[PATCH AUTOSEL 6.5 1/6] printk: Reduce console_unblank() usage in unsafe scenarios

[PATCH AUTOSEL 6.5 1/6] printk: Reduce console_unblank() usage in unsafe scenarios

[Sasha Levin]

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit 7b23a66db55ed0a55b020e913f0d6f6d52a1ad2c ]

A semaphore is not NMI-safe, even when using down_trylock(). Both
down_trylock() and up() are using internal spinlocks and up()
might even call wake_up_process().

In the panic() code path it gets even worse because the internal
spinlocks of the semaphore may have been taken by a CPU that has
been stopped.

To reduce the risk of deadlocks caused by the console semaphore in
the panic path, make the following changes:

- First check if any consoles have implemented the unblank()
  callback. If not, then there is no reason to take the console
  semaphore anyway. (This check is also useful for the non-panic
  path since the locking/unlocking of the console lock can be
  quite expensive due to console printing.)

- If the panic path is in NMI context, bail out without attempting
  to take the console semaphore or calling any unblank() callbacks.
  Bailing out is acceptable because console_unblank() would already
  bail out if the console semaphore is contended. The alternative of
  ignoring the console semaphore and calling the unblank() callbacks
  anyway is a bad idea because these callbacks are also not NMI-safe.

If consoles with unblank() callbacks exist and console_unblank() is
called from a non-NMI panic context, it will still attempt a
down_trylock(). This could still result in a deadlock if one of the
stopped CPUs is holding the semaphore internal spinlock. But this
is a risk that the kernel has been (and continues to be) willing
to take.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-3-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/printk/printk.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 357a4d18f6387..7d3f30eb35862 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3045,9 +3045,27 @@ EXPORT_SYMBOL(console_conditional_schedule);
 
 void console_unblank(void)
 {
+	bool found_unblank = false;
 	struct console *c;
 	int cookie;
 
+	/*
+	 * First check if there are any consoles implementing the unblank()
+	 * callback. If not, there is no reason to continue and take the
+	 * console lock, which in particular can be dangerous if
+	 * @oops_in_progress is set.
+	 */
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
+		if ((console_srcu_read_flags(c) & CON_ENABLED) && c->unblank) {
+			found_unblank = true;
+			break;
+		}
+	}
+	console_srcu_read_unlock(cookie);
+	if (!found_unblank)
+		return;
+
 	/*
 	 * Stop console printing because the unblank() callback may
 	 * assume the console is not within its write() callback.
@@ -3056,6 +3074,16 @@ void console_unblank(void)
 	 * In that case, attempt a trylock as best-effort.
 	 */
 	if (oops_in_progress) {
+		/* Semaphores are not NMI-safe. */
+		if (in_nmi())
+			return;
+
+		/*
+		 * Attempting to trylock the console lock can deadlock
+		 * if another CPU was stopped while modifying the
+		 * semaphore. "Hope and pray" that this is not the
+		 * current situation.
+		 */
 		if (down_trylock_console_sem() != 0)
 			return;
 	} else
-- 
2.40.1

[PATCH AUTOSEL 6.5 2/6] printk: Keep non-panic-CPUs out of console lock

[Sasha Levin]

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit 51a1d258e50e03a0216bf42b6af9ff34ec402ac1 ]

When in a panic situation, non-panic CPUs should avoid holding the
console lock so as not to contend with the panic CPU. This is already
implemented with abandon_console_lock_in_panic(), which is checked
after each printed line. However, non-panic CPUs should also avoid
trying to acquire the console lock during a panic.

Modify console_trylock() to fail and console_lock() to block() when
called from a non-panic CPU during a panic.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-4-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/printk/printk.c | 45 ++++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7d3f30eb35862..591c11888200d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2585,6 +2585,25 @@ static int console_cpu_notify(unsigned int cpu)
 	return 0;
 }
 
+/*
+ * Return true when this CPU should unlock console_sem without pushing all
+ * messages to the console. This reduces the chance that the console is
+ * locked when the panic CPU tries to use it.
+ */
+static bool abandon_console_lock_in_panic(void)
+{
+	if (!panic_in_progress())
+		return false;
+
+	/*
+	 * We can use raw_smp_processor_id() here because it is impossible for
+	 * the task to be migrated to the panic_cpu, or away from it. If
+	 * panic_cpu has already been set, and we're not currently executing on
+	 * that CPU, then we never will be.
+	 */
+	return atomic_read(&panic_cpu) != raw_smp_processor_id();
+}
+
 /**
  * console_lock - block the console subsystem from printing
  *
@@ -2597,6 +2616,10 @@ void console_lock(void)
 {
 	might_sleep();
 
