Re: [PATCH] ptrace: use fsuid, fsgid, effective creds for fs access checks

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On Sat, Dec 5, 2015 at 6:04 PM, Jann Horn <jann@xxxxxxxxx> wrote:
> By checking the effective credentials instead of the real UID /
> permitted capabilities, ensure that the calling process actually
> intended to use its credentials.
>
> To ensure that all ptrace checks use the correct caller
> credentials (e.g. in case out-of-tree code or newly added code
> omits the PTRACE_MODE_*CREDS flag), use two new flags and
> require one of them to be set.
>
> The problem was that when a privileged task had temporarily dropped
> its privileges, e.g. by calling setreuid(0, user_uid), with the
> intent to perform following syscalls with the credentials of
> a user, it still passed ptrace access checks that the user would
> not be able to pass.
>
> While an attacker should not be able to convince the privileged
> task to perform a ptrace() syscall, this is a problem because the
> ptrace access check is reused for things in procfs.
>
> In particular, the following somewhat interesting procfs entries
> only rely on ptrace access checks:
>
>  /proc/$pid/stat - uses the check for determining whether pointers
>      should be visible, useful for bypassing ASLR
>  /proc/$pid/maps - also useful for bypassing ASLR
>  /proc/$pid/cwd - useful for gaining access to restricted
>      directories that contain files with lax permissions, e.g. in
>      this scenario:
>      lrwxrwxrwx root root /proc/13020/cwd -> /root/foobar
>      drwx------ root root /root
>      drwxr-xr-x root root /root/foobar
>      -rw-r--r-- root root /root/foobar/secret
>
> Therefore, on a system where a root-owned mode 6755 binary
> changes its effective credentials as described and then dumps a
> user-specified file, this could be used by an attacker to reveal
> the memory layout of root's processes or reveal the contents of
> files he is not allowed to access (through /proc/$pid/cwd).
>
> Signed-off-by: Jann Horn <jann@xxxxxxxxx>
> ---
>  fs/proc/array.c        |  2 +-
>  fs/proc/base.c         | 21 +++++++++++----------
>  fs/proc/namespaces.c   |  4 ++--
>  include/linux/ptrace.h | 24 +++++++++++++++++++++++-
>  kernel/events/core.c   |  2 +-
>  kernel/futex.c         |  2 +-
>  kernel/futex_compat.c  |  2 +-
>  kernel/kcmp.c          |  4 ++--
>  kernel/ptrace.c        | 36 +++++++++++++++++++++++++++++-------
>  mm/process_vm_access.c |  2 +-
>  security/commoncap.c   |  7 ++++++-
>  11 files changed, 78 insertions(+), 28 deletions(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index d73291f..b6c00ce 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -395,7 +395,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>
>         state = *get_task_state(task);
>         vsize = eip = esp = 0;
> -       permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
> +       permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS | PTRACE_MODE_NOAUDIT);
>         mm = get_task_mm(task);
>         if (mm) {
>                 vsize = task_vsize(mm);
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index bd3e9e6..c0a2f29 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -403,7 +403,7 @@ static const struct file_operations proc_pid_cmdline_ops = {
>  static int proc_pid_auxv(struct seq_file *m, struct pid_namespace *ns,
>                          struct pid *pid, struct task_struct *task)
>  {
> -       struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ);
> +       struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
>         if (mm && !IS_ERR(mm)) {
>                 unsigned int nwords = 0;
>                 do {
> @@ -430,7 +430,8 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>
>         wchan = get_wchan(task);
>
> -       if (wchan && ptrace_may_access(task, PTRACE_MODE_READ) && !lookup_symbol_name(wchan, symname))
> +       if (wchan && ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
> +                       && !lookup_symbol_name(wchan, symname))
>                 seq_printf(m, "%s", symname);
>         else
>                 seq_putc(m, '0');
> @@ -444,7 +445,7 @@ static int lock_trace(struct task_struct *task)
>         int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
>         if (err)
>                 return err;
> -       if (!ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
> +       if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_FSCREDS)) {
>                 mutex_unlock(&task->signal->cred_guard_mutex);
>                 return -EPERM;
>         }
> @@ -697,7 +698,7 @@ static int proc_fd_access_allowed(struct inode *inode)
>          */
>         task = get_proc_task(inode);
>         if (task) {
> -               allowed = ptrace_may_access(task, PTRACE_MODE_READ);
> +               allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
>                 put_task_struct(task);
>         }
>         return allowed;
> @@ -732,7 +733,7 @@ static bool has_pid_permissions(struct pid_namespace *pid,
>                 return true;
>         if (in_group_p(pid->pid_gid))
>                 return true;
> -       return ptrace_may_access(task, PTRACE_MODE_READ);
> +       return ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
>  }
>
>
> @@ -809,7 +810,7 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
>         struct mm_struct *mm = ERR_PTR(-ESRCH);
>
>         if (task) {
> -               mm = mm_access(task, mode);
> +               mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
>                 put_task_struct(task);
>
>                 if (!IS_ERR_OR_NULL(mm)) {
> @@ -1856,7 +1857,7 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
>         if (!task)
>                 goto out_notask;
>
> -       mm = mm_access(task, PTRACE_MODE_READ);
> +       mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
>         if (IS_ERR_OR_NULL(mm))
>                 goto out;
>
> @@ -2007,7 +2008,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
>                 goto out;
>
>         result = -EACCES;
> -       if (!ptrace_may_access(task, PTRACE_MODE_READ))
> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
>                 goto out_put_task;
>
>         result = -ENOENT;
> @@ -2060,7 +2061,7 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
>                 goto out;
>
>         ret = -EACCES;
> -       if (!ptrace_may_access(task, PTRACE_MODE_READ))
> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
>                 goto out_put_task;
>
>         ret = 0;
> @@ -2529,7 +2530,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
>         if (result)
>                 return result;
>
> -       if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
>                 result = -EACCES;
>                 goto out_unlock;
>         }
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index f6e8354..0cbe012 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -42,7 +42,7 @@ static const char *proc_ns_follow_link(struct dentry *dentry, void **cookie)
>         if (!task)
>                 return error;
>
> -       if (ptrace_may_access(task, PTRACE_MODE_READ)) {
> +       if (ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)) {

