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ptrace: use fsuid, fsgid, effective creds for fs access checks

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commit caaee6234d05a58c5b4d05e7bf766131b810a657 upstream.

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).

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Jann Horn 2016-01-20 15:00:04 -08:00 committed by Greg Kroah-Hartman
parent 05c5582559
commit 414f6fbc84
11 changed files with 79 additions and 29 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
fs/proc/array.c
View file

@ -398,7 +398,7 @@ 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);

20
fs/proc/base.c
View file

@ -239,7 +239,7 @@ out:
static int proc_pid_auxv(struct task_struct *task, char *buffer)
{
struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ);
struct mm_struct *mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
int res = PTR_ERR(mm);
if (mm && !IS_ERR(mm)) {
unsigned int nwords = 0;
@ -269,7 +269,7 @@ static int proc_pid_wchan(struct task_struct *task, char *buffer)
wchan = get_wchan(task);
if (lookup_symbol_name(wchan, symname) < 0)
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
return 0;
else
return sprintf(buffer, "%lu", wchan);
@ -283,7 +283,7 @@ 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;
}
@ -557,7 +557,7 @@ 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;
@ -592,7 +592,7 @@ 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);
}
@ -707,7 +707,7 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
if (!task)
return -ESRCH;
mm = mm_access(task, mode);
mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
put_task_struct(task);
if (IS_ERR(mm))
@ -1761,7 +1761,7 @@ 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;
@ -1896,7 +1896,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
goto out;
result = ERR_PTR(-EACCES);
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out_put_task;
result = ERR_PTR(-ENOENT);
@ -1952,7 +1952,7 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
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;
@ -2488,7 +2488,7 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
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;
}

4
fs/proc/namespaces.c
View file

@ -125,7 +125,7 @@ static void *proc_ns_follow_link(struct dentry *dentry, struct nameidata *nd)
if (!task)
goto out;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out_put_task;
ns_path.dentry = proc_ns_get_dentry(sb, task, ei->ns.ns_ops);
@ -158,7 +158,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
if (!task)
goto out;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out_put_task;
len = -ENOENT;

24
include/linux/ptrace.h
View file

@ -56,7 +56,29 @@ extern void exit_ptrace(struct task_struct *tracer);
#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)

2
kernel/events/core.c
View file

@ -2938,7 +2938,7 @@ 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;

2
kernel/futex.c
View file

@ -2620,7 +2620,7 @@ 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;

2
kernel/futex_compat.c
View file

@ -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;

4
kernel/kcmp.c
View file

@ -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;
}

39
kernel/ptrace.c
View file

@ -225,6 +225,14 @@ 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;
int dumpable = 0;
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
@ -234,18 +242,33 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
* because setting up the necessary parent/child relationship
* or halting the specified task is impossible.
*/
int dumpable = 0;
/* Don't let security modules deny introspection */
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;
@ -312,7 +335,7 @@ 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;

2
mm/process_vm_access.c
View file

@ -298,7 +298,7 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
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;
/*

7
security/commoncap.c
View file

@ -142,12 +142,17 @@ 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;