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    Subject[tip:sched/core] sched/core: Convert sighand_struct.count to refcount_t
    Commit-ID:  d036bda7d0e7269c2982eb979acfef855f5d7977
    Gitweb: https://git.kernel.org/tip/d036bda7d0e7269c2982eb979acfef855f5d7977
    Author: Elena Reshetova <elena.reshetova@intel.com>
    AuthorDate: Fri, 18 Jan 2019 14:27:26 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 4 Feb 2019 08:53:52 +0100

    sched/core: Convert sighand_struct.count to refcount_t

    atomic_t variables are currently used to implement reference
    counters with the following properties:

    - counter is initialized to 1 using atomic_set()
    - a resource is freed upon counter reaching zero
    - once counter reaches zero, its further
    increments aren't allowed
    - counter schema uses basic atomic operations
    (set, inc, inc_not_zero, dec_and_test, etc.)

    Such atomic variables should be converted to a newly provided
    refcount_t type and API that prevents accidental counter overflows
    and underflows. This is important since overflows and underflows
    can lead to use-after-free situation and be exploitable.

    The variable sighand_struct.count is used as pure reference counter.
    Convert it to refcount_t and fix up the operations.

    ** Important note for maintainers:

    Some functions from refcount_t API defined in lib/refcount.c
    have different memory ordering guarantees than their atomic
    counterparts.

    The full comparison can be seen in
    https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
    in state to be merged to the documentation tree.

    Normally the differences should not matter since refcount_t provides
    enough guarantees to satisfy the refcounting use cases, but in
    some rare cases it might matter.

    Please double check that you don't have some undocumented
    memory guarantees for this variable usage.

    For the sighand_struct.count it might make a difference
    in following places:

    - __cleanup_sighand: decrement in refcount_dec_and_test() only
    provides RELEASE ordering and control dependency on success
    vs. fully ordered atomic counterpart

    Suggested-by: Kees Cook <keescook@chromium.org>
    Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: David Windsor <dwindsor@gmail.com>
    Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
    Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
    Reviewed-by: Oleg Nesterov <oleg@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: akpm@linux-foundation.org
    Cc: viro@zeniv.linux.org.uk
    Link: https://lkml.kernel.org/r/1547814450-18902-2-git-send-email-elena.reshetova@intel.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    fs/exec.c | 4 ++--
    fs/proc/task_nommu.c | 2 +-
    include/linux/sched/signal.h | 3 ++-
    kernel/fork.c | 8 ++++----
    4 files changed, 9 insertions(+), 8 deletions(-)

    diff --git a/fs/exec.c b/fs/exec.c
    index fb72d36f7823..966cd98a2ce2 100644
    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -1189,7 +1189,7 @@ no_thread_group:
    flush_itimer_signals();
    #endif

    - if (atomic_read(&oldsighand->count) != 1) {
    + if (refcount_read(&oldsighand->count) != 1) {
    struct sighand_struct *newsighand;
    /*
    * This ->sighand is shared with the CLONE_SIGHAND
    @@ -1199,7 +1199,7 @@ no_thread_group:
    if (!newsighand)
    return -ENOMEM;

    - atomic_set(&newsighand->count, 1);
    + refcount_set(&newsighand->count, 1);
    memcpy(newsighand->action, oldsighand->action,
    sizeof(newsighand->action));

    diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
    index 0b63d68dedb2..f912872fbf91 100644
    --- a/fs/proc/task_nommu.c
    +++ b/fs/proc/task_nommu.c
    @@ -64,7 +64,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
    else
    bytes += kobjsize(current->files);

    - if (current->sighand && atomic_read(&current->sighand->count) > 1)
    + if (current->sighand && refcount_read(&current->sighand->count) > 1)
    sbytes += kobjsize(current->sighand);
    else
    bytes += kobjsize(current->sighand);
    diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
    index 13789d10a50e..37eeb1a28eba 100644
    --- a/include/linux/sched/signal.h
    +++ b/include/linux/sched/signal.h
    @@ -8,13 +8,14 @@
    #include <linux/sched/jobctl.h>
    #include <linux/sched/task.h>
    #include <linux/cred.h>
    +#include <linux/refcount.h>

    /*
    * Types defining task->signal and task->sighand and APIs using them:
    */

    struct sighand_struct {
    - atomic_t count;
    + refcount_t count;
    struct k_sigaction action[_NSIG];
    spinlock_t siglock;
    wait_queue_head_t signalfd_wqh;
    diff --git a/kernel/fork.c b/kernel/fork.c
    index b69248e6f0e0..370856d4c0b3 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -1463,7 +1463,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
    struct sighand_struct *sig;

    if (clone_flags & CLONE_SIGHAND) {
    - atomic_inc(&current->sighand->count);
    + refcount_inc(&current->sighand->count);
    return 0;
    }
    sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
    @@ -1471,7 +1471,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
    if (!sig)
    return -ENOMEM;

    - atomic_set(&sig->count, 1);
    + refcount_set(&sig->count, 1);
    spin_lock_irq(&current->sighand->siglock);
    memcpy(sig->action, current->sighand->action, sizeof(sig->action));
    spin_unlock_irq(&current->sighand->siglock);
    @@ -1480,7 +1480,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)

    void __cleanup_sighand(struct sighand_struct *sighand)
    {
    - if (atomic_dec_and_test(&sighand->count)) {
    + if (refcount_dec_and_test(&sighand->count)) {
    signalfd_cleanup(sighand);
    /*
    * sighand_cachep is SLAB_TYPESAFE_BY_RCU so we can free it
    @@ -2439,7 +2439,7 @@ static int check_unshare_flags(unsigned long unshare_flags)
    return -EINVAL;
    }
    if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
    - if (atomic_read(&current->sighand->count) > 1)
    + if (refcount_read(&current->sighand->count) > 1)
    return -EINVAL;
    }
    if (unshare_flags & CLONE_VM) {
    \
     
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