[PATCH 2/2] powerpc/tm: Avoid SPR flush if TM is disabled

[Breno Leitao]

There is a bug in the flush_tmregs_to_thread() function, where it forces
TM SPRs to be saved to the thread even if the TM facility is disabled.

This bug could be reproduced using a simple test case:

  mtspr(SPRN_TEXASR, XX);
  sleep until load_tm == 0
  cause a coredump
  read SPRN_TEXASR in the coredump

In this case, the coredump may contain an invalid SPR, because the
current code is flushing live SPRs (Used by the last thread with TM
active) into the current thread, overwriting the latest SPRs (which were
valid).

This patch checks if TM is enabled for current task before
saving the SPRs, otherwise, the TM is lazily disabled and the thread
value is already up-to-date and could be used directly, and saving is
not required.

Fixes: cd63f3cf1d5 ("powerpc/tm: Fix saving of TM SPRs in core dump")
Signed-off-by: Breno Leitao <leitao at debian.org>
---
 arch/powerpc/kernel/ptrace.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index 9667666eb18e..e0a2ee865032 100644
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -138,7 +138,12 @@ static void flush_tmregs_to_thread(struct task_struct *tsk)
 
 	if (MSR_TM_SUSPENDED(mfmsr())) {
 		tm_reclaim_current(TM_CAUSE_SIGNAL);
-	} else {
+	} else if (tm_enabled(tsk)) {
+		/*
+		 * Only flush TM SPRs to the thread if TM was enabled,
+		 * otherwise (TM lazily disabled), the thread already
+		 * contains the latest SPR value
+		 */
 		tm_enable();
 		tm_save_sprs(&(tsk->thread));
 	}
-- 
2.19.0