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![Jann Horn](/assets/img/avatar_default.png)
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>
198 lines
4.4 KiB
C
198 lines
4.4 KiB
C
#include <linux/kernel.h>
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#include <linux/syscalls.h>
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#include <linux/fdtable.h>
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#include <linux/string.h>
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#include <linux/random.h>
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#include <linux/module.h>
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#include <linux/ptrace.h>
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#include <linux/init.h>
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#include <linux/errno.h>
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#include <linux/cache.h>
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#include <linux/bug.h>
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#include <linux/err.h>
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#include <linux/kcmp.h>
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#include <asm/unistd.h>
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/*
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* We don't expose the real in-memory order of objects for security reasons.
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* But still the comparison results should be suitable for sorting. So we
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* obfuscate kernel pointers values and compare the production instead.
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*
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* The obfuscation is done in two steps. First we xor the kernel pointer with
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* a random value, which puts pointer into a new position in a reordered space.
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* Secondly we multiply the xor production with a large odd random number to
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* permute its bits even more (the odd multiplier guarantees that the product
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* is unique ever after the high bits are truncated, since any odd number is
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* relative prime to 2^n).
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*
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* Note also that the obfuscation itself is invisible to userspace and if needed
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* it can be changed to an alternate scheme.
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*/
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static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
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static long kptr_obfuscate(long v, int type)
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{
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return (v ^ cookies[type][0]) * cookies[type][1];
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}
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/*
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* 0 - equal, i.e. v1 = v2
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* 1 - less than, i.e. v1 < v2
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* 2 - greater than, i.e. v1 > v2
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* 3 - not equal but ordering unavailable (reserved for future)
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*/
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static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
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{
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long t1, t2;
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t1 = kptr_obfuscate((long)v1, type);
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t2 = kptr_obfuscate((long)v2, type);
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return (t1 < t2) | ((t1 > t2) << 1);
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}
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/* The caller must have pinned the task */
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static struct file *
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get_file_raw_ptr(struct task_struct *task, unsigned int idx)
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{
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struct file *file = NULL;
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task_lock(task);
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rcu_read_lock();
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if (task->files)
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file = fcheck_files(task->files, idx);
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rcu_read_unlock();
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task_unlock(task);
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return file;
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}
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static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
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{
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if (likely(m2 != m1))
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mutex_unlock(m2);
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mutex_unlock(m1);
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}
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static int kcmp_lock(struct mutex *m1, struct mutex *m2)
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{
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int err;
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if (m2 > m1)
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swap(m1, m2);
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err = mutex_lock_killable(m1);
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if (!err && likely(m1 != m2)) {
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err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
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if (err)
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mutex_unlock(m1);
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}
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return err;
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}
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SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
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unsigned long, idx1, unsigned long, idx2)
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{
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struct task_struct *task1, *task2;
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int ret;
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rcu_read_lock();
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/*
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* Tasks are looked up in caller's PID namespace only.
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*/
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task1 = find_task_by_vpid(pid1);
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task2 = find_task_by_vpid(pid2);
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if (!task1 || !task2)
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goto err_no_task;
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get_task_struct(task1);
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get_task_struct(task2);
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rcu_read_unlock();
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/*
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* One should have enough rights to inspect task details.
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*/
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ret = kcmp_lock(&task1->signal->cred_guard_mutex,
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&task2->signal->cred_guard_mutex);
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if (ret)
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goto err;
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if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
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!ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
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ret = -EPERM;
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goto err_unlock;
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}
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switch (type) {
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case KCMP_FILE: {
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struct file *filp1, *filp2;
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filp1 = get_file_raw_ptr(task1, idx1);
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filp2 = get_file_raw_ptr(task2, idx2);
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if (filp1 && filp2)
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ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
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else
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ret = -EBADF;
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break;
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}
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case KCMP_VM:
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ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
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break;
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case KCMP_FILES:
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ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
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break;
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case KCMP_FS:
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ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
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break;
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case KCMP_SIGHAND:
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ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
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break;
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case KCMP_IO:
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ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
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break;
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case KCMP_SYSVSEM:
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#ifdef CONFIG_SYSVIPC
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ret = kcmp_ptr(task1->sysvsem.undo_list,
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task2->sysvsem.undo_list,
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KCMP_SYSVSEM);
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#else
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ret = -EOPNOTSUPP;
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#endif
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break;
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default:
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ret = -EINVAL;
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break;
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}
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err_unlock:
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kcmp_unlock(&task1->signal->cred_guard_mutex,
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&task2->signal->cred_guard_mutex);
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err:
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put_task_struct(task1);
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put_task_struct(task2);
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return ret;
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err_no_task:
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rcu_read_unlock();
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return -ESRCH;
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}
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static __init int kcmp_cookies_init(void)
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{
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int i;
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get_random_bytes(cookies, sizeof(cookies));
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for (i = 0; i < KCMP_TYPES; i++)
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cookies[i][1] |= (~(~0UL >> 1) | 1);
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return 0;
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}
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arch_initcall(kcmp_cookies_init);
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