Fix: filter interpreter early-exits on uninitialized value

I observed that syscall filtering on string arguments wouldn't work on
my development machines, both running 5.11.2-arch1-1 (Arch Linux).

For instance, enabling the tracing of the `openat()` syscall with the
'filename == "/proc/cpuinfo"' filter would not produce events even
though matching events were present in another session that had no
filtering active. The same problem occurred with `execve()`.

I tried a couple of kernel versions before (5.11.1 and 5.10.13, if
memory serves me well) and I had the same problem. Meanwhile, I couldn't
reproduce the problem on various Debian machines (the LTTng CI) nor on a
fresh Ubuntu 20.04 with both the stock kernel and with an updated 5.11.2
kernel.

I built the lttng-modules with the interpreter debugging printout and
saw the following warning:
  LTTng: [debug bytecode in /home/jgalar/EfficiOS/src/lttng-modules/src/lttng-bytecode-interpreter.c:bytecode_interpret@1508] Bytecode warning: loading a NULL string.

After a shedload (yes, a _shed_load) of digging, I figured that the
problem was hidden in plain sight near that logging statement.

In the `BYTECODE_OP_LOAD_FIELD_REF_USER_STRING` operation, the 'ax'
register's 'user_str' is initialized with the stack value (the user
space string's address in our case). However, a NULL check is performed
against the register's 'str' member.

I initialy suspected that both members would be part of the same union
and alias each-other, but they are actually contiguous in a structure.

On the unaffected machines, I could confirm that the `str` member was
uninitialized to a non-zero value causing the condition to evaluate to
false.

Francis Deslauriers reproduced the problem by initializing the
interpreter stack to zero.

I am unsure of the exact kernel configuration option that reveals this
issue on Arch Linux, but my kernel has the following option enabled:

CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL:
   Zero-initialize any stack variables that may be passed by reference
   and had not already been explicitly initialized. This is intended to
   eliminate all classes of uninitialized stack variable exploits and
   information exposures.

I have not tried to build without this enabled as, anyhow, this seems
to be a legitimate issue.

I have spotted what appears to be an identical problem in
`BYTECODE_OP_LOAD_FIELD_REF_USER_SEQUENCE` and corrected it. However,
I have not exercised that code path.

The commit that introduced this problem is 5b4ad89.

The debug print-out of the `BYTECODE_OP_LOAD_FIELD_REF_USER_STRING`
operation is modified to print the user string (truncated to 31 chars).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2da3c31b9e3ce0e1b164cf3d2711c0893cbec273

diff --git a/lttng-filter-interpreter.c b/lttng-filter-interpreter.c
index 5d5724379749a00a8a80fd042b770a59c950e5a4..6e5a5139e3dfd285fe0500c2558b8c1982fb49ba 100644 (file)
--- a/lttng-filter-interpreter.c
+++ b/lttng-filter-interpreter.c
@@ -22,7 +22,7 @@ LTTNG_STACK_FRAME_NON_STANDARD(lttng_filter_interpret_bytecode);
  * to handle user-space read.
  */
 static
-char get_char(struct estack_entry *reg, size_t offset)
+char get_char(const struct estack_entry *reg, size_t offset)
 {
        if (unlikely(offset >= reg->u.s.seq_len))
                return '\0';
@@ -593,6 +593,39 @@ end:
        return ret;
 }
 
+#ifdef DEBUG
+
+#define DBG_USER_STR_CUTOFF 32
+
+/*
+ * In debug mode, print user string (truncated, if necessary).
+ */
+static inline
+void dbg_load_ref_user_str_printk(const struct estack_entry *user_str_reg)
+{
+       size_t pos = 0;
+       char last_char;
+       char user_str[DBG_USER_STR_CUTOFF];
+
+       pagefault_disable();
+       do {
+               last_char = get_char(user_str_reg, pos);
+               user_str[pos] = last_char;
+               pos++;
+       } while (last_char != '\0' && pos < sizeof(user_str));
+       pagefault_enable();
+
+       user_str[sizeof(user_str) - 1] = '\0';
+       dbg_printk("load field ref user string: '%s%s'\n", user_str,
+               last_char != '\0' ? "[...]" : "");
+}
+#else
+static inline
+void dbg_load_ref_user_str_printk(const struct estack_entry *user_str_reg)
+{
+}
+#endif
+
 /*
  * Return 0 (discard), or raise the 0x1 flag (log event).
  * Currently, other flags are kept for future extensions and have no
@@ -1313,7 +1346,7 @@ uint64_t lttng_filter_interpret_bytecode(void *filter_data,
                        estack_push(stack, top, ax, bx);
                        estack_ax(stack, top)->u.s.user_str =
                                *(const char * const *) &filter_stack_data[ref->offset];
-                       if (unlikely(!estack_ax(stack, top)->u.s.str)) {
+                       if (unlikely(!estack_ax(stack, top)->u.s.user_str)) {
                                dbg_printk("Filter warning: loading a NULL string.\n");
                                ret = -EINVAL;
                                goto end;
@@ -1322,7 +1355,7 @@ uint64_t lttng_filter_interpret_bytecode(void *filter_data,
                        estack_ax(stack, top)->u.s.literal_type =
                                ESTACK_STRING_LITERAL_TYPE_NONE;
                        estack_ax(stack, top)->u.s.user = 1;
-                       dbg_printk("ref load string %s\n", estack_ax(stack, top)->u.s.str);
+                       dbg_load_ref_user_str_printk(estack_ax(stack, top));
                        next_pc += sizeof(struct load_op) + sizeof(struct field_ref);
                        PO;
                }
@@ -1340,7 +1373,7 @@ uint64_t lttng_filter_interpret_bytecode(void *filter_data,
                        estack_ax(stack, top)->u.s.user_str =
                                *(const char **) (&filter_stack_data[ref->offset
                                                                + sizeof(unsigned long)]);
-                       if (unlikely(!estack_ax(stack, top)->u.s.str)) {
+                       if (unlikely(!estack_ax(stack, top)->u.s.user_str)) {
                                dbg_printk("Filter warning: loading a NULL sequence.\n");
                                ret = -EINVAL;
                                goto end;