parent d06e9ca8
From ab6f762f0f53162d41497708b33c9a3236d3609e Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <[email protected]>
Date: Tue, 3 Mar 2020 20:30:02 +0900
Subject: [PATCH] printk: queue wake_up_klogd irq_work only if per-CPU areas
are ready
printk_deferred(), similarly to printk_safe/printk_nmi, does not
immediately attempt to print a new message on the consoles, avoiding
calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
which potentially can deadlock the system.
Those printk() flavors, instead, rely on per-CPU flush irq_work to print
messages from safer contexts. For same reasons (recursive scheduler or
timekeeping calls) printk() uses per-CPU irq_work in order to wake up
user space syslog/kmsg readers.
However, only printk_safe/printk_nmi do make sure that per-CPU areas
have been initialised and that it's safe to modify per-CPU irq_work.
This means that, for instance, should printk_deferred() be invoked "too
early", that is before per-CPU areas are initialised, printk_deferred()
will perform illegal per-CPU access.
Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.
The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).
Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.
Link: https://lore.kernel.org/lkml/[email protected]/
Reported-by: Lech Perczak <[email protected]>
Signed-off-by: Sergey Senozhatsky <[email protected]>
Tested-by: Jann Horn <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: John Ogness <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
---
include/linux/printk.h | 5 -----
init/main.c | 1 -
kernel/printk/internal.h | 5 +++++
kernel/printk/printk.c | 34 ++++++++++++++++++++++++++++++++++
kernel/printk/printk_safe.c | 11 +----------
5 files changed, 40 insertions(+), 16 deletions(-)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 1e6108b8d15fc..e061635e04091 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -202,7 +202,6 @@ __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
void dump_stack_print_info(const char *log_lvl);
void show_regs_print_info(const char *log_lvl);
extern asmlinkage void dump_stack(void) __cold;
-extern void printk_safe_init(void);
extern void printk_safe_flush(void);
extern void printk_safe_flush_on_panic(void);
#else
@@ -269,10 +268,6 @@ static inline void dump_stack(void)
{
}
-static inline void printk_safe_init(void)
-{
-}
-
static inline void printk_safe_flush(void)
{
}
diff --git a/init/main.c b/init/main.c
index e488213857e2a..a48617f2e5e5b 100644
--- a/init/main.c
+++ b/init/main.c
@@ -913,7 +913,6 @@ asmlinkage __visible void __init start_kernel(void)
boot_init_stack_canary();
time_init();
- printk_safe_init();
perf_event_init();
profile_init();
call_function_init();
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index c8e6ab689d42f..b2b0f526f249e 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -23,6 +23,9 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args);
void __printk_safe_enter(void);
void __printk_safe_exit(void);
+void printk_safe_init(void);
+bool printk_percpu_data_ready(void);
+
#define printk_safe_enter_irqsave(flags) \
do { \
local_irq_save(flags); \
@@ -64,4 +67,6 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
#define printk_safe_enter_irq() local_irq_disable()
#define printk_safe_exit_irq() local_irq_enable()
+static inline void printk_safe_init(void) { }
+static inline bool printk_percpu_data_ready(void) { return false; }
#endif /* CONFIG_PRINTK */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 633f41a11d759..9a9b6156270bb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -460,6 +460,18 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
+/*
+ * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
+ * per_cpu_areas are initialised. This variable is set to true when
+ * it's safe to access per-CPU data.
+ */
+static bool __printk_percpu_data_ready __read_mostly;
+
+bool printk_percpu_data_ready(void)
+{
+ return __printk_percpu_data_ready;
+}
+
/* Return log buffer address */
char *log_buf_addr_get(void)
{
@@ -1146,12 +1158,28 @@ static void __init log_buf_add_cpu(void)
static inline void log_buf_add_cpu(void) {}
#endif /* CONFIG_SMP */
+static void __init set_percpu_data_ready(void)
+{
+ printk_safe_init();
+ /* Make sure we set this flag only after printk_safe() init is done */
+ barrier();
+ __printk_percpu_data_ready = true;
+}
+
void __init setup_log_buf(int early)
{
unsigned long flags;
char *new_log_buf;
unsigned int free;
+ /*
+ * Some archs call setup_log_buf() multiple times - first is very
+ * early, e.g. from setup_arch(), and second - when percpu_areas
+ * are initialised.
+ */
+ if (!early)
+ set_percpu_data_ready();
+
if (log_buf != __log_buf)
return;
@@ -2975,6 +3003,9 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
void wake_up_klogd(void)
{
+ if (!printk_percpu_data_ready())
+ return;
+
preempt_disable();
if (waitqueue_active(&log_wait)) {
this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
@@ -2985,6 +3016,9 @@ void wake_up_klogd(void)
void defer_console_output(void)
{
+ if (!printk_percpu_data_ready())
+ return;
+
preempt_disable();
__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index b4045e7827431..d9a659a686f31 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -27,7 +27,6 @@
* There are situations when we want to make sure that all buffers
* were handled or when IRQs are blocked.
*/
-static int printk_safe_irq_ready __read_mostly;
#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
sizeof(atomic_t) - \
@@ -51,7 +50,7 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
/* Get flushed in a more safe context. */
static void queue_flush_work(struct printk_safe_seq_buf *s)
{
- if (printk_safe_irq_ready)
+ if (printk_percpu_data_ready())
irq_work_queue(&s->work);
}
@@ -402,14 +401,6 @@ void __init printk_safe_init(void)
#endif
}
- /*
- * In the highly unlikely event that a NMI were to trigger at
- * this moment. Make sure IRQ work is set up before this
- * variable is set.
- */
- barrier();
- printk_safe_irq_ready = 1;
-
/* Flush pending messages that did not have scheduled IRQ works. */
printk_safe_flush();
}
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