[PATCH] powerpc/pseries: Fix UAF reference for src_info after list_add

[Haren Myneni <haren@linux.ibm.com>]

Getting the following kernel panic in papr_hvpipe_dev_create_handle()
when trying to add src_info to the list.
 Kernel attempted to write user page (0) - exploit attempt? (uid: 0)
 BUG: Kernel NULL pointer dereference on write at 0x00000000
 Faulting instruction address: 0xc0000000001b44a0
 Oops: Kernel access of bad area, sig: 11 [#1]
 ...
 Call Trace:
 papr_hvpipe_dev_ioctl+0x1f4/0x48c (unreliable)
 sys_ioctl+0x528/0x1064
 system_call_exception+0x128/0x360
 system_call_vectored_common+0x15c/0x2ec

The current code adds src_info to the list after UAF for src_info.
So move the retain_and_null_ptr(src_info) after this list add.

Fixes: 6d3789d347a7 ("papr-hvpipe: convert papr_hvpipe_dev_create_handle() to FD_PREPARE()")
Signed-off-by: Haren Myneni <haren@linux.ibm.com>
---
 arch/powerpc/platforms/pseries/papr-hvpipe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/papr-hvpipe.c b/arch/powerpc/platforms/pseries/papr-hvpipe.c
index 14ae480d060a..5121c87d1fad 100644
--- a/arch/powerpc/platforms/pseries/papr-hvpipe.c
+++ b/arch/powerpc/platforms/pseries/papr-hvpipe.c
@@ -509,7 +509,6 @@ static int papr_hvpipe_dev_create_handle(u32 srcID)
 	if (fdf.err)
 		return fdf.err;
 
-	retain_and_null_ptr(src_info);
 	spin_lock(&hvpipe_src_list_lock);
 	/*
 	 * If two processes are executing ioctl() for the same
@@ -522,6 +521,7 @@ static int papr_hvpipe_dev_create_handle(u32 srcID)
 	}
 	list_add(&src_info->list, &hvpipe_src_list);
 	spin_unlock(&hvpipe_src_list_lock);
+	retain_and_null_ptr(src_info);
 	return fd_publish(fdf);
 }
 
-- 
2.50.1