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    Subject[ 71/75] futex: Do not leak robust list to unprivileged process
    3.3-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Kees Cook <keescook@chromium.org>

    commit bdbb776f882f5ad431aa1e694c69c1c3d6a4a5b8 upstream.

    It was possible to extract the robust list head address from a setuid
    process if it had used set_robust_list(), allowing an ASLR info leak. This
    changes the permission checks to be the same as those used for similar
    info that comes out of /proc.

    Running a setuid program that uses robust futexes would have had:
    cred->euid != pcred->euid
    cred->euid == pcred->uid
    so the old permissions check would allow it. I'm not aware of any setuid
    programs that use robust futexes, so this is just a preventative measure.

    (This patch is based on changes from grsecurity.)

    Signed-off-by: Kees Cook <keescook@chromium.org>
    Cc: Darren Hart <dvhart@linux.intel.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Jiri Kosina <jkosina@suse.cz>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
    Cc: kernel-hardening@lists.openwall.com
    Cc: spender@grsecurity.net
    Link: http://lkml.kernel.org/r/20120319231253.GA20893@www.outflux.net
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/futex.c | 36 +++++++++++++-----------------------
    kernel/futex_compat.c | 36 +++++++++++++-----------------------
    2 files changed, 26 insertions(+), 46 deletions(-)

    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -59,6 +59,7 @@
    #include <linux/magic.h>
    #include <linux/pid.h>
    #include <linux/nsproxy.h>
    +#include <linux/ptrace.h>

    #include <asm/futex.h>

    @@ -2443,40 +2444,29 @@ SYSCALL_DEFINE3(get_robust_list, int, pi
    {
    struct robust_list_head __user *head;
    unsigned long ret;
    - const struct cred *cred = current_cred(), *pcred;
    + struct task_struct *p;

    if (!futex_cmpxchg_enabled)
    return -ENOSYS;

    + rcu_read_lock();
    +
    + ret = -ESRCH;
    if (!pid)
    - head = current->robust_list;
    + p = current;
    else {
    - struct task_struct *p;
    -
    - ret = -ESRCH;
    - rcu_read_lock();
    p = find_task_by_vpid(pid);
    if (!p)
    goto err_unlock;
    - ret = -EPERM;
    - pcred = __task_cred(p);
    - /* If victim is in different user_ns, then uids are not
    - comparable, so we must have CAP_SYS_PTRACE */
    - if (cred->user->user_ns != pcred->user->user_ns) {
    - if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
    - goto err_unlock;
    - goto ok;
    - }
    - /* If victim is in same user_ns, then uids are comparable */
    - if (cred->euid != pcred->euid &&
    - cred->euid != pcred->uid &&
    - !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
    - goto err_unlock;
    -ok:
    - head = p->robust_list;
    - rcu_read_unlock();
    }

    + ret = -EPERM;
    + if (!ptrace_may_access(p, PTRACE_MODE_READ))
    + goto err_unlock;
    +
    + head = p->robust_list;
    + rcu_read_unlock();
    +
    if (put_user(sizeof(*head), len_ptr))
    return -EFAULT;
    return put_user(head, head_ptr);
    --- a/kernel/futex_compat.c
    +++ b/kernel/futex_compat.c
    @@ -10,6 +10,7 @@
    #include <linux/compat.h>
    #include <linux/nsproxy.h>
    #include <linux/futex.h>
    +#include <linux/ptrace.h>

    #include <asm/uaccess.h>

    @@ -136,40 +137,29 @@ compat_sys_get_robust_list(int pid, comp
    {
    struct compat_robust_list_head __user *head;
    unsigned long ret;
    - const struct cred *cred = current_cred(), *pcred;
    + struct task_struct *p;

    if (!futex_cmpxchg_enabled)
    return -ENOSYS;

    + rcu_read_lock();
    +
    + ret = -ESRCH;
    if (!pid)
    - head = current->compat_robust_list;
    + p = current;
    else {
    - struct task_struct *p;
    -
    - ret = -ESRCH;
    - rcu_read_lock();
    p = find_task_by_vpid(pid);
    if (!p)
    goto err_unlock;
    - ret = -EPERM;
    - pcred = __task_cred(p);
    - /* If victim is in different user_ns, then uids are not
    - comparable, so we must have CAP_SYS_PTRACE */
    - if (cred->user->user_ns != pcred->user->user_ns) {
    - if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
    - goto err_unlock;
    - goto ok;
    - }
    - /* If victim is in same user_ns, then uids are comparable */
    - if (cred->euid != pcred->euid &&
    - cred->euid != pcred->uid &&
    - !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
    - goto err_unlock;
    -ok:
    - head = p->compat_robust_list;
    - rcu_read_unlock();
    }

    + ret = -EPERM;
    + if (!ptrace_may_access(p, PTRACE_MODE_READ))
    + goto err_unlock;
    +
    + head = p->compat_robust_list;
    + rcu_read_unlock();
    +
    if (put_user(sizeof(*head), len_ptr))
    return -EFAULT;
    return put_user(ptr_to_compat(head), head_ptr);



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