[RFC v1 1/6] pseries/papr-hvpipe: Fix null ptr deref in papr_hvpipe_dev_create_handle

["Ritesh Harjani (IBM)" <ritesh.list@gmail.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 error handling with FD_PREPARE's file cleanup and __free(kfree) auto
cleanup is getting too convoluted. This is mainly because we need to
ensure only 1 user get the srcID handle. To simplify this, we allocate
prepare the src_info in the beginning and add it to the global list
under a spinlock after checking that no duplicates exist.

This simplify the error handling where if the FD_ADD fails, we can
simply remove the src_info from the list.

Cc: Christian Brauner <brauner@kernel.org>
Fixes: 6d3789d347a7 ("papr-hvpipe: convert papr_hvpipe_dev_create_handle() to FD_PREPARE()")
Reported-by: Haren Myneni <haren@linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 arch/powerpc/platforms/pseries/papr-hvpipe.c | 50 +++++++++-----------
 1 file changed, 22 insertions(+), 28 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/papr-hvpipe.c b/arch/powerpc/platforms/pseries/papr-hvpipe.c
index 14ae480d060a..ef10f5a5a4fa 100644
--- a/arch/powerpc/platforms/pseries/papr-hvpipe.c
+++ b/arch/powerpc/platforms/pseries/papr-hvpipe.c
@@ -479,21 +479,8 @@ static const struct file_operations papr_hvpipe_handle_ops = {

 static int papr_hvpipe_dev_create_handle(u32 srcID)
 {
-	struct hvpipe_source_info *src_info __free(kfree) = NULL;
-
-	spin_lock(&hvpipe_src_list_lock);
-	/*
-	 * Do not allow more than one process communicates with
-	 * each source.
-	 */
-	src_info = hvpipe_find_source(srcID);
-	if (src_info) {
-		spin_unlock(&hvpipe_src_list_lock);
-		pr_err("pid(%d) is already using the source(%d)\n",
-				src_info->tsk->pid, srcID);
-		return -EALREADY;
-	}
-	spin_unlock(&hvpipe_src_list_lock);
+	struct hvpipe_source_info *src_info;
+	int fd;

 	src_info = kzalloc_obj(*src_info, GFP_KERNEL_ACCOUNT);
 	if (!src_info)
@@ -503,26 +490,33 @@ static int papr_hvpipe_dev_create_handle(u32 srcID)
 	src_info->tsk = current;
 	init_waitqueue_head(&src_info->recv_wqh);

-	FD_PREPARE(fdf, O_RDONLY | O_CLOEXEC,
-		   anon_inode_getfile("[papr-hvpipe]", &papr_hvpipe_handle_ops,
-				      (void *)src_info, O_RDWR));
-	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
-	 * source ID concurrently, prevent the second process to
-	 * acquire FD.
+	 * Do not allow more than one process communicates with
+	 * each source.
 	 */
-	if (hvpipe_find_source(srcID)) {
+	spin_lock(&hvpipe_src_list_lock);
+	if(hvpipe_find_source(srcID)) {
 		spin_unlock(&hvpipe_src_list_lock);
+		pr_err("pid(%d) could not get the source(%d)\n",
+				src_info->tsk->pid, srcID);
+		kfree(src_info);
 		return -EALREADY;
 	}
 	list_add(&src_info->list, &hvpipe_src_list);
 	spin_unlock(&hvpipe_src_list_lock);
-	return fd_publish(fdf);
+
+	fd = FD_ADD(O_RDONLY | O_CLOEXEC,
+		   anon_inode_getfile("[papr-hvpipe]", &papr_hvpipe_handle_ops,
+				      (void *)src_info, O_RDWR));
+	if (fd < 0) {
+		spin_lock(&hvpipe_src_list_lock);
+		list_del(&src_info->list);
+		spin_unlock(&hvpipe_src_list_lock);
+		kfree(src_info);
+		return fd;
+	}
+
+	return fd;
 }

 /*
--
2.39.5