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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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