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Subject[PATCH v2] twist: allow converting pr_devel()/pr_debug() into snprintf()
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);

Thus, let's allow fuzzers to always evaluate pr_devel()/pr_debug()
messages, by redirecting no-op pr_devel()/pr_debug() calls to snprintf().

[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/dev_printk.h | 16 ++++++++++++++++
include/linux/dynamic_debug.h | 14 ++++++++++++--
include/linux/printk.h | 10 ++++++++++
lib/Kconfig.twist | 12 ++++++++++++
4 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/include/linux/dev_printk.h b/include/linux/dev_printk.h
index 3028b644b4fb..ed5d5bb3b5b6 100644
--- a/include/linux/dev_printk.h
+++ b/include/linux/dev_printk.h
@@ -121,6 +121,8 @@ void _dev_info(const struct device *dev, const char *fmt, ...)
({ \
if (0) \
dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, dev_fmt(fmt), ##__VA_ARGS__); \
})
#endif

@@ -133,12 +135,16 @@ do { \
__print_once = true; \
dev_level(dev, fmt, ##__VA_ARGS__); \
} \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
} while (0)
#else
#define dev_level_once(dev_level, dev, fmt, ...) \
do { \
if (0) \
dev_level(dev, fmt, ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
} while (0)
#endif

@@ -166,6 +172,8 @@ do { \
DEFAULT_RATELIMIT_BURST); \
if (__ratelimit(&_rs)) \
dev_level(dev, fmt, ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
} while (0)

#define dev_emerg_ratelimited(dev, fmt, ...) \
@@ -195,6 +203,8 @@ do { \
__ratelimit(&_rs)) \
__dynamic_dev_dbg(&descriptor, dev, dev_fmt(fmt), \
##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, dev_fmt(fmt), ##__VA_ARGS__); \
} while (0)
#elif defined(DEBUG)
#define dev_dbg_ratelimited(dev, fmt, ...) \
@@ -204,12 +214,16 @@ do { \
DEFAULT_RATELIMIT_BURST); \
if (__ratelimit(&_rs)) \
dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, dev_fmt(fmt), ##__VA_ARGS__); \
} while (0)
#else
#define dev_dbg_ratelimited(dev, fmt, ...) \
do { \
if (0) \
dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, dev_fmt(fmt), ##__VA_ARGS__); \
} while (0)
#endif

@@ -220,6 +234,8 @@ do { \
({ \
if (0) \
dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, dev_fmt(fmt), ##__VA_ARGS__); \
})
#endif

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index abcd5fde30eb..ede42ebb38e5 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -201,9 +201,19 @@ static inline int ddebug_dyndbg_module_param_cb(char *param, char *val,
}

#define dynamic_pr_debug(fmt, ...) \
- do { if (0) printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__); } while (0)
+ do { \
+ if (0) \
+ printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, pr_fmt(fmt), ##__VA_ARGS__); \
+ } while (0)
#define dynamic_dev_dbg(dev, fmt, ...) \
- do { if (0) dev_printk(KERN_DEBUG, dev, fmt, ##__VA_ARGS__); } while (0)
+ do { \
+ if (0) \
+ dev_printk(KERN_DEBUG, dev, fmt, ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
+ } while (0)
#define dynamic_hex_dump(prefix_str, prefix_type, rowsize, \
groupsize, buf, len, ascii) \
do { if (0) \
diff --git a/include/linux/printk.h b/include/linux/printk.h
index fc8f03c54543..517c7cdec265 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -135,6 +135,8 @@ struct va_format {
({ \
if (0) \
printk(fmt, ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
0; \
})

@@ -439,6 +441,8 @@ extern int kptr_restrict;
__print_once = true; \
printk(fmt, ##__VA_ARGS__); \
} \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
unlikely(__ret_print_once); \
})
#define printk_deferred_once(fmt, ...) \
@@ -450,6 +454,8 @@ extern int kptr_restrict;
__print_once = true; \
printk_deferred(fmt, ##__VA_ARGS__); \
} \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
unlikely(__ret_print_once); \
})
#else
@@ -505,6 +511,8 @@ extern int kptr_restrict;
\
if (__ratelimit(&_rs)) \
printk(fmt, ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, fmt, ##__VA_ARGS__); \
})
#else
#define printk_ratelimited(fmt, ...) \
@@ -548,6 +556,8 @@ do { \
if (DYNAMIC_DEBUG_BRANCH(descriptor) && \
__ratelimit(&_rs)) \
__dynamic_pr_debug(&descriptor, pr_fmt(fmt), ##__VA_ARGS__); \
+ else if (IS_BUILTIN(CONFIG_TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS)) \
+ snprintf(NULL, 0, pr_fmt(fmt), ##__VA_ARGS__); \
} while (0)
#elif defined(DEBUG)
#define pr_debug_ratelimited(fmt, ...) \
diff --git a/lib/Kconfig.twist b/lib/Kconfig.twist
index f20e0d003581..a45a631819c3 100644
--- a/lib/Kconfig.twist
+++ b/lib/Kconfig.twist
@@ -12,10 +12,22 @@ if TWIST_KERNEL_BEHAVIOR

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

+config TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS
+ bool "Always evaluate printk() arguments"
+ help
+ Currently, only format string of printk() arguments is checked
+ by compiler if pr_devel()/pr_debug() are disabled. Therefore,
+ fuzz testing cannot catch runtime bugs (e.g. NULL pointer
+ dereference, use-after-free/out-of-bounds/uninitialized read)
+ in disabled printk() calls. This option redirects disabled
+ printk(...) to snprintf(NULL, 0, ...) in order to evaluate
+ arguments without printing.
+
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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