| From 23a2f61ffc6a656f136fa2044c0c3b8f79766779 Mon Sep 17 00:00:00 2001 |
| From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20Galarneau?= |
| <jeremie.galarneau@efficios.com> |
| Date: Wed, 3 Mar 2021 18:52:19 -0500 |
| Subject: [PATCH 2/4] Fix: filter interpreter early-exits on uninitialized |
| value |
| MIME-Version: 1.0 |
| Content-Type: text/plain; charset=UTF-8 |
| Content-Transfer-Encoding: 8bit |
| |
| 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). |
| |
| Upstream-status: backport |
| |
| Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com> |
| Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| Change-Id: I2da3c31b9e3ce0e1b164cf3d2711c0893cbec273 |
| --- |
| lttng-filter-interpreter.c | 41 ++++++++++++++++++++++++++++++++++---- |
| 1 file changed, 37 insertions(+), 4 deletions(-) |
| |
| diff --git a/lttng-filter-interpreter.c b/lttng-filter-interpreter.c |
| index 5d572437..6e5a5139 100644 |
| --- 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; |
| -- |
| 2.19.1 |
| |