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  1. 05 Feb, 2020 2 commits
    • Xiaochen Shen's avatar
      x86/resctrl: Fix use-after-free when deleting resource groups · 1b006f8c
      Xiaochen Shen authored 
      commit b8511ccc upstream.

A resource group (rdtgrp) contains a reference count (rdtgrp->waitcount)
that indicates how many waiters expect this rdtgrp to exist. Waiters
could be waiting on rdtgroup_mutex or some work sitting on a task's
workqueue for when the task returns from kernel mode or exits.

The deletion of a rdtgrp is intended to have two phases:

  (1) while holding rdtgroup_mutex the necessary cleanup is done and
  rdtgrp->flags is set to RDT_DELETED,

  (2) after releasing the rdtgroup_mutex, the rdtgrp structure is freed
  only if there are no waiters and its flag is set to RDT_DELETED. Upon
  gaining access to rdtgroup_mutex or rdtgrp, a waiter is required to check
  for the RDT_DELETED flag.

When unmounting the resctrl file system or deleting ctrl_mon groups,
all of the subdirectories are removed and the data structure of rdtgrp
is forcibly freed without checking rdtgrp->waitcount. If at this point
there was a waiter on rdtgrp then a use-after-free issue occurs when the
waiter starts running and accesses the rdtgrp structure it was waiting
on.

See kfree() calls in [1], [2] and [3] in these two call paths in
following scenarios:
(1) rdt_kill_sb() -> rmdir_all_sub() -> free_all_child_rdtgrp()
(2) rdtgroup_rmdir() -> rdtgroup_rmdir_ctrl() -> free_all_child_rdtgrp()

There are several scenarios that result in use-after-free issue in
following:

Scenario 1:
-----------
In Thread 1, rdtgroup_tasks_write() adds a task_work callback
move_myself(). If move_myself() is scheduled to execute after Thread 2
rdt_kill_sb() is finished, referring to earlier rdtgrp memory
(rdtgrp->waitcount) which was already freed in Thread 2 results in
use-after-free issue.

Thread 1 (rdtgroup_tasks_write)        Thread 2 (rdt_kill_sb)
-------------------------------        ----------------------
rdtgroup_kn_lock_live
  atomic_inc(&rdtgrp->waitcount)
  mutex_lock
rdtgroup_move_task
  __rdtgroup_move_task
    /*
     * Take an extra refcount, so rdtgrp cannot be freed
     * before the call back move_myself has been invoked
     */
    atomic_inc(&rdtgrp->waitcount)
    /* Callback move_myself will be scheduled for later */
    task_work_add(move_myself)
rdtgroup_kn_unlock
  mutex_unlock
  atomic_dec_and_test(&rdtgrp->waitcount)
  && (flags & RDT_DELETED)
                                       mutex_lock
                                       rmdir_all_sub
                                         /*
                                          * sentry and rdtgrp are freed
                                          * without checking refcount
                                          */
                                         free_all_child_rdtgrp
                                           kfree(sentry)*[1]
                                         kfree(rdtgrp)*[2]
                                       mutex_unlock
/*
 * Callback is scheduled to execute
 * after rdt_kill_sb is finished
 */
move_myself
  /*
   * Use-after-free: refer to earlier rdtgrp
   * memory which was freed in [1] or [2].
   */
  atomic_dec_and_test(&rdtgrp->waitcount)
  && (flags & RDT_DELETED)
    kfree(rdtgrp)

Scenario 2:
-----------
In Thread 1, rdtgroup_tasks_write() adds a task_work callback
move_myself(). If move_myself() is scheduled to execute after Thread 2
rdtgroup_rmdir() is finished, referring to earlier rdtgrp memory
(rdtgrp->waitcount) which was already freed in Thread 2 results in
use-after-free issue.

Thread 1 (rdtgroup_tasks_write)        Thread 2 (rdtgroup_rmdir)
-------------------------------        -------------------------
rdtgroup_kn_lock_live
  atomic_inc(&rdtgrp->waitcount)
  mutex_lock
rdtgroup_move_task
  __rdtgroup_move_task
    /*
     * Take an extra refcount, so rdtgrp cannot be freed
     * before the call back move_myself has been invoked
     */
    atomic_inc(&rdtgrp->waitcount)
    /* Callback move_myself will be scheduled for later */
    task_work_add(move_myself)
rdtgroup_kn_unlock
  mutex_unlock
  atomic_dec_and_test(&rdtgrp->waitcount)
  && (flags & RDT_DELETED)
                                       rdtgroup_kn_lock_live
                                         atomic_inc(&rdtgrp->waitcount)
                                         mutex_lock
                                       rdtgroup_rmdir_ctrl
                                         free_all_child_rdtgrp
                                           /*
                                            * sentry is freed without
                                            * checking refcount
                                            */
                                           kfree(sentry)*[3]
                                         rdtgroup_ctrl_remove
                                           rdtgrp->flags = RDT_DELETED
                                       rdtgroup_kn_unlock
                                         mutex_unlock
                                         atomic_dec_and_test(
                                                     &rdtgrp->waitcount)
                                         && (flags & RDT_DELETED)
                                           kfree(rdtgrp)
/*
 * Callback is scheduled to execute
 * after rdt_kill_sb is finished
 */
move_myself
  /*
   * Use-after-free: refer to earlier rdtgrp
   * memory which was freed in [3].
   */
  atomic_dec_and_test(&rdtgrp->waitcount)
  && (flags & RDT_DELETED)
    kfree(rdtgrp)

