The patch below does not apply to the 4.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8126b1c73108bc691f5643df19071a59a69d0bc6 Mon Sep 17 00:00:00 2001 From: Jann Horn jannh@google.com Date: Mon, 14 Mar 2022 19:59:53 +0100 Subject: [PATCH] pstore: Don't use semaphores in always-atomic-context code

pstore_dump() is *always* invoked in atomic context (nowadays in an RCU read-side critical section, before that under a spinlock). It doesn't make sense to try to use semaphores here.

This is mostly a revert of commit ea84b580b955 ("pstore: Convert buf_lock to semaphore"), except that two parts aren't restored back exactly as they were:

- keep the lock initialization in pstore_register - in efi_pstore_write(), always set the "block" flag to false - omit "is_locked", that was unnecessary since commit 959217c84c27 ("pstore: Actually give up during locking failure") - fix the bailout message

The actual problem that the buggy commit was trying to address may have been that the use of preemptible() in efi_pstore_write() was wrong - it only looks at preempt_count() and the state of IRQs, but __rcu_read_lock() doesn't touch either of those under CONFIG_PREEMPT_RCU. (Sidenote: CONFIG_PREEMPT_RCU means that the scheduler can preempt tasks in RCU read-side critical sections, but you're not allowed to actively block/reschedule.)

Lockdep probably never caught the problem because it's very rare that you actually hit the contended case, so lockdep always just sees the down_trylock(), not the down_interruptible(), and so it can't tell that there's a problem.

Fixes: ea84b580b955 ("pstore: Convert buf_lock to semaphore") Cc: stable@vger.kernel.org Acked-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Kees Cook keescook@chromium.org Link: https://lore.kernel.org/r/20220314185953.2068993-1-jannh@google.com

diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c index 0ef086e43090..7e771c56c13c 100644 --- a/drivers/firmware/efi/efi-pstore.c +++ b/drivers/firmware/efi/efi-pstore.c @@ -266,7 +266,7 @@ static int efi_pstore_write(struct pstore_record *record) efi_name[i] = name[i];

ret = efivar_entry_set_safe(efi_name, vendor, PSTORE_EFI_ATTRIBUTES, - preemptible(), record->size, record->psi->buf); + false, record->size, record->psi->buf);

if (record->reason == KMSG_DUMP_OOPS && try_module_get(THIS_MODULE)) if (!schedule_work(&efivar_work)) diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c index f243cb5e6a4f..e26162f102ff 100644 --- a/fs/pstore/platform.c +++ b/fs/pstore/platform.c @@ -143,21 +143,22 @@ static void pstore_timer_kick(void) mod_timer(&pstore_timer, jiffies + msecs_to_jiffies(pstore_update_ms)); }

-/* - * Should pstore_dump() wait for a concurrent pstore_dump()? If - * not, the current pstore_dump() will report a failure to dump - * and return. - */ -static bool pstore_cannot_wait(enum kmsg_dump_reason reason) +static bool pstore_cannot_block_path(enum kmsg_dump_reason reason) { - /* In NMI path, pstore shouldn't block regardless of reason. */ + /* + * In case of NMI path, pstore shouldn't be blocked + * regardless of reason. + */ if (in_nmi()) return true;

switch (reason) { /* In panic case, other cpus are stopped by smp_send_stop(). */ case KMSG_DUMP_PANIC: - /* Emergency restart shouldn't be blocked. */ + /* + * Emergency restart shouldn't be blocked by spinning on + * pstore_info::buf_lock. + */ case KMSG_DUMP_EMERG: return true; default: @@ -389,21 +390,19 @@ static void pstore_dump(struct kmsg_dumper *dumper, unsigned long total = 0; const char *why; unsigned int part = 1; + unsigned long flags = 0; int ret;

why = kmsg_dump_reason_str(reason);

- if (down_trylock(&psinfo->buf_lock)) { - /* Failed to acquire lock: give up if we cannot wait. */ - if (pstore_cannot_wait(reason)) { - pr_err("dump skipped in %s path: may corrupt error record\n", - in_nmi() ? "NMI" : why); - return; - } - if (down_interruptible(&psinfo->buf_lock)) { - pr_err("could not grab semaphore?!\n"); + if (pstore_cannot_block_path(reason)) { + if (!spin_trylock_irqsave(&psinfo->buf_lock, flags)) { + pr_err("dump skipped in %s path because of concurrent dump\n", + in_nmi() ? "NMI" : why); return; } + } else { + spin_lock_irqsave(&psinfo->buf_lock, flags); }

kmsg_dump_rewind(&iter); @@ -467,8 +466,7 @@ static void pstore_dump(struct kmsg_dumper *dumper, total += record.size; part++; } - - up(&psinfo->buf_lock); + spin_unlock_irqrestore(&psinfo->buf_lock, flags); }

static struct kmsg_dumper pstore_dumper = { @@ -594,7 +592,7 @@ int pstore_register(struct pstore_info *psi) psi->write_user = pstore_write_user_compat; psinfo = psi; mutex_init(&psinfo->read_mutex); - sema_init(&psinfo->buf_lock, 1); + spin_lock_init(&psinfo->buf_lock);

if (psi->flags & PSTORE_FLAGS_DMESG) allocate_buf_for_compression(); diff --git a/include/linux/pstore.h b/include/linux/pstore.h index eb93a54cff31..e97a8188f0fd 100644 --- a/include/linux/pstore.h +++ b/include/linux/pstore.h @@ -14,7 +14,7 @@ #include <linux/errno.h> #include <linux/kmsg_dump.h> #include <linux/mutex.h> -#include <linux/semaphore.h> +#include <linux/spinlock.h> #include <linux/time.h> #include <linux/types.h>

@@ -87,7 +87,7 @@ struct pstore_record { * @owner: module which is responsible for this backend driver * @name: name of the backend driver * - * @buf_lock: semaphore to serialize access to @buf + * @buf_lock: spinlock to serialize access to @buf * @buf: preallocated crash dump buffer * @bufsize: size of @buf available for crash dump bytes (must match * smallest number of bytes available for writing to a @@ -178,7 +178,7 @@ struct pstore_info { struct module *owner; const char *name;

- struct semaphore buf_lock; + spinlock_t buf_lock; char *buf; size_t bufsize;