this error code to firmware.
  *  virtual-address -  success, return to this address.
+ *
+ * vbar is the interrupted context's VBAR_EL1, for the IRQ-like resume.
  */
 unsigned long __kprobes do_sdei_event(struct pt_regs *regs,
-				      struct sdei_registered_event *arg)
+				      struct sdei_registered_event *arg,
+				      unsigned long vbar)
 {
 	u32 mode;
 	int i, err = 0;
 	int clobbered_registers = 4;
 	u64 elr = read_sysreg(elr_el1);
 	u32 kernel_mode = read_sysreg(CurrentEL) | 1;	/* +SPSel */
-	unsigned long vbar = read_sysreg(vbar_el1);
 
 	if (arm64_kernel_unmapped_at_el0())
 		clobbered_registers++;