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Subject[PATCH] twist: allow converting pr_devel()/pr_debug() into printk(KERN_DEBUG)
Date
syzbot found a NULL pointer dereference bug inside mptcp_recvmsg() due to
ssock == NULL, but this bug manifested inside selinux_socket_recvmsg()
because pr_debug() was no-op [1].

pr_debug("fallback-read subflow=%p",
mptcp_subflow_ctx(ssock->sk));
copied = sock_recvmsg(ssock, msg, flags);

Since console loglevel used by syzkaller will not print KERN_DEBUG
messages to consoles, always evaluating pr_devel()/pr_debug() messages
will not cause too much console output. Thus, let's allow fuzzers to
always evaluate pr_devel()/pr_debug() messages.

[1] https://syzkaller.appspot.com/bug?id=12be9aa373be9d8727cdd172f190de39528a413a

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
---
include/linux/printk.h | 25 ++++++++++++++++++-------
lib/Kconfig.twist | 10 ++++++++++
2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 38beb97e7018..2562ffb438ed 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -389,7 +389,7 @@ extern int kptr_restrict;
*
* It uses pr_fmt() to generate the format string.
*/
-#ifdef DEBUG
+#if defined(DEBUG) || defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
#define pr_devel(fmt, ...) \
printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
@@ -399,7 +399,10 @@ extern int kptr_restrict;


/* If you are writing a driver, please use dev_dbg instead */
-#if defined(CONFIG_DYNAMIC_DEBUG) || \
+#if defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
+#define pr_debug(fmt, ...) \
+ printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+#elif defined(CONFIG_DYNAMIC_DEBUG) || \
(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
#include <linux/dynamic_debug.h>

@@ -476,7 +479,7 @@ extern int kptr_restrict;
#define pr_cont_once(fmt, ...) \
printk_once(KERN_CONT pr_fmt(fmt), ##__VA_ARGS__)

-#if defined(DEBUG)
+#if defined(DEBUG) || defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
#define pr_devel_once(fmt, ...) \
printk_once(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
@@ -485,7 +488,7 @@ extern int kptr_restrict;
#endif

/* If you are writing a driver, please use dev_dbg instead */
-#if defined(DEBUG)
+#if defined(DEBUG) || defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
#define pr_debug_once(fmt, ...) \
printk_once(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
@@ -528,7 +531,7 @@ extern int kptr_restrict;
printk_ratelimited(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
/* no pr_cont_ratelimited, don't do that... */

-#if defined(DEBUG)
+#if defined(DEBUG) || defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
#define pr_devel_ratelimited(fmt, ...) \
printk_ratelimited(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
@@ -537,7 +540,10 @@ extern int kptr_restrict;
#endif

/* If you are writing a driver, please use dev_dbg instead */
-#if defined(CONFIG_DYNAMIC_DEBUG) || \
+#if defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
+#define pr_debug_ratelimited(fmt, ...) \
+ printk_ratelimited(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+#elif defined(CONFIG_DYNAMIC_DEBUG) || \
(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
/* descriptor check is first to prevent flooding with "callbacks suppressed" */
#define pr_debug_ratelimited(fmt, ...) \
@@ -585,7 +591,12 @@ static inline void print_hex_dump_bytes(const char *prefix_str, int prefix_type,

#endif

-#if defined(CONFIG_DYNAMIC_DEBUG) || \
+#if defined(CONFIG_TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK)
+#define print_hex_dump_debug(prefix_str, prefix_type, rowsize, \
+ groupsize, buf, len, ascii) \
+ print_hex_dump(KERN_DEBUG, prefix_str, prefix_type, rowsize, \
+ groupsize, buf, len, ascii)
+#elif defined(CONFIG_DYNAMIC_DEBUG) || \
(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
#define print_hex_dump_debug(prefix_str, prefix_type, rowsize, \
groupsize, buf, len, ascii) \
diff --git a/lib/Kconfig.twist b/lib/Kconfig.twist
index f20e0d003581..710202f4b15d 100644
--- a/lib/Kconfig.twist
+++ b/lib/Kconfig.twist
@@ -12,10 +12,20 @@ if TWIST_KERNEL_BEHAVIOR

config TWIST_FOR_SYZKALLER_TESTING
bool "Select all twist options suitable for syzkaller testing"
+ select TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK
select TWIST_DISABLE_KBD_K_SPEC_HANDLER
help
Say N unless you are building kernels for syzkaller's testing.

+config TWIST_ALWAYS_EVALUATE_DEBUG_PRINTK
+ bool "Always evaluate pr_devel() and pr_debug()"
+ help
+ When pr_devel()/pr_debug() are no-op, only format string is checked
+ by compiler, and runtime bugs (such as NULL pointer dereference)
+ cannot be reported by fuzz testing.
+ This option will unconditionally convert pr_devel() and pr_debug()
+ into printk(KERN_DEBUG) in order to evaluate printk() arguments.
+
config TWIST_DISABLE_KBD_K_SPEC_HANDLER
bool "Disable k_spec() function in drivers/tty/vt/keyboard.c"
help
--
2.18.2
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