[PATCH] dlm: check the maximum size of a request from user

[Sasha Levin <levinsasha928@gmail.com>]

device_write only checks whether the request size is big enough, but it doesn't
check if the size is too big.

At that point, it also tries to allocate as much memory as the user has requested
even if it's too much. This can lead to OOM killer kicking in, or memory corruption
if (count + 1) overflows.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
---
 fs/dlm/user.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/dlm/user.c b/fs/dlm/user.c
index eb4ed9b..7ff4985 100644
--- a/fs/dlm/user.c
+++ b/fs/dlm/user.c
@@ -503,6 +503,13 @@ static ssize_t device_write(struct file *file, const char __user *buf,
 #endif
 		return -EINVAL;
 
+#ifdef CONFIG_COMPAT
+	if (count > sizeof(struct dlm_write_request32) + DLM_RESNAME_MAXLEN)
+#else
+	if (count > sizeof(struct dlm_write_request) + DLM_RESNAME_MAXLEN)
+#endif
+		return -EINVAL;
+
 	kbuf = kzalloc(count + 1, GFP_NOFS);
 	if (!kbuf)
 		return -ENOMEM;
-- 
1.7.12