1 files changed, 422 insertions, 0 deletions
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
new file mode 100644
index 000000000..2e9e3ed7d
--- /dev/null
+++ b/kernel/printk/printk_safe.c
@@ -0,0 +1,422 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * printk_safe.c - Safe printk for printk-deadlock-prone contexts
+ */
+
+#include <linux/preempt.h>
+#include <linux/spinlock.h>
+#include <linux/debug_locks.h>
+#include <linux/kdb.h>
+#include <linux/smp.h>
+#include <linux/cpumask.h>
+#include <linux/irq_work.h>
+#include <linux/printk.h>
+#include <linux/kprobes.h>
+
+#include "internal.h"
+
+/*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it uses an alternative implementation that temporary stores
+ * the strings into a per-CPU buffer. The content of the buffer
+ * is later flushed into the main ring buffer via IRQ work.
+ *
+ * The alternative implementation is chosen transparently
+ * by examining current printk() context mask stored in @printk_context
+ * per-CPU variable.
+ *
+ * The implementation allows to flush the strings also from another CPU.
+ * There are situations when we want to make sure that all buffers
+ * were handled or when IRQs are blocked.
+ */
+
+#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
+				sizeof(atomic_t) -			\
+				sizeof(atomic_t) -			\
+				sizeof(struct irq_work))
+
+struct printk_safe_seq_buf {
+	atomic_t		len;	/* length of written data */
+	atomic_t		message_lost;
+	struct irq_work		work;	/* IRQ work that flushes the buffer */
+	unsigned char		buffer[SAFE_LOG_BUF_LEN];
+};
+
+static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
+static DEFINE_PER_CPU(int, printk_context);
+
+static DEFINE_RAW_SPINLOCK(safe_read_lock);
+
+#ifdef CONFIG_PRINTK_NMI
+static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
+#endif
+
+/* Get flushed in a more safe context. */
+static void queue_flush_work(struct printk_safe_seq_buf *s)
+{
+	if (printk_percpu_data_ready())
+		irq_work_queue(&s->work);
+}
+
+/*
+ * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
+ * have dedicated buffers, because otherwise printk-safe preempted by
+ * NMI-printk would have overwritten the NMI messages.
+ *
+ * The messages are flushed from irq work (or from panic()), possibly,
+ * from other CPU, concurrently with printk_safe_log_store(). Should this
+ * happen, printk_safe_log_store() will notice the buffer->len mismatch
+ * and repeat the write.
+ */
+static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
+						const char *fmt, va_list args)
+{
+	int add;
+	size_t len;
+	va_list ap;
+
+again:
+	len = atomic_read(&s->len);
+
+	/* The trailing '\0' is not counted into len. */
+	if (len >= sizeof(s->buffer) - 1) {
+		atomic_inc(&s->message_lost);
+		queue_flush_work(s);
+		return 0;
+	}
+
+	/*
+	 * Make sure that all old data have been read before the buffer
+	 * was reset. This is not needed when we just append data.
+	 */
+	if (!len)
+		smp_rmb();
+
+	va_copy(ap, args);
+	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
+	va_end(ap);
+	if (!add)
+		return 0;
+
+	/*
+	 * Do it once again if the buffer has been flushed in the meantime.
+	 * Note that atomic_cmpxchg() is an implicit memory barrier that
+	 * makes sure that the data were written before updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, len + add) != len)
+		goto again;
+
+	queue_flush_work(s);
+	return add;
+}
+
+static inline void printk_safe_flush_line(const char *text, int len)
+{
+	/*
+	 * Avoid any console drivers calls from here, because we may be
+	 * in NMI or printk_safe context (when in panic). The messages
+	 * must go only into the ring buffer at this stage.  Consoles will
+	 * get explicitly called later when a crashdump is not generated.
+	 */
+	printk_deferred("%.*s", len, text);
+}
+
+/* printk part of the temporary buffer line by line */
+static int printk_safe_flush_buffer(const char *start, size_t len)
+{
+	const char *c, *end;
+	bool header;
+
+	c = start;
+	end = start + len;
+	header = true;
+
+	/* Print line by line. */
+	while (c < end) {
+		if (*c == '\n') {
+			printk_safe_flush_line(start, c - start + 1);
+			start = ++c;
+			header = true;
+			continue;
+		}
+
+		/* Handle continuous lines or missing new line. */
+		if ((c + 1 < end) && printk_get_level(c)) {
+			if (header) {
+				c = printk_skip_level(c);
+				continue;
+			}
+
+			printk_safe_flush_line(start, c - start);
+			start = c++;
+			header = true;
+			continue;
+		}
+
+		header = false;
+		c++;
+	}
+
+	/* Check if there was a partial line. Ignore pure header. */
+	if (start < end && !header) {
+		static const char newline[] = KERN_CONT "\n";
+
+		printk_safe_flush_line(start, end - start);
+		printk_safe_flush_line(newline, strlen(newline));
+	}
+
+	return len;
+}
+
+static void report_message_lost(struct printk_safe_seq_buf *s)
+{
+	int lost = atomic_xchg(&s->message_lost, 0);
+
+	if (lost)
+		printk_deferred("Lost %d message(s)!\n", lost);
+}
+
+/*
+ * Flush data from the associated per-CPU buffer. The function
+ * can be called either via IRQ work or independently.
+ */
+static void __printk_safe_flush(struct irq_work *work)
+{
+	struct printk_safe_seq_buf *s =
+		container_of(work, struct printk_safe_seq_buf, work);
+	unsigned long flags;
+	size_t len;
+	int i;
+
+	/*
+	 * The lock has two functions. First, one reader has to flush all
+	 * available message to make the lockless synchronization with
+	 * writers easier. Second, we do not want to mix messages from
+	 * different CPUs. This is especially important when printing
+	 * a backtrace.
