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Subject[PATCH 14/15] kernel: convert futex_pi_state.refcount from atomic_t to refcount_t
Date
refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
kernel/futex.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 16dbe4c..1cc7641 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -67,6 +67,7 @@
#include <linux/freezer.h>
#include <linux/bootmem.h>
#include <linux/fault-inject.h>
+#include <linux/refcount.h>

#include <asm/futex.h>

@@ -209,7 +210,7 @@ struct futex_pi_state {
struct rt_mutex pi_mutex;

struct task_struct *owner;
- atomic_t refcount;
+ refcount_t refcount;

union futex_key key;
} __randomize_layout;
@@ -794,7 +795,7 @@ static int refill_pi_state_cache(void)
INIT_LIST_HEAD(&pi_state->list);
/* pi_mutex gets initialized later */
pi_state->owner = NULL;
- atomic_set(&pi_state->refcount, 1);
+ refcount_set(&pi_state->refcount, 1);
pi_state->key = FUTEX_KEY_INIT;

current->pi_state_cache = pi_state;
@@ -814,7 +815,7 @@ static struct futex_pi_state *alloc_pi_state(void)

static void get_pi_state(struct futex_pi_state *pi_state)
{
- WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
+ refcount_inc(&pi_state->refcount);
}

/*
@@ -828,7 +829,7 @@ static void put_pi_state(struct futex_pi_state *pi_state)
if (!pi_state)
return;

- if (!atomic_dec_and_test(&pi_state->refcount))
+ if (!refcount_dec_and_test(&pi_state->refcount))
return;

/*
@@ -852,7 +853,7 @@ static void put_pi_state(struct futex_pi_state *pi_state)
* refcount is at 0 - put it back to 1.
*/
pi_state->owner = NULL;
- atomic_set(&pi_state->refcount, 1);
+ refcount_set(&pi_state->refcount, 1);
current->pi_state_cache = pi_state;
}
}
@@ -1046,7 +1047,7 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
* and futex_wait_requeue_pi() as it cannot go to 0 and consequently
* free pi_state before we can take a reference ourselves.
*/
- WARN_ON(!atomic_read(&pi_state->refcount));
+ WARN_ON(!refcount_read(&pi_state->refcount));

/*
* Now that we have a pi_state, we can acquire wait_lock
--
2.7.4
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