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pstore: Don't use semaphores in always-atomic-context code

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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 ea84b580b9 ("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 959217c84c ("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: ea84b580b9 ("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
This commit is contained in:
Jann Horn 2022-03-14 19:59:53 +01:00 committed by Kees Cook
parent 023bbde3db
commit 8126b1c731
3 changed files with 22 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
drivers/firmware/efi/efi-pstore.c
View File

@ -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))

38
fs/pstore/platform.c
View File

@ -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();

6
include/linux/pstore.h
View File

@ -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;