+	/* On panic, the console_lock must be left to the panic cpu. */
+	while (abandon_console_lock_in_panic())
+		msleep(1000);
+
 	down_console_sem();
 	if (console_suspended)
 		return;
@@ -2615,6 +2638,9 @@ EXPORT_SYMBOL(console_lock);
  */
 int console_trylock(void)
 {
+	/* On panic, the console_lock must be left to the panic cpu. */
+	if (abandon_console_lock_in_panic())
+		return 0;
 	if (down_trylock_console_sem())
 		return 0;
 	if (console_suspended) {
@@ -2633,25 +2659,6 @@ int is_console_locked(void)
 }
 EXPORT_SYMBOL(is_console_locked);
 
-/*
- * Return true when this CPU should unlock console_sem without pushing all
- * messages to the console. This reduces the chance that the console is
- * locked when the panic CPU tries to use it.
- */
-static bool abandon_console_lock_in_panic(void)
-{
-	if (!panic_in_progress())
-		return false;
-
-	/*
-	 * We can use raw_smp_processor_id() here because it is impossible for
-	 * the task to be migrated to the panic_cpu, or away from it. If
-	 * panic_cpu has already been set, and we're not currently executing on
-	 * that CPU, then we never will be.
-	 */
-	return atomic_read(&panic_cpu) != raw_smp_processor_id();
-}
-
 /*
  * Check if the given console is currently capable and allowed to print
  * records.
-- 
2.40.1

[PATCH AUTOSEL 6.5 3/6] printk: Do not take console lock for console_flush_on_panic()

[Sasha Levin]

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit eacb04ff3c5b8662a65f380ae450250698448cff ]

Currently console_flush_on_panic() will attempt to acquire the
console lock when flushing the buffer on panic. If it fails to
acquire the lock, it continues anyway because this is the last
chance to get any pending records printed.

The reason why the console lock was attempted at all was to
prevent any other CPUs from acquiring the console lock for
printing while the panic CPU was printing. But as of the
previous commit, non-panic CPUs will no longer attempt to
acquire the console lock in a panic situation. Therefore it is
no longer strictly necessary for a panic CPU to acquire the
console lock.

Avoiding taking the console lock when flushing in panic has
the additional benefit of avoiding possible deadlocks due to
semaphore usage in NMI context (semaphores are not NMI-safe)
and avoiding possible deadlocks if another CPU accesses the
semaphore and is stopped while holding one of the semaphore's
internal spinlocks.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-5-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/printk/printk.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 591c11888200d..88770561c4350 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3120,14 +3120,24 @@ void console_unblank(void)
  */
 void console_flush_on_panic(enum con_flush_mode mode)
 {
+	bool handover;
+	u64 next_seq;
+
 	/*
-	 * If someone else is holding the console lock, trylock will fail
-	 * and may_schedule may be set.  Ignore and proceed to unlock so
-	 * that messages are flushed out.  As this can be called from any
-	 * context and we don't want to get preempted while flushing,
-	 * ensure may_schedule is cleared.
+	 * Ignore the console lock and flush out the messages. Attempting a
+	 * trylock would not be useful because:
+	 *
+	 *   - if it is contended, it must be ignored anyway
+	 *   - console_lock() and console_trylock() block and fail
+	 *     respectively in panic for non-panic CPUs
+	 *   - semaphores are not NMI-safe
+	 */
+
+	/*
+	 * If another context is holding the console lock,
+	 * @console_may_schedule might be set. Clear it so that
+	 * this context does not call cond_resched() while flushing.
 	 */
-	console_trylock();
 	console_may_schedule = 0;
 
 	if (mode == CONSOLE_REPLAY_ALL) {
@@ -3140,15 +3150,15 @@ void console_flush_on_panic(enum con_flush_mode mode)
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(c) {
 			/*
-			 * If the above console_trylock() failed, this is an
-			 * unsynchronized assignment. But in that case, the
+			 * This is an unsynchronized assignment, but the
 			 * kernel is in "hope and pray" mode anyway.
 			 */
 			c->seq = seq;
 		}
 		console_srcu_read_unlock(cookie);
 	}
-	console_unlock();
+
+	console_flush_all(false, &next_seq, &handover);
 }
 
 /*
-- 
2.40.1

[PATCH AUTOSEL 6.5 4/6] printk: Consolidate console deferred printing

[Sasha Levin]

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit 696ffaf50e1f8dbc66223ff614473f945f5fb8d8 ]

Printing to consoles can be deferred for several reasons:

- explicitly with printk_deferred()
- printk() in NMI context
- recursive printk() calls

The current implementation is not consistent. For printk_deferred(),
irq work is scheduled twice. For NMI und recursive, panic CPU
suppression and caller delays are not properly enforced.

