rexec: Avoid invalid free in rexec failure path

Commit "rexec: free argv array on failure" used __do_free_string_list
as a destructor for argv, which is an array of pointers to a single
buffer and not an array of pointers to independent buffers, which leads
to an attempt to free invalid pointer whenever argv has more than one
element.

Structure argv as one memory block and use __do_free as the destructor.

Signed-off-by: Petr Malat <oss@malat.biz>

diff --git a/src/lxc/rexec.c b/src/lxc/rexec.c
index 6a017da9c62cb51575b7ac84e4008c8766e33b36..09c9da20b1f13e88c044620500a9e4d3cb7700ff 100644 (file)
--- a/src/lxc/rexec.c
+++ b/src/lxc/rexec.c
@@ -23,25 +23,39 @@
 #define LXC_MEMFD_REXEC_SEALS \
        (F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
 
-static int push_vargs(char *data, int data_length, char ***output)
+/**
+ * Transforms null separated string elements into an array of pointers to these
+ * elements.
+ * @param[in] data - null separated string elements
+ * @param[in] data_length - length of data
+ * @param[out] output - NULL terminated array of pointers to a copy of elements
+ *                      passed in data. Data are copied behind the array and the
+ *                      whole output resides in one chunk of memory and should
+ *                      be freed with free(*output).
+ * @return Number of elements returned or -EINVAL if data or output is NULL or
+ *         if *output is not NULL.
+ */
+static int push_vargs(const char *data, int data_length, char ***output)
 {
-       int num = 0;
-       char *cur = data;
+       int i, j, nmemb;
+       char *end;
 
-       if (!data || *output)
-               return -1;
+       if (!data || !output || *output)
+               return -EINVAL;
 
-       *output = must_realloc(NULL, sizeof(**output));
+       for (nmemb = i = 0; i < data_length; i++)
+               if (!data[i]) nmemb++;
 
-       while (cur < data + data_length) {
-               num++;
-               *output = must_realloc(*output, (num + 1) * sizeof(**output));
+       *output = must_realloc(NULL, (nmemb + 1) * sizeof(char*) + data_length);
+       end = (char *)&(*output)[nmemb + 1];
+       memcpy(end, data, data_length);
 
-               (*output)[num - 1] = cur;
-               cur += strlen(cur) + 1;
-       }
-       (*output)[num] = NULL;
-       return num;
+       (*output)[0] = end;
+       for (i = j = 0; i < data_length; i++)
+               if (!end[i]) (*output)[++j] = &end[i + 1];
+       (*output)[j] = NULL;
+
+       return nmemb;
 }
 
 static int parse_argv(char ***argv)
@@ -55,11 +69,7 @@ static int parse_argv(char ***argv)
                return -1;
 
        ret = push_vargs(cmdline, cmdline_size, argv);
-       if (ret <= 0)
-               return -1;
-
-       move_ptr(cmdline);
-       return 0;
+       return ret <= 0 ? -1 : 0;
 }
 
 static int is_memfd(void)
@@ -171,7 +181,7 @@ extern char **environ;
 
 int lxc_rexec(const char *memfd_name)
 {
-       __do_free_string_list char **argv = NULL;
+       __do_free char **argv = NULL;
        int ret;
 
        ret = is_memfd();