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futex: Make lookup_pi_state more robust

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The current implementation of lookup_pi_state has ambigous handling of
the TID value 0 in the user space futex.  We can get into the kernel
even if the TID value is 0, because either there is a stale waiters bit
or the owner died bit is set or we are called from the requeue_pi path
or from user space just for fun.

The current code avoids an explicit sanity check for pid = 0 in case
that kernel internal state (waiters) are found for the user space
address.  This can lead to state leakage and worse under some
circumstances.

Handle the cases explicit:

       Waiter | pi_state | pi->owner | uTID      | uODIED | ?

  [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
  [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid

  [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid

  [4]  Found  | Found    | NULL      | 0         | 1      | Valid
  [5]  Found  | Found    | NULL      | >0        | 1      | Invalid

  [6]  Found  | Found    | task      | 0         | 1      | Valid

  [7]  Found  | Found    | NULL      | Any       | 0      | Invalid

  [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
  [9]  Found  | Found    | task      | 0         | 0      | Invalid
  [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid

 [1] Indicates that the kernel can acquire the futex atomically. We
     came came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.

 [2] Valid, if TID does not belong to a kernel thread. If no matching
     thread is found then it indicates that the owner TID has died.

 [3] Invalid. The waiter is queued on a non PI futex

 [4] Valid state after exit_robust_list(), which sets the user space
     value to FUTEX_WAITERS | FUTEX_OWNER_DIED.

 [5] The user space value got manipulated between exit_robust_list()
     and exit_pi_state_list()

 [6] Valid state after exit_pi_state_list() which sets the new owner in
     the pi_state but cannot access the user space value.

 [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.

 [8] Owner and user space value match

 [9] There is no transient state which sets the user space TID to 0
     except exit_robust_list(), but this is indicated by the
     FUTEX_OWNER_DIED bit. See [4]

[10] There is no transient state which leaves owner and user space
     TID out of sync.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Drewry <wad@chromium.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Thomas Gleixner 2014-06-03 12:27:08 +00:00 committed by Linus Torvalds
parent 13fbca4c6e
commit 54a217887a
1 changed files with 106 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

134
kernel/futex.c
View File

@ -743,10 +743,58 @@ void exit_pi_state_list(struct task_struct *curr)
raw_spin_unlock_irq(&curr->pi_lock);
}
/*
* We need to check the following states:
*
* Waiter | pi_state | pi->owner | uTID | uODIED | ?
*
* [1] NULL | --- | --- | 0 | 0/1 | Valid
* [2] NULL | --- | --- | >0 | 0/1 | Valid
*
* [3] Found | NULL | -- | Any | 0/1 | Invalid
*
* [4] Found | Found | NULL | 0 | 1 | Valid
* [5] Found | Found | NULL | >0 | 1 | Invalid
*
* [6] Found | Found | task | 0 | 1 | Valid
*
* [7] Found | Found | NULL | Any | 0 | Invalid
*
* [8] Found | Found | task | ==taskTID | 0/1 | Valid
* [9] Found | Found | task | 0 | 0 | Invalid
* [10] Found | Found | task | !=taskTID | 0/1 | Invalid
*
* [1] Indicates that the kernel can acquire the futex atomically. We
* came came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
*
* [2] Valid, if TID does not belong to a kernel thread. If no matching
* thread is found then it indicates that the owner TID has died.
*
* [3] Invalid. The waiter is queued on a non PI futex
*
* [4] Valid state after exit_robust_list(), which sets the user space
* value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
*
* [5] The user space value got manipulated between exit_robust_list()
* and exit_pi_state_list()
*
* [6] Valid state after exit_pi_state_list() which sets the new owner in
* the pi_state but cannot access the user space value.
*
* [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.
*
* [8] Owner and user space value match
*
* [9] There is no transient state which sets the user space TID to 0
* except exit_robust_list(), but this is indicated by the
* FUTEX_OWNER_DIED bit. See [4]
*
* [10] There is no transient state which leaves owner and user space
* TID out of sync.
*/
static int
lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
union futex_key *key, struct futex_pi_state **ps,
struct task_struct *task)
union futex_key *key, struct futex_pi_state **ps)
{
struct futex_pi_state *pi_state = NULL;
struct futex_q *this, *next;
@ -756,12 +804,13 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
plist_for_each_entry_safe(this, next, &hb->chain, list) {
if (match_futex(&this->key, key)) {
/*
* Another waiter already exists - bump up
* the refcount and return its pi_state:
* Sanity check the waiter before increasing
* the refcount and attaching to it.
*/
pi_state = this->pi_state;
/*
* Userspace might have messed up non-PI and PI futexes
* Userspace might have messed up non-PI and
* PI futexes [3]
*/
if (unlikely(!pi_state))
return -EINVAL;
@ -769,44 +818,70 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
WARN_ON(!atomic_read(&pi_state->refcount));
/*
* When pi_state->owner is NULL then the owner died
* and another waiter is on the fly. pi_state->owner
* is fixed up by the task which acquires
* pi_state->rt_mutex.
*
* We do not check for pid == 0 which can happen when
* the owner died and robust_list_exit() cleared the
* TID.
* Handle the owner died case:
*/
if (pid && pi_state->owner) {
if (uval & FUTEX_OWNER_DIED) {
/*
* Bail out if user space manipulated the
* futex value.
* exit_pi_state_list sets owner to NULL and
* wakes the topmost waiter. The task which
* acquires the pi_state->rt_mutex will fixup
* owner.
*/
if (pid != task_pid_vnr(pi_state->owner))
if (!pi_state->owner) {
/*
* No pi state owner, but the user
* space TID is not 0. Inconsistent
* state. [5]
*/
if (pid)
return -EINVAL;
/*
* Take a ref on the state and
* return. [4]
*/
goto out_state;
}
/*
* If TID is 0, then either the dying owner
* has not yet executed exit_pi_state_list()
* or some waiter acquired the rtmutex in the
* pi state, but did not yet fixup the TID in
* user space.
*
* Take a ref on the state and return. [6]
*/
if (!pid)
goto out_state;
} else {
/*
* If the owner died bit is not set,
* then the pi_state must have an
* owner. [7]
*/
if (!pi_state->owner)
return -EINVAL;
}
/*
* Protect against a corrupted uval. If uval
* is 0x80000000 then pid is 0 and the waiter
* bit is set. So the deadlock check in the
* calling code has failed and we did not fall
* into the check above due to !pid.
* Bail out if user space manipulated the
* futex value. If pi state exists then the
* owner TID must be the same as the user
* space TID. [9/10]
*/
if (task && pi_state->owner == task)
return -EDEADLK;
if (pid != task_pid_vnr(pi_state->owner))
return -EINVAL;
out_state:
atomic_inc(&pi_state->refcount);
*ps = pi_state;
return 0;
}
}
/*
* We are the first waiter - try to look up the real owner and attach
* the new pi_state to it, but bail out when TID = 0
* the new pi_state to it, but bail out when TID = 0 [1]
*/
if (!pid)
return -ESRCH;
@ -839,6 +914,9 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
return ret;
}
/*
* No existing pi state. First waiter. [2]
*/
pi_state = alloc_pi_state();
/*
@ -959,7 +1037,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
* We dont have the lock. Look up the PI state (or create it if
* we are the first waiter):
*/
ret = lookup_pi_state(uval, hb, key, ps, task);
ret = lookup_pi_state(uval, hb, key, ps);
if (unlikely(ret)) {
switch (ret) {
@ -1565,7 +1643,7 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
* rereading and handing potential crap to
* lookup_pi_state.
*/
ret = lookup_pi_state(ret, hb2, &key2, &pi_state, NULL);
ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
}
switch (ret) {