powerpc/64: Prevent stack protection in early boot

The previous commit reduced the amount of code that is run before we
setup a paca. However there are still a few remaining functions that
run with no paca, or worse, with an arbitrary value in r13 that will
be used as a paca pointer.

In particular the stack protector canary is stored in the paca, so if
stack protector is activated for any of these functions we will read
the stack canary from wherever r13 points. If r13 happens to point
outside of memory we will get a machine check / checkstop.

For example if we modify initialise_paca() to trigger stack
protection, and then boot in the mambo simulator with r13 poisoned in
skiboot before calling the kernel:

  DEBUG: 19952232: (19952232): INSTRUCTION: PC=0xC0000000191FC1E8: [0x3C4C006D]: addis   r2,r12,0x6D [fetch]
  DEBUG: 19952236: (19952236): INSTRUCTION: PC=0xC00000001807EAD8: [0x7D8802A6]: mflr    r12 [fetch]
  FATAL ERROR: 19952276: (19952276): Check Stop for 0:0: Machine Check with ME bit of MSR off
  DEBUG: 19952276: (19952276): INSTRUCTION: PC=0xC0000000191FCA7C: [0xE90D0CF8]: ld      r8,0xCF8(r13) [Instruction Failed]
  INFO: 19952276: (19952277): ** Execution stopped: Mambo Error, Machine Check Stop,  **
  systemsim % bt
  pc:                             0xC0000000191FCA7C      initialise_paca+0x54
  lr:                             0xC0000000191FC22C      early_setup+0x44
  stack:0x00000000198CBED0        0x0     +0x0
  stack:0x00000000198CBF00        0xC0000000191FC22C      early_setup+0x44
  stack:0x00000000198CBF90        0x1801C968      +0x1801C968

So annotate the relevant functions to ensure stack protection is never
enabled for them.

Fixes: 06ec27ae ("powerpc/64: add stack protector support")
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200320032116.1024773-2-mpe@ellerman.id.au

arch/powerpc/kernel/paca.c

@@ -176,7 +176,7 @@ static struct slb_shadow * __init new_slb_shadow(int cpu, unsigned long limit)
 struct paca_struct **paca_ptrs __read_mostly;
 EXPORT_SYMBOL(paca_ptrs);
 
-void __init initialise_paca(struct paca_struct *new_paca, int cpu)
+void __init __nostackprotector initialise_paca(struct paca_struct *new_paca, int cpu)
 {
 #ifdef CONFIG_PPC_PSERIES
 	new_paca->lppaca_ptr = NULL;
@@ -205,7 +205,7 @@ void __init initialise_paca(struct paca_struct *new_paca, int cpu)
 }
 
 /* Put the paca pointer into r13 and SPRG_PACA */
-void setup_paca(struct paca_struct *new_paca)
+void __nostackprotector setup_paca(struct paca_struct *new_paca)
 {
 	/* Setup r13 */
 	local_paca = new_paca;

arch/powerpc/kernel/setup.h

@@ -8,6 +8,12 @@
 #ifndef __ARCH_POWERPC_KERNEL_SETUP_H
 #define __ARCH_POWERPC_KERNEL_SETUP_H
 
+#ifdef CONFIG_CC_IS_CLANG
+#define __nostackprotector
+#else
+#define __nostackprotector __attribute__((__optimize__("no-stack-protector")))
+#endif
+
 void initialize_cache_info(void);
 void irqstack_early_init(void);
 

arch/powerpc/kernel/setup_64.c

@@ -279,7 +279,7 @@ void __init record_spr_defaults(void)
  * device-tree is not accessible via normal means at this point.
  */
 
-void __init early_setup(unsigned long dt_ptr)
+void __init __nostackprotector early_setup(unsigned long dt_ptr)
 {
 	static __initdata struct paca_struct boot_paca;