Correct these inconsistencies by consolidating the deferred printing
code so that vprintk_deferred() is the top-level function for
deferred printing and vprintk_emit() will perform whichever irq_work
queueing is appropriate.

Also add kerneldoc for wake_up_klogd() and defer_console_output() to
clarify their differences and appropriate usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-6-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/printk/printk.c      | 35 ++++++++++++++++++++++++++++-------
 kernel/printk/printk_safe.c |  9 ++-------
 2 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 88770561c4350..d5e29fad84234 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2308,7 +2308,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	wake_up_klogd();
+	if (in_sched)
+		defer_console_output();
+	else
+		wake_up_klogd();
+
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
@@ -3843,11 +3847,33 @@ static void __wake_up_klogd(int val)
 	preempt_enable();
 }
 
+/**
+ * wake_up_klogd - Wake kernel logging daemon
+ *
+ * Use this function when new records have been added to the ringbuffer
+ * and the console printing of those records has already occurred or is
+ * known to be handled by some other context. This function will only
+ * wake the logging daemon.
+ *
+ * Context: Any context.
+ */
 void wake_up_klogd(void)
 {
 	__wake_up_klogd(PRINTK_PENDING_WAKEUP);
 }
 
+/**
+ * defer_console_output - Wake kernel logging daemon and trigger
+ *	console printing in a deferred context
+ *
+ * Use this function when new records have been added to the ringbuffer,
+ * this context is responsible for console printing those records, but
+ * the current context is not allowed to perform the console printing.
+ * Trigger an irq_work context to perform the console printing. This
+ * function also wakes the logging daemon.
+ *
+ * Context: Any context.
+ */
 void defer_console_output(void)
 {
 	/*
@@ -3864,12 +3890,7 @@ void printk_trigger_flush(void)
 
 int vprintk_deferred(const char *fmt, va_list args)
 {
-	int r;
-
-	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
-	defer_console_output();
-
-	return r;
+	return vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
 }
 
 int _printk_deferred(const char *fmt, ...)
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index ef0f9a2044da1..6d10927a07d83 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -38,13 +38,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	 * Use the main logbuf even in NMI. But avoid calling console
 	 * drivers that might have their own locks.
 	 */
-	if (this_cpu_read(printk_context) || in_nmi()) {
-		int len;
-
-		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
-		defer_console_output();
-		return len;
-	}
+	if (this_cpu_read(printk_context) || in_nmi())
+		return vprintk_deferred(fmt, args);
 
 	/* No obstacles. */
 	return vprintk_default(fmt, args);
-- 
2.40.1

[PATCH AUTOSEL 6.5 5/6] printk: Rename abandon_console_lock_in_panic() to other_cpu_in_panic()

[Sasha Levin]

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit 132a90d1527fedba2d95085c951ccf00dbbebe41 ]

Currently abandon_console_lock_in_panic() is only used to determine if
the current CPU should immediately release the console lock because
another CPU is in panic. However, later this function will be used by
the CPU to immediately release other resources in this situation.

Rename the function to other_cpu_in_panic(), which is a better
description and does not assume it is related to the console lock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230717194607.145135-8-john.ogness@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/printk/internal.h |  2 ++
 kernel/printk/printk.c   | 15 ++++++++-------
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 2a17704136f1d..7d4979d5c3ce6 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -103,3 +103,5 @@ struct printk_message {
 	u64			seq;
 	unsigned long		dropped;
 };
+
+bool other_cpu_in_panic(void);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d5e29fad84234..08a9419046b65 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2590,11 +2590,12 @@ static int console_cpu_notify(unsigned int cpu)
 }
 
 /*
- * Return true when this CPU should unlock console_sem without pushing all
- * messages to the console. This reduces the chance that the console is
- * locked when the panic CPU tries to use it.
+ * Return true if a panic is in progress on a remote CPU.
+ *
+ * On true, the local CPU should immediately release any printing resources
+ * that may be needed by the panic CPU.
  */
-static bool abandon_console_lock_in_panic(void)
+bool other_cpu_in_panic(void)
 {
 	if (!panic_in_progress())
 		return false;
@@ -2621,7 +2622,7 @@ void console_lock(void)
 	might_sleep();
 
