powerpc: Avoid signed to unsigned conversion in set_thread_tidr()

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>
Reviewed-by: Christophe Lombard clombard@linux.vnet.ibm.com
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index bfdd783e39166143e205fd2424bae692fdbe5aa8..d205b52e3850a07c42b13eef712ab37862e0adae 100644 (file)
--- 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;