rust/pvimg: Fix possible 'range start index out of range for slice' error

Fix possible 'range start index 16 out of range for slice of length 0'
error by adding a check of the slice data length.

Fixes: f4cf4ae6eb ("rust: Add a new tool called 'pvimg'")
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pvimg/src/pv_utils/se_hdr/brb.rs

@@ -259,6 +259,10 @@ impl SeHdr {
             return Err(Error::InvalidSeHdr);
         }
 
+        if sehs <= common_size {
+            return Err(Error::InvalidSeHdr);
+        }
+
         data.resize(sehs, 0);
         reader.read_exact(&mut data[common_size..])?;
         Self::try_from_data(&data)
@@ -366,3 +370,22 @@ impl AeadCipherTrait for SeHdrPlain {
         self.data.aead_tag_size()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::io::Cursor;
+
+    use super::SeHdr;
+    use crate::error::Error;
+
+    #[test]
+    fn test_sehdr_try_from_io() {
+        // Invalid SeHdr as `sehs` is set to 0
+        assert!(matches!(
+            SeHdr::try_from_io(Cursor::new([
+                73, 66, 77, 83, 101, 99, 69, 120, 0, 0, 1, 0, 0, 0, 0, 0, 2, 0, 8
+            ])),
+            Err(Error::InvalidSeHdr)
+        ));
+    }
+}