 	/* On panic, the console_lock must be left to the panic cpu. */
-	while (abandon_console_lock_in_panic())
+	while (other_cpu_in_panic())
 		msleep(1000);
 
 	down_console_sem();
@@ -2643,7 +2644,7 @@ EXPORT_SYMBOL(console_lock);
 int console_trylock(void)
 {
 	/* On panic, the console_lock must be left to the panic cpu. */
-	if (abandon_console_lock_in_panic())
+	if (other_cpu_in_panic())
 		return 0;
 	if (down_trylock_console_sem())
 		return 0;
@@ -2959,7 +2960,7 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 			any_progress = true;
 
 			/* Allow panic_cpu to take over the consoles safely. */
-			if (abandon_console_lock_in_panic())
+			if (other_cpu_in_panic())
 				goto abandon;
 
 			if (do_cond_resched)
-- 
2.40.1

[PATCH AUTOSEL 6.5 6/6] thermal/drivers/sun8i: Free calibration nvmem after reading it

[Sasha Levin]

From: Mark Brown <broonie@kernel.org>

[ Upstream commit c51592a95f360aabf2b8a5691c550e1749dc41eb ]

The sun8i thermal driver reads calibration data via the nvmem API at
startup, updating the device configuration and not referencing the data
again.  Rather than explicitly freeing the nvmem data the driver relies
on devm_ to release it, even though the data is never referenced again.
The allocation is still tracked so it's not leaked but this is notable
when looking at the code and is a little wasteful so let's instead
explicitly free the nvmem after we're done with it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230719-thermal-sun8i-free-nvmem-v1-1-f553d5afef79@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/thermal/sun8i_thermal.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/thermal/sun8i_thermal.c b/drivers/thermal/sun8i_thermal.c
index 195f3c5d0b388..af3098717e3c1 100644
--- a/drivers/thermal/sun8i_thermal.c
+++ b/drivers/thermal/sun8i_thermal.c
@@ -286,7 +286,7 @@ static int sun8i_ths_calibrate(struct ths_device *tmdev)
 	size_t callen;
 	int ret = 0;
 
-	calcell = devm_nvmem_cell_get(dev, "calibration");
+	calcell = nvmem_cell_get(dev, "calibration");
 	if (IS_ERR(calcell)) {
 		if (PTR_ERR(calcell) == -EPROBE_DEFER)
 			return -EPROBE_DEFER;
@@ -316,6 +316,8 @@ static int sun8i_ths_calibrate(struct ths_device *tmdev)
 
 	kfree(caldata);
 out:
+	if (!IS_ERR(calcell))
+		nvmem_cell_put(calcell);
 	return ret;
 }
 
-- 
2.40.1

Re: [PATCH AUTOSEL 6.5 6/6] thermal/drivers/sun8i: Free calibration nvmem after reading it

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 793 bytes --]

On Fri, Sep 08, 2023 at 09:12:54PM -0400, Sasha Levin wrote:
> From: Mark Brown <broonie@kernel.org>
> 
> [ Upstream commit c51592a95f360aabf2b8a5691c550e1749dc41eb ]
> 
> The sun8i thermal driver reads calibration data via the nvmem API at
> startup, updating the device configuration and not referencing the data
> again.  Rather than explicitly freeing the nvmem data the driver relies
> on devm_ to release it, even though the data is never referenced again.
> The allocation is still tracked so it's not leaked but this is notable
> when looking at the code and is a little wasteful so let's instead
> explicitly free the nvmem after we're done with it.

This is a minor cleanup which as with so much of what's come in today's
backports seems very questionable for stable.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH AUTOSEL 6.5 6/6] thermal/drivers/sun8i: Free calibration nvmem after reading it

[Sasha Levin]

On Sat, Sep 09, 2023 at 01:37:19PM +0100, Mark Brown wrote:
>On Fri, Sep 08, 2023 at 09:12:54PM -0400, Sasha Levin wrote:
>> From: Mark Brown <broonie@kernel.org>
>>
>> [ Upstream commit c51592a95f360aabf2b8a5691c550e1749dc41eb ]
>>
>> The sun8i thermal driver reads calibration data via the nvmem API at
>> startup, updating the device configuration and not referencing the data
>> again.  Rather than explicitly freeing the nvmem data the driver relies
>> on devm_ to release it, even though the data is never referenced again.
>> The allocation is still tracked so it's not leaked but this is notable
>> when looking at the code and is a little wasteful so let's instead
>> explicitly free the nvmem after we're done with it.
>
>This is a minor cleanup which as with so much of what's come in today's
>backports seems very questionable for stable.

I'll drop this, thanks!

-- 
Thanks,
Sasha