linux-stable/kernel/kcmp.c
Eric W. Biederman 117433236a exec: Transform exec_update_mutex into a rw_semaphore
[ Upstream commit f7cfd871ae ]

Recently syzbot reported[0] that there is a deadlock amongst the users
of exec_update_mutex.  The problematic lock ordering found by lockdep
was:

   perf_event_open  (exec_update_mutex -> ovl_i_mutex)
   chown            (ovl_i_mutex       -> sb_writes)
   sendfile         (sb_writes         -> p->lock)
     by reading from a proc file and writing to overlayfs
   proc_pid_syscall (p->lock           -> exec_update_mutex)

While looking at possible solutions it occured to me that all of the
users and possible users involved only wanted to state of the given
process to remain the same.  They are all readers.  The only writer is
exec.

There is no reason for readers to block on each other.  So fix
this deadlock by transforming exec_update_mutex into a rw_semaphore
named exec_update_lock that only exec takes for writing.

Cc: Jann Horn <jannh@google.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Christopher Yeoh <cyeoh@au1.ibm.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Fixes: eea9673250 ("exec: Add exec_update_mutex to replace cred_guard_mutex")
[0] https://lkml.kernel.org/r/00000000000063640c05ade8e3de@google.com
Reported-by: syzbot+db9cdf3dd1f64252c6ef@syzkaller.appspotmail.com
Link: https://lkml.kernel.org/r/87ft4mbqen.fsf@x220.int.ebiederm.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2021-01-09 13:44:55 +01:00

256 lines
5.7 KiB
C

// SPDX-License-Identifier: GPL-2.0
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/fdtable.h>
#include <linux/string.h>
#include <linux/random.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/cache.h>
#include <linux/bug.h>
#include <linux/err.h>
#include <linux/kcmp.h>
#include <linux/capability.h>
#include <linux/list.h>
#include <linux/eventpoll.h>
#include <linux/file.h>
#include <asm/unistd.h>
/*
* We don't expose the real in-memory order of objects for security reasons.
* But still the comparison results should be suitable for sorting. So we
* obfuscate kernel pointers values and compare the production instead.
*
* The obfuscation is done in two steps. First we xor the kernel pointer with
* a random value, which puts pointer into a new position in a reordered space.
* Secondly we multiply the xor production with a large odd random number to
* permute its bits even more (the odd multiplier guarantees that the product
* is unique ever after the high bits are truncated, since any odd number is
* relative prime to 2^n).
*
* Note also that the obfuscation itself is invisible to userspace and if needed
* it can be changed to an alternate scheme.
*/
static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
static long kptr_obfuscate(long v, int type)
{
return (v ^ cookies[type][0]) * cookies[type][1];
}
/*
* 0 - equal, i.e. v1 = v2
* 1 - less than, i.e. v1 < v2
* 2 - greater than, i.e. v1 > v2
* 3 - not equal but ordering unavailable (reserved for future)
*/
static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
{
long t1, t2;
t1 = kptr_obfuscate((long)v1, type);
t2 = kptr_obfuscate((long)v2, type);
return (t1 < t2) | ((t1 > t2) << 1);
}
/* The caller must have pinned the task */
static struct file *
get_file_raw_ptr(struct task_struct *task, unsigned int idx)
{
struct file *file = NULL;
task_lock(task);
rcu_read_lock();
if (task->files)
file = fcheck_files(task->files, idx);
rcu_read_unlock();
task_unlock(task);
return file;
}
static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{
if (likely(l2 != l1))
up_read(l2);
up_read(l1);
}
static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{
int err;
if (l2 > l1)
swap(l1, l2);
err = down_read_killable(l1);
if (!err && likely(l1 != l2)) {
err = down_read_killable_nested(l2, SINGLE_DEPTH_NESTING);
if (err)
up_read(l1);
}
return err;
}
#ifdef CONFIG_EPOLL
static int kcmp_epoll_target(struct task_struct *task1,
struct task_struct *task2,
unsigned long idx1,
struct kcmp_epoll_slot __user *uslot)
{
struct file *filp, *filp_epoll, *filp_tgt;
struct kcmp_epoll_slot slot;
struct files_struct *files;
if (copy_from_user(&slot, uslot, sizeof(slot)))
return -EFAULT;
filp = get_file_raw_ptr(task1, idx1);
if (!filp)
return -EBADF;
files = get_files_struct(task2);
if (!files)
return -EBADF;
spin_lock(&files->file_lock);
filp_epoll = fcheck_files(files, slot.efd);
if (filp_epoll)
get_file(filp_epoll);
else
filp_tgt = ERR_PTR(-EBADF);
spin_unlock(&files->file_lock);
put_files_struct(files);
if (filp_epoll) {
filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
fput(filp_epoll);
}
if (IS_ERR(filp_tgt))
return PTR_ERR(filp_tgt);
return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
}
#else
static int kcmp_epoll_target(struct task_struct *task1,
struct task_struct *task2,
unsigned long idx1,
struct kcmp_epoll_slot __user *uslot)
{
return -EOPNOTSUPP;
}
#endif
SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
unsigned long, idx1, unsigned long, idx2)
{
struct task_struct *task1, *task2;
int ret;
rcu_read_lock();
/*
* Tasks are looked up in caller's PID namespace only.
*/
task1 = find_task_by_vpid(pid1);
task2 = find_task_by_vpid(pid2);
if (!task1 || !task2)
goto err_no_task;
get_task_struct(task1);
get_task_struct(task2);
rcu_read_unlock();
/*
* One should have enough rights to inspect task details.
*/
ret = kcmp_lock(&task1->signal->exec_update_lock,
&task2->signal->exec_update_lock);
if (ret)
goto err;
if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
!ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
ret = -EPERM;
goto err_unlock;
}
switch (type) {
case KCMP_FILE: {
struct file *filp1, *filp2;
filp1 = get_file_raw_ptr(task1, idx1);
filp2 = get_file_raw_ptr(task2, idx2);
if (filp1 && filp2)
ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
else
ret = -EBADF;
break;
}
case KCMP_VM:
ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
break;
case KCMP_FILES:
ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
break;
case KCMP_FS:
ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
break;
case KCMP_SIGHAND:
ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
break;
case KCMP_IO:
ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
break;
case KCMP_SYSVSEM:
#ifdef CONFIG_SYSVIPC
ret = kcmp_ptr(task1->sysvsem.undo_list,
task2->sysvsem.undo_list,
KCMP_SYSVSEM);
#else
ret = -EOPNOTSUPP;
#endif
break;
case KCMP_EPOLL_TFD:
ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
break;
default:
ret = -EINVAL;
break;
}
err_unlock:
kcmp_unlock(&task1->signal->exec_update_lock,
&task2->signal->exec_update_lock);
err:
put_task_struct(task1);
put_task_struct(task2);
return ret;
err_no_task:
rcu_read_unlock();
return -ESRCH;
}
static __init int kcmp_cookies_init(void)
{
int i;
get_random_bytes(cookies, sizeof(cookies));
for (i = 0; i < KCMP_TYPES; i++)
cookies[i][1] |= (~(~0UL >> 1) | 1);
return 0;
}
arch_initcall(kcmp_cookies_init);