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    SubjectRe: [PATCH] futex: do not leak robust list to unprivileged process
    Quoting Kees Cook (keescook@chromium.org):
    > 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>

    I like the change. Much cleaner. I'm not 100% sure though that
    there are no legitimate cases of robust futexes use which would now
    be forbidden. (Explicitly cc:ing Ingo)

    Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

    > ---
    > kernel/futex.c | 36 +++++++++++++-----------------------
    > kernel/futex_compat.c | 36 +++++++++++++-----------------------
    > 2 files changed, 26 insertions(+), 46 deletions(-)
    >
    > diff --git a/kernel/futex.c b/kernel/futex.c
    > index 1614be2..439440d 100644
    > --- 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, pid,
    > {
    > 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);
    > diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
    > index 5f9e689..a9642d5 100644
    > --- 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, compat_uptr_t __user *head_ptr,
    > {
    > 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);
    > --
    > 1.7.0.4
    >


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