[PATCH v1 03/11] fs/proc/vmcore: disallow vmcore modifications after the vmcore was opened

[David Hildenbrand <david@redhat.com>]

Let's protect all vmcore modifications by the vmcore_mutex and
disallow vmcore modifications after the vmcore was opened: modifications
would no longer be safe. Properly synchronize against concurrent opening
of the vmcore.

As a nice side-effect, we now properly protect concurrent vmcore
modifications.

No need to grab the mutex during mmap()/read(): after we opened the
vmcore, modifications are impossible.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 fs/proc/vmcore.c | 42 +++++++++++++++++++-----------------------
 1 file changed, 19 insertions(+), 23 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index b91c304463c9..6371dbaa21be 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -243,33 +243,27 @@ static int vmcoredd_copy_dumps(struct iov_iter *iter, u64 start, size_t size)
 {
 	struct vmcoredd_node *dump;
 	u64 offset = 0;
-	int ret = 0;
 	size_t tsz;
 	char *buf;
 
-	mutex_lock(&vmcore_mutex);
 	list_for_each_entry(dump, &vmcoredd_list, list) {
 		if (start < offset + dump->size) {
 			tsz = min(offset + (u64)dump->size - start, (u64)size);
 			buf = dump->buf + start - offset;
-			if (copy_to_iter(buf, tsz, iter) < tsz) {
-				ret = -EFAULT;
-				goto out_unlock;
-			}
+			if (copy_to_iter(buf, tsz, iter) < tsz)
+				return -EFAULT;
 
 			size -= tsz;
 			start += tsz;
 
 			/* Leave now if buffer filled already */
 			if (!size)
-				goto out_unlock;
+				return 0;
 		}
 		offset += dump->size;
 	}
 
-out_unlock:
-	mutex_unlock(&vmcore_mutex);
-	return ret;
+	return 0;
 }
 
 #ifdef CONFIG_MMU
@@ -278,20 +272,16 @@ static int vmcoredd_mmap_dumps(struct vm_area_struct *vma, unsigned long dst,
 {
 	struct vmcoredd_node *dump;
 	u64 offset = 0;
-	int ret = 0;
 	size_t tsz;
 	char *buf;
 
-	mutex_lock(&vmcore_mutex);
 	list_for_each_entry(dump, &vmcoredd_list, list) {
 		if (start < offset + dump->size) {
 			tsz = min(offset + (u64)dump->size - start, (u64)size);
 			buf = dump->buf + start - offset;
 			if (remap_vmalloc_range_partial(vma, dst, buf, 0,
-							tsz)) {
-				ret = -EFAULT;
-				goto out_unlock;
-			}
+							tsz))
+				return -EFAULT;
 
 			size -= tsz;
 			start += tsz;
@@ -299,14 +289,12 @@ static int vmcoredd_mmap_dumps(struct vm_area_struct *vma, unsigned long dst,
 
 			/* Leave now if buffer filled already */
 			if (!size)
-				goto out_unlock;
+				return 0;
 		}
 		offset += dump->size;
 	}
 
-out_unlock:
-	mutex_unlock(&vmcore_mutex);
-	return ret;
+	return 0;
 }
 #endif /* CONFIG_MMU */
 #endif /* CONFIG_PROC_VMCORE_DEVICE_DUMP */
@@ -1482,6 +1470,10 @@ int vmcore_add_device_dump(struct vmcoredd_data *data)
 		return -EINVAL;
 	}
 
+	/* We'll recheck under lock later. */
+	if (data_race(vmcore_opened))
+		return -EBUSY;
+
 	if (!data || !strlen(data->dump_name) ||
 	    !data->vmcoredd_callback || !data->size)
 		return -EINVAL;
@@ -1515,12 +1507,16 @@ int vmcore_add_device_dump(struct vmcoredd_data *data)
 	dump->buf = buf;
 	dump->size = data_size;
 
-	/* Add the dump to driver sysfs list */
+	/* Add the dump to driver sysfs list and update the elfcore hdr */
 	mutex_lock(&vmcore_mutex);
-	list_add_tail(&dump->list, &vmcoredd_list);
-	mutex_unlock(&vmcore_mutex);
+	if (vmcore_opened) {
+		ret = -EBUSY;
+		goto out_err;
+	}
 
+	list_add_tail(&dump->list, &vmcoredd_list);
 	vmcoredd_update_size(data_size);
+	mutex_unlock(&vmcore_mutex);
 	return 0;
 
 out_err:
-- 
2.46.1