It's very easy to crash the kernel right now by simply trying to enable memtrace concurrently, hammering on the "enable" interface

loop.sh: #!/bin/bash

dmesg --console-off

while true; do echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable done

[root@localhost ~]# loop.sh & [root@localhost ~]# loop.sh &

Resulting quickly in a kernel crash. Let's properly protect using a mutex.

Fixes: 9d5171a8f248 ("powerpc/powernv: Enable removal of memory for in memory tracing") Cc: stable@vger.kernel.org# v4.14+ Cc: Michael Ellerman mpe@ellerman.id.au Cc: Benjamin Herrenschmidt benh@kernel.crashing.org Cc: Paul Mackerras paulus@samba.org Cc: Rashmica Gupta rashmica.g@gmail.com Signed-off-by: David Hildenbrand david@redhat.com --- arch/powerpc/platforms/powernv/memtrace.c | 22 +++++++++++++++------- 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/memtrace.c b/arch/powerpc/platforms/powernv/memtrace.c index eea1f94482ff..0e42fe2d7b6a 100644 --- a/arch/powerpc/platforms/powernv/memtrace.c +++ b/arch/powerpc/platforms/powernv/memtrace.c @@ -30,6 +30,7 @@ struct memtrace_entry { char name[16]; };

+static DEFINE_MUTEX(memtrace_mutex); static u64 memtrace_size;

static struct memtrace_entry *memtrace_array; @@ -279,6 +280,7 @@ static int memtrace_online(void)

static int memtrace_enable_set(void *data, u64 val) { + int rc = -EAGAIN; u64 bytes;

/* @@ -291,25 +293,31 @@ static int memtrace_enable_set(void *data, u64 val) return -EINVAL; }

+ mutex_lock(&memtrace_mutex); + /* Re-add/online previously removed/offlined memory */ if (memtrace_size) { if (memtrace_online()) - return -EAGAIN; + goto out_unlock; }

- if (!val) - return 0; + if (!val) { + rc = 0; + goto out_unlock; + }

/* Offline and remove memory */ if (memtrace_init_regions_runtime(val)) - return -EINVAL; + goto out_unlock;

if (memtrace_init_debugfs()) - return -EINVAL; + goto out_unlock;

memtrace_size = val; - - return 0; + rc = 0; +out_unlock: + mutex_unlock(&memtrace_mutex); + return rc; }

static int memtrace_enable_get(void *data, u64 *val)