[PATCH v2] powerpc: setup_64: set up PACA earlier to avoid kcov problems

[Daniel Axtens]

kcov instrumentation is collected the __sanitizer_cov_trace_pc hook in
kernel/kcov.c. The compiler inserts these hooks into every basic block
unless kcov is disabled for that file.

We then have a deep call-chain:
 - __sanitizer_cov_trace_pc calls to check_kcov_mode()
 - check_kcov_mode() (kernel/kcov.c) calls in_task()
 - in_task() (include/linux/preempt.h) calls preempt_count().
 - preempt_count() (include/asm-generic/preempt.h) calls
     current_thread_info()
 - because powerpc has THREAD_INFO_IN_TASK, current_thread_info()
     (include/linux/thread_info.h) is defined to 'current'
 - current (arch/powerpc/include/asm/current.h) is defined to
     get_current().
 - get_current (same file) loads an offset of r13.
 - arch/powerpc/include/asm/paca.h makes r13 a register variable
     called local_paca - it is the PACA for the current CPU, so
     this has the effect of loading the current task from PACA.
 - get_current returns the current task from PACA,
 - current_thread_info returns the task cast to a thread_info
 - preempt_count dereferences the thread_info to load preempt_count
 - that value is used by in_task and so on up the chain

The problem is:

 - kcov instrumentation is enabled for arch/powerpc/kernel/dt_cpu_ftrs.c

 - even if it were not, dt_cpu_ftrs_init calls generic dt parsing code
   which should definitely have instrumentation enabled.

 - setup_64.c calls dt_cpu_ftrs_init before it sets up a PACA.

 - If we don't set up a paca, r13 will contain unpredictable data.

 - In a zImage compiled with kcov and KASAN, we see r13 containing a value
   that leads to dereferencing invalid memory (something like
   912a72603d420015).

 - Weirdly, the same kernel as a vmlinux loaded directly by qemu does not
   crash. Investigating with gdb, it seems that in the vmlinux boot case,
   r13 is near enough to zero that we just happen to be able to read that
   part of memory (we're operating with translation off at this point) and
   the current pointer also happens to land in readable memory and
   everything just works.

 - PACA setup refers to CPU features - setup_paca() looks at
   early_cpu_has_feature(CPU_FTR_HVMODE)

There's no generic kill switch for kcov (as far as I can tell), and we
don't want to have to turn off instrumentation in the generic dt parsing
code (which lives outside arch/powerpc/) just because we don't have a real
paca or task yet.

So:
 - change the test when setting up a PACA to consider the actual value of
   the MSR rather than the CPU feature.

 - move the PACA setup to before the cpu feature parsing.

Translations get switched on once we leave early_setup, so I think we'd
already catch any other cases where the PACA or task aren't set up.

Boot tested on a P9 guest and host.

Fixes: fb0b0a73b223 ("powerpc: Enable kcov")
Cc: Andrew Donnellan <ajd at linux.ibm.com>
Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Daniel Axtens <dja at axtens.net>

---

Regarding moving the comment about printk()-safety:
I am about 75% sure that the thing that makes printk() safe is the PACA,
not the CPU features. That's what commit 24d9649574fb ("[POWERPC] Document
when printk is useable") seems to indicate, but as someone wise recently
told me, "bootstrapping is hard", so I may be totally wrong.
---
 arch/powerpc/kernel/paca.c     |  2 +-
 arch/powerpc/kernel/setup_64.c | 17 ++++++++++++-----
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index 949eceb254d8..347e947b9d4b 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -218,7 +218,7 @@ void setup_paca(struct paca_struct *new_paca)
 	 * if we do a GET_PACA() before the feature fixups have been
 	 * applied
 	 */
-	if (early_cpu_has_feature(CPU_FTR_HVMODE))
+	if (mfmsr() & MSR_HV)
 		mtspr(SPRN_SPRG_HPACA, local_paca);
 #endif
 	mtspr(SPRN_SPRG_PACA, local_paca);
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index e05e6dd67ae6..ef455da7efa3 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -285,18 +285,25 @@ void __init early_setup(unsigned long dt_ptr)
 
 	/* -------- printk is _NOT_ safe to use here ! ------- */
 
-	/* Try new device tree based feature discovery ... */
-	if (!dt_cpu_ftrs_init(__va(dt_ptr)))
-		/* Otherwise use the old style CPU table */
-		identify_cpu(0, mfspr(SPRN_PVR));
+	/* Assume we're on cpu 0 for now. Don't write to the paca yet!
+	 *
+	 * We need to load a PACA very early if we are using kcov. kcov will
+	 * call in_task() in its instrumentation, which relies on the current
+	 * task from the PACA. dt_cpu_ftrs_init is coveraged-enabled and also
+	 * calls into the coverage-enabled generic dt library.
+	 */
 
-	/* Assume we're on cpu 0 for now. Don't write to the paca yet! */
 	initialise_paca(&boot_paca, 0);
 	setup_paca(&boot_paca);
 	fixup_boot_paca();
 
 	/* -------- printk is now safe to use ------- */
 
+	/* Try new device tree based feature discovery ... */
+	if (!dt_cpu_ftrs_init(__va(dt_ptr)))
+		/* Otherwise use the old style CPU table */
+		identify_cpu(0, mfspr(SPRN_PVR));
+
 	/* Enable early debugging if any specified (see udbg.h) */
 	udbg_early_init();
 
-- 
2.20.1