If CONFIG_DEBUG_SLAB=y, Slab corruption on kmalloc-2k can be observed
like following. Note that "0x6b" is POISON_FREE after kfree(). The
corrupted bits "0x6a", "0x64" at offset 0x424 correspond to
waitcount member of struct rdtgroup which was freed:

  Slab corruption (Not tainted): kmalloc-2k start=ffff9504c5b0d000, len=2048
  420: 6b 6b 6b 6b 6a 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkjkkkkkkkkkkk
  Single bit error detected. Probably bad RAM.
  Run memtest86+ or a similar memory test tool.
  Next obj: start=ffff9504c5b0d800, len=2048
  000: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  010: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk

  Slab corruption (Not tainted): kmalloc-2k start=ffff9504c58ab800, len=2048
  420: 6b 6b 6b 6b 64 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkdkkkkkkkkkkk
  Prev obj: start=ffff9504c58ab000, len=2048
  000: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  010: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk

Fix this by taking reference count (waitcount) of rdtgrp into account in
the two call paths that currently do not do so. Instead of always
freeing the resource group it will only be freed if there are no waiters
on it. If there are waiters, the resource group will have its flags set
to RDT_DELETED.

It will be left to the waiter to free the resource group when it starts
running and finding that it was the last waiter and the resource group
has been removed (rdtgrp->flags & RDT_DELETED) since. (1) rdt_kill_sb()
-> rmdir_all_sub() -> free_all_child_rdtgrp() (2) rdtgroup_rmdir() ->
rdtgroup_rmdir_ctrl() -> free_all_child_rdtgrp()

Backporting notes:

Since upstream commit fa7d9493 ("x86/resctrl: Rename and move rdt
files to a separate directory"), the file
arch/x86/kernel/cpu/intel_rdt_rdtgroup.c has been renamed and moved to
arch/x86/kernel/cpu/resctrl/rdtgroup.c.

Apply the change against file arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
in older stable trees.

Fixes: f3cbeaca ("x86/intel_rdt/cqm: Add rmdir support")
Fixes: 60cf5e10 ("x86/intel_rdt: Add mkdir to resctrl file system")
Suggested-by: default avatarReinette Chatre <reinette.chatre@intel.com>
Signed-off-by: default avatarXiaochen Shen <xiaochen.shen@intel.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarReinette Chatre <reinette.chatre@intel.com>
Reviewed-by: default avatarTony Luck <tony.luck@intel.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1578500886-21771-2-git-send-email-xiaochen.shen@intel.comSigned-off-by: default avatarSasha Levin <sashal@kernel.org>
      1b006f8c
    • Al Viro's avatar
      vfs: fix do_last() regression · 8d7a5100
      Al Viro authored 
      commit 6404674a upstream.

Brown paperbag time: fetching ->i_uid/->i_mode really should've been
done from nd->inode.  I even suggested that, but the reason for that has
slipped through the cracks and I went for dir->d_inode instead - made
for more "obvious" patch.

Analysis:

 - at the entry into do_last() and all the way to step_into(): dir (aka
   nd->path.dentry) is known not to have been freed; so's nd->inode and
   it's equal to dir->d_inode unless we are already doomed to -ECHILD.
   inode of the file to get opened is not known.

 - after step_into(): inode of the file to get opened is known; dir
   might be pointing to freed memory/be negative/etc.

 - at the call of may_create_in_sticky(): guaranteed to be out of RCU
   mode; inode of the file to get opened is known and pinned; dir might
   be garbage.

The last was the reason for the original patch.  Except that at the
do_last() entry we can be in RCU mode and it is possible that
nd->path.dentry->d_inode has already changed under us.

In that case we are going to fail with -ECHILD, but we need to be
careful; nd->inode is pointing to valid struct inode and it's the same
as nd->path.dentry->d_inode in "won't fail with -ECHILD" case, so we
should use that.
Reported-by: default avatar"Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@nokia.com>
Reported-by: syzbot+190005201ced78a74ad6@syzkaller.appspotmail.com
Wearing-brown-paperbag: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org
Fixes: d0cb5018 ("do_last(): fetch directory ->i_