[PATCH v4] powerpc: Avoid signed to unsigned conversion in set_thread_tidr()

[Vaibhav Jain <vaibhav@linux.vnet.ibm.com>]

There is an unsafe signed to unsigned conversion in set_thread_tidr()
that may cause an error value to be assigned to SPRN_TIDR register and
used as thread-id.

The issue happens as assign_thread_tidr() returns an int and
thread.tidr is an unsigned-long. So a negative error code returned
from assign_thread_tidr() will fail the error check and gets assigned
as tidr as a large positive value.

To fix this the patch assigns the return value of assign_thread_tidr()
to a temporary int and assigns it to thread.tidr iff its '> 0'.

The patch shouldn't impact the calling convention of set_thread_tidr()
i.e all -ve return-values are error codes and a return value of '0'
indicates success.

Fixes: ec233ede4c86("powerpc: Add support for setting SPRN_TIDR")
Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>

---
Changelog:

v4  ->  Simplified the code flow [Sukadev]

v3  ->  Updated the patch to not impact the calling convention [Mpe, Christophe]

v2  ->	* Update the patch description to document the calling
	convention of set_thread_tidr(). [Mpe]
	* Fix a tidr allocation leak.
---
 arch/powerpc/kernel/process.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index bfdd783e3916..d205b52e3850 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1569,16 +1569,19 @@ void arch_release_task_struct(struct task_struct *t)
  */
 int set_thread_tidr(struct task_struct *t)
 {
+	int rc;
+
 	if (!cpu_has_feature(CPU_FTR_ARCH_300))
 		return -EINVAL;
 
 	if (t != current)
 		return -EINVAL;
 
-	t->thread.tidr = assign_thread_tidr();
-	if (t->thread.tidr < 0)
-		return t->thread.tidr;
+	rc = assign_thread_tidr();
+	if (rc < 0)
+		return rc;
 
+	t->thread.tidr = rc;
 	mtspr(SPRN_TIDR, t->thread.tidr);
 
 	return 0;
-- 
2.14.3