+	 */
+	raw_spin_lock_irqsave(&safe_read_lock, flags);
+
+	i = 0;
+more:
+	len = atomic_read(&s->len);
+
+	/*
+	 * This is just a paranoid check that nobody has manipulated
+	 * the buffer an unexpected way. If we printed something then
+	 * @len must only increase. Also it should never overflow the
+	 * buffer size.
+	 */
+	if ((i && i >= len) || len > sizeof(s->buffer)) {
+		const char *msg = "printk_safe_flush: internal error\n";
+
+		printk_safe_flush_line(msg, strlen(msg));
+		len = 0;
+	}
+
+	if (!len)
+		goto out; /* Someone else has already flushed the buffer. */
+
+	/* Make sure that data has been written up to the @len */
+	smp_rmb();
+	i += printk_safe_flush_buffer(s->buffer + i, len - i);
+
+	/*
+	 * Check that nothing has got added in the meantime and truncate
+	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
+	 * barrier that makes sure that the data were copied before
+	 * updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, 0) != len)
+		goto more;
+
+out:
+	report_message_lost(s);
+	raw_spin_unlock_irqrestore(&safe_read_lock, flags);
+}
+
+/**
+ * printk_safe_flush - flush all per-cpu nmi buffers.
+ *
+ * The buffers are flushed automatically via IRQ work. This function
+ * is useful only when someone wants to be sure that all buffers have
+ * been flushed at some point.
+ */
+void printk_safe_flush(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+#ifdef CONFIG_PRINTK_NMI
+		__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
+#endif
+		__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
+	}
+}
+
+/**
+ * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
+ *	goes down.
+ *
+ * Similar to printk_safe_flush() but it can be called even in NMI context when
+ * the system goes down. It does the best effort to get NMI messages into
+ * the main ring buffer.
+ *
+ * Note that it could try harder when there is only one CPU online.
+ */
+void printk_safe_flush_on_panic(void)
+{
+	/*
+	 * Make sure that we could access the main ring buffer.
+	 * Do not risk a double release when more CPUs are up.
+	 */
+	if (raw_spin_is_locked(&logbuf_lock)) {
+		if (num_online_cpus() > 1)
+			return;
+
+		debug_locks_off();
+		raw_spin_lock_init(&logbuf_lock);
+	}
+
+	if (raw_spin_is_locked(&safe_read_lock)) {
+		if (num_online_cpus() > 1)
+			return;
+
+		debug_locks_off();
+		raw_spin_lock_init(&safe_read_lock);
+	}
+
+	printk_safe_flush();
+}
+
+#ifdef CONFIG_PRINTK_NMI
+/*
+ * Safe printk() for NMI context. It uses a per-CPU buffer to
+ * store the message. NMIs are not nested, so there is always only
+ * one writer running. But the buffer might get flushed from another
+ * CPU, so we need to be careful.
+ */
+static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
+{
+	struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+
+	return printk_safe_log_store(s, fmt, args);
+}
+
+void noinstr printk_nmi_enter(void)
+{
+	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
+}
+
+void noinstr printk_nmi_exit(void)
+{
+	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
+}
+
+/*
+ * Marks a code that might produce many messages in NMI context
+ * and the risk of losing them is more critical than eventual
+ * reordering.
+ *
+ * It has effect only when called in NMI context. Then printk()
+ * will try to store the messages into the main logbuf directly
+ * and use the per-CPU buffers only as a fallback when the lock
+ * is not available.
+ */
+void printk_nmi_direct_enter(void)
+{
+	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
+		this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
+}
+
+void printk_nmi_direct_exit(void)
+{
+	this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
+}
+
+#else
+
+static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
+{
+	return 0;
+}
+
+#endif /* CONFIG_PRINTK_NMI */
+
+/*
+ * Lock-less printk(), to avoid deadlocks should the printk() recurse
+ * into itself. It uses a per-CPU buffer to store the message, just like
+ * NMI.
+ */
+static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
+{
+	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
+
+	return printk_safe_log_store(s, fmt, args);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_enter(void)
+{
+	this_cpu_inc(printk_context);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_exit(void)
+{
+	this_cpu_dec(printk_context);
+}
+
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+#ifdef CONFIG_KGDB_KDB
+	/* Allow to pass printk() to kdb but avoid a recursion. */
+	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
+		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
+#endif
+
+	/*
+	 * Try to use the main logbuf even in NMI. But avoid calling console
+	 * drivers that might have their own locks.
+	 */
+	if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
+	    raw_spin_trylock(&logbuf_lock)) {
+		int len;
+
+		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
+		raw_spin_unlock(&logbuf_lock);
+		defer_console_output();
+		return len;
+	}
+
+	/* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
+	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
+		return vprintk_nmi(fmt, args);
+
+	/* Use extra buffer to prevent a recursion deadlock in safe mode. */
+	if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
+		return vprintk_safe(fmt, args);
+
+	/* No obstacles. */
+	return vprintk_default(fmt, args);
+}
+
+void __init printk_safe_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct printk_safe_seq_buf *s;
+
+		s = &per_cpu(safe_print_seq, cpu);
+		init_irq_work(&s->work, __printk_safe_flush);
+
+#ifdef CONFIG_PRINTK_NMI
+		s = &per_cpu(nmi_print_seq, cpu);
+		init_irq_work(&s->work, __printk_safe_flush);
+#endif
+	}
+
+	/* Flush pending messages that did not have scheduled IRQ works. */
+	printk_safe_flush();
+}