This should maybe use the PTRACE_MODE_READ_FSCREDS macro?

>                 error = ns_get_path(&ns_path, task, ns_ops);
>                 if (!error)
>                         nd_jump_link(&ns_path);
> @@ -63,7 +63,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
>         if (!task)
>                 return res;
>
> -       if (ptrace_may_access(task, PTRACE_MODE_READ)) {
> +       if (ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)) {

same here?

>                 res = ns_get_name(name, sizeof(name), task, ns_ops);
>                 if (res >= 0)
>                         res = readlink_copy(buffer, buflen, name);
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index 061265f..504c98a 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -57,7 +57,29 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
>  #define PTRACE_MODE_READ       0x01
>  #define PTRACE_MODE_ATTACH     0x02
>  #define PTRACE_MODE_NOAUDIT    0x04
> -/* Returns true on success, false on denial. */
> +#define PTRACE_MODE_FSCREDS 0x08
> +#define PTRACE_MODE_REALCREDS 0x10
> +
> +/* shorthands for READ/ATTACH and FSCREDS/REALCREDS combinations */
> +#define PTRACE_MODE_READ_FSCREDS (PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)
> +#define PTRACE_MODE_READ_REALCREDS (PTRACE_MODE_READ | PTRACE_MODE_REALCREDS)
> +#define PTRACE_MODE_ATTACH_FSCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS)
> +#define PTRACE_MODE_ATTACH_REALCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS)
> +
> +/**
> + * ptrace_may_access - check whether the caller is permitted to access
> + * a target task.
> + * @task: target task
> + * @mode: selects type of access and caller credentials
> + *
> + * Returns true on success, false on denial.
> + *
> + * One of the flags PTRACE_MODE_FSCREDS and PTRACE_MODE_REALCREDS must
> + * be set in @mode to specify whether the access was requested through
> + * a filesystem syscall (should use effective capabilities and fsuid
> + * of the caller) or through an explicit syscall such as
> + * process_vm_writev or ptrace (and should use the real credentials).
> + */
>  extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
>
>  static inline int ptrace_reparented(struct task_struct *child)
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 36babfd..565e41a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3430,7 +3430,7 @@ find_lively_task_by_vpid(pid_t vpid)
>
>         /* Reuse ptrace permission checks for now. */
>         err = -EACCES;
> -       if (!ptrace_may_access(task, PTRACE_MODE_READ))
> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
>                 goto errout;
>
>         return task;
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 684d754..495a1d0 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2881,7 +2881,7 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
>         }
>
>         ret = -EPERM;
> -       if (!ptrace_may_access(p, PTRACE_MODE_READ))
> +       if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
>                 goto err_unlock;
>
>         head = p->robust_list;
> diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
> index 55c8c93..4ae3232 100644
> --- a/kernel/futex_compat.c
> +++ b/kernel/futex_compat.c
> @@ -155,7 +155,7 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
>         }
>
>         ret = -EPERM;
> -       if (!ptrace_may_access(p, PTRACE_MODE_READ))
> +       if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
>                 goto err_unlock;
>
>         head = p->compat_robust_list;
> diff --git a/kernel/kcmp.c b/kernel/kcmp.c
> index 0aa69ea..3a47fa9 100644
> --- a/kernel/kcmp.c
> +++ b/kernel/kcmp.c
> @@ -122,8 +122,8 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
>                         &task2->signal->cred_guard_mutex);
>         if (ret)
>                 goto err;
> -       if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
> -           !ptrace_may_access(task2, PTRACE_MODE_READ)) {
> +       if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
> +           !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
>                 ret = -EPERM;
>                 goto err_unlock;
>         }
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index b760bae..21aef51 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -219,6 +219,13 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
>  static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  {
>         const struct cred *cred = current_cred(), *tcred;
> +       kuid_t caller_uid;
> +       kgid_t caller_gid;
> +
> +       if (!(mode & PTRACE_MODE_FSCREDS) == !(mode & PTRACE_MODE_REALCREDS)) {
> +               WARN(1, "denying ptrace access check without PTRACE_MODE_*CREDS\n");
> +               return -EPERM;
> +       }
>
>         /* May we inspect the given task?
>          * This check is used both for attaching with ptrace
> @@ -233,13 +240,28 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>         if (same_thread_group(task, current))
>                 return 0;
>         rcu_read_lock();
> +       if (mode & PTRACE_MODE_FSCREDS) {
> +               caller_uid = cred->fsuid;
> +               caller_gid = cred->fsgid;
> +       } else {
> +               /*
> +                * Using the euid would make more sense here, but something
> +                * in userland might rely on the old behavior, and this
> +                * shouldn't be a security problem since
> +                * PTRACE_MODE_REALCREDS implies that the caller explicitly
> +                * used a syscall that requests access to another process
> +                * (and not a filesystem syscall to procfs).
> +                */
> +               caller_uid = cred->uid;
> +               caller_gid = cred->gid;
> +       }
>         tcred = __task_cred(task);
> -       if (uid_eq(cred->uid, tcred->euid) &&
> -           uid_eq(cred->uid, tcred->suid) &&
> -           uid_eq(cred->uid, tcred->uid)  &&
> -           gid_eq(cred->gid, tcred->egid) &&
> -           gid_eq(cred->gid, tcred->sgid) &&
> -           gid_eq(cred->gid, tcred->gid))
> +       if (uid_eq(caller_uid, tcred->euid) &&
> +           uid_eq(caller_uid, tcred->suid) &&
> +           uid_eq(caller_uid, tcred->uid)  &&
> +           gid_eq(caller_gid, tcred->egid) &&
> +           gid_eq(caller_gid, tcred->sgid) &&
> +           gid_eq(caller_gid, tcred->gid))
>                 goto ok;
>         if (ptrace_has_cap(tcred->user_ns, mode))
>                 goto ok;
> @@ -306,7 +328,7 @@ static int ptrace_attach(struct task_struct *task, long request,
>                 goto out;
>
>         task_lock(task);
> -       retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
> +       retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
>         task_unlock(task);
>         if (retval)
>                 goto unlock_creds;
> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
> index e88d071..5d453e5 100644
> --- a/mm/process_vm_access.c
> +++ b/mm/process_vm_access.c
> @@ -194,7 +194,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter,
>                 goto free_proc_pages;
>         }
>
> -       mm = mm_access(task, PTRACE_MODE_ATTACH);
> +       mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS);
>         if (!mm || IS_ERR(mm)) {
>                 rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
>                 /*
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 1832cf7..48071ed 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -137,12 +137,17 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
>  {
>         int ret = 0;
>         const struct cred *cred, *child_cred;
> +       const kernel_cap_t *caller_caps;
>
>         rcu_read_lock();
>         cred = current_cred();
>         child_cred = __task_cred(child);
> +       if (mode & PTRACE_MODE_FSCREDS)
> +               caller_caps = &cred->cap_effective;
> +       else
> +               caller_caps = &cred->cap_permitted;
>         if (cred->user_ns == child_cred->user_ns &&
> -           cap_issubset(child_cred->cap_permitted, cred->cap_permitted))
> +           cap_issubset(child_cred->cap_permitted, *caller_caps))
>                 goto out;
>         if (ns_capable(child_cred->user_ns, CAP_SYS_PTRACE))
>                 goto out;
> --
> 2.1.4
>

Thanks for working on this! I think it's done after the macro fix.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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