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    Subject[PATCH] futex: futex_find_get_task remove credentails check
    On Wed 30-06-10 09:01:15, Michal Hocko wrote:
    > On Tue 29-06-10 09:41:02, Linus Torvalds wrote:
    > > On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko <mhocko@suse.cz> wrote:
    > > >
    > > > futex_find_get_task is currently used (through lookup_pi_state) from two
    > > > contexts, futex_requeue and futex_lock_pi_atomic. While credentials check
    > > > makes sense in the first code path, the second one is more problematic
    > > > because this check requires that the PI lock holder (pid parameter) has
    > > > the same uid and euid as the process's euid which is trying to lock the
    > > > same futex (current).
    > >
    > > So exactly why does it make sense to check the credentials in the
    > > first code path then?
    >
    > I though that requeue needs this for security reasons (don't let requeue
    > process for other user), but when I thought about that again you are
    > right and the only what matters should be accessibility of the shared
    > memory.

    And here is the patch which does the thing.

    --

    From 082c5ad2c482a8e78b61b17e213e750b006176aa Mon Sep 17 00:00:00 2001
    From: Michal Hocko <mhocko@suse.cz>
    Date: Wed, 30 Jun 2010 09:51:19 +0200
    Subject: [PATCH] futex: futex_find_get_task remove credentails check

    futex_find_get_task is currently used (through lookup_pi_state) from two
    contexts, futex_requeue and futex_lock_pi_atomic. None of the paths
    looks it needs the credentials check, though. Different (e)uids
    shouldn't matter at all because the only thing that is important for
    shared futex is the accessibility of the shared memory.

    The credentail check results in glibc assert failure or process hang (if
    glibc is compiled without assert support) for shared robust pthread
    mutex with priority inheritance if a process tries to lock already held
    lock owned by a process with a different euid:

    pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.

    The problem is that futex_lock_pi_atomic which is called when we try to
    lock already held lock checks the current holder (tid is stored in the
    futex value) to get the PI state. It uses lookup_pi_state which in turn
    gets task struct from futex_find_get_task. ESRCH is returned either when
    the task is not found or if credentials check fails.
    futex_lock_pi_atomic simply returns if it gets ESRCH. glibc code,
    however, doesn't expect that robust lock returns with ESRCH because it
    should get either success or owner died.

    Signed-off-by: Michal Hocko <mhocko@suse.cz>
    ---
    kernel/futex.c | 17 ++++-------------
    1 files changed, 4 insertions(+), 13 deletions(-)

    diff --git a/kernel/futex.c b/kernel/futex.c
    index e7a35f1..6a3a5fa 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -429,20 +429,11 @@ static void free_pi_state(struct futex_pi_state *pi_state)
    static struct task_struct * futex_find_get_task(pid_t pid)
    {
    struct task_struct *p;
    - const struct cred *cred = current_cred(), *pcred;

    rcu_read_lock();
    p = find_task_by_vpid(pid);
    - if (!p) {
    - p = ERR_PTR(-ESRCH);
    - } else {
    - pcred = __task_cred(p);
    - if (cred->euid != pcred->euid &&
    - cred->euid != pcred->uid)
    - p = ERR_PTR(-ESRCH);
    - else
    - get_task_struct(p);
    - }
    + if (p)
    + get_task_struct(p);

    rcu_read_unlock();

    @@ -564,8 +555,8 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
    if (!pid)
    return -ESRCH;
    p = futex_find_get_task(pid);
    - if (IS_ERR(p))
    - return PTR_ERR(p);
    + if (!p)
    + return -ESRCH;

    /*
    * We need to look at the task state flags to figure out,
    --
    1.7.1

    --
    Michal Hocko
    L3 team
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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