Al suggested that if we allocate enough space to add in the '\0'
character at the end of our strings, we could just use scnprintf() in
our ->proc_handler functions without having to be fancy about keeping
track of space. There are a lot of these handlers, so the follow ups
will be separate, but start with allocating the extra byte to handle the
null termination of strings.
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
fs/proc/proc_sysctl.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 8e19bad83b45..446e7a949025 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -548,6 +548,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
struct ctl_table *table = PROC_I(inode)->sysctl_entry;
void *kbuf;
ssize_t error;
+ size_t orig_count = count;
if (IS_ERR(head))
return PTR_ERR(head);
@@ -577,9 +578,23 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
goto out;
}
} else {
- kbuf = kvzalloc(count, GFP_KERNEL);
+ /*
+ * To make our lives easier in ->proc_handler, we allocate an
+ * extra byte to allow us to use scnprintf() for handling the
+ * buffer output. This works properly because scnprintf() will
+ * only return the number of bytes that it was able to write
+ * out, _NOT_ including the NULL byte. This means the handler's
+ * will only ever return a maximum of count as what they've
+ * copied.
+ *
+ * HOWEVER, we do not assume that ->proc_handlers are without
+ * bugs, so further down we'll do an extra check to make sure
+ * that count isn't larger than the orig_count.
+ */
+ kbuf = kvzalloc(count + 1, GFP_KERNEL);
if (!kbuf)
goto out;
+ count += 1;
}
error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, &kbuf, &count,
@@ -593,6 +608,13 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
goto out_free_buf;
if (!write) {
+ /*
+ * This shouldn't happen, but those are the last words before
+ * somebody adds a security vulnerability, so just make sure
+ * that count isn't larger than orig_count.
+ */
+ if (count > orig_count)
+ count = orig_count;
error = -EFAULT;
if (copy_to_user(ubuf, kbuf, count))
goto out_free_buf;
--
2.24.1
