[PATCH] powerpc/hv-gpci: fix preempt count leak in sysfs show paths

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

Four sysfs show() callbacks in hv-gpci take get_cpu_var(hv_gpci_reqb)
(which calls preempt_disable()) but only call the matching put_cpu_var()
on the error path under the 'out:' label. Every successful read leaks
one preempt_disable():

  processor_bus_topology_show()
  processor_config_show()
  affinity_domain_via_virtual_processor_show()
  affinity_domain_via_domain_show()

(affinity_domain_via_partition_show() was already correct.)

On a CONFIG_PREEMPT=y kernel, repeated reads raise preempt_count and
eventually return to userspace with preemption still disabled. The
next user-mode page fault then hits faulthandler_disabled() == 1,
gets forced to SIGSEGV, and the resulting coredump trips
'BUG: scheduling while atomic' in call_usermodehelper_exec ->
wait_for_completion_state -> schedule:

  BUG: scheduling while atomic: <task>/<pid>/0x00000004
  ...
  __schedule_bug+0x6c/0x90
  __schedule+0x58c/0x13a0
  schedule+0x48/0x1a0
  schedule_timeout+0x104/0x170
  wait_for_completion_state+0x16c/0x330
  call_usermodehelper_exec+0x254/0x2d0
  vfs_coredump+0x1050/0x2590
  get_signal+0xb9c/0xc80
  do_notify_resume+0xf8/0x470

Add an out_success label that calls put_cpu_var() before returning
the byte count, mirroring affinity_domain_via_partition_show().

Fixes: 71f1c39647d8 ("powerpc/hv_gpci: Add sysfs file inside hv_gpci device to show processor bus topology information")
Fixes: 1a160c2a13c6 ("powerpc/hv_gpci: Add sysfs file inside hv_gpci device to show processor config information")
Fixes: 71a7ccb478fc ("powerpc/hv_gpci: Add sysfs file inside hv_gpci device to show affinity domain via virtual processor information")
Fixes: a69a57cac1ec ("powerpc/hv_gpci: Add sysfs file inside hv_gpci device to show affinity domain via domain information")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
 arch/powerpc/perf/hv-gpci.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/perf/hv-gpci.c b/arch/powerpc/perf/hv-gpci.c
index 5cac2cf3bd1e5..10c82cf8f5b39 100644
--- a/arch/powerpc/perf/hv-gpci.c
+++ b/arch/powerpc/perf/hv-gpci.c
@@ -210,7 +210,7 @@ static ssize_t processor_bus_topology_show(struct device *dev, struct device_att
 			0, 0, buf, &n, arg);
 
 	if (!ret)
-		return n;
+		goto out_success;
 
 	if (ret != H_PARAMETER)
 		goto out;
@@ -244,12 +244,14 @@ static ssize_t processor_bus_topology_show(struct device *dev, struct device_att
 				starting_index, 0, buf, &n, arg);
 
 		if (!ret)
-			return n;
+			goto out_success;
 
 		if (ret != H_PARAMETER)
 			goto out;
 	}
 
+out_success:
+	put_cpu_var(hv_gpci_reqb);
 	return n;
 
 out:
@@ -278,7 +280,7 @@ static ssize_t processor_config_show(struct device *dev, struct device_attribute
 			0, 0, buf, &n, arg);
 
 	if (!ret)
-		return n;
+		goto out_success;
 
 	if (ret != H_PARAMETER)
 		goto out;
@@ -312,12 +314,14 @@ static ssize_t processor_config_show(struct device *dev, struct device_attribute
 				starting_index, 0, buf, &n, arg);
 
 		if (!ret)
-			return n;
+			goto out_success;
 
 		if (ret != H_PARAMETER)
 			goto out;
 	}
 
+out_success:
+	put_cpu_var(hv_gpci_reqb);
 	return n;
 
 out:
@@ -346,7 +350,7 @@ static ssize_t affinity_domain_via_virtual_processor_show(struct device *dev,
 			0, 0, buf, &n, arg);
 
 	if (!ret)
-		return n;
+		goto out_success;
 
 	if (ret != H_PARAMETER)
 		goto out;
@@ -382,12 +386,14 @@ static ssize_t affinity_domain_via_virtual_processor_show(struct device *dev,
 				starting_index, secondary_index, buf, &n, arg);
 
 		if (!ret)
-			return n;
+			goto out_success;
 
 		if (ret != H_PARAMETER)
 			goto out;
 	}
 
+out_success:
+	put_cpu_var(hv_gpci_reqb);
 	return n;
 
 out:
@@ -416,7 +422,7 @@ static ssize_t affinity_domain_via_domain_show(struct device *dev, struct device
 			0, 0, buf, &n, arg);
 
 	if (!ret)
-		return n;
+		goto out_success;
 
 	if (ret != H_PARAMETER)
 		goto out;
@@ -448,12 +454,14 @@ static ssize_t affinity_domain_via_domain_show(struct device *dev, struct device
 					starting_index, 0, buf, &n, arg);
 
 		if (!ret)
-			return n;
+			goto out_success;
 
 		if (ret != H_PARAMETER)
 			goto out;
 	}
 
+out_success:
+	put_cpu_var(hv_gpci_reqb);
 	return n;
 
 out:
-- 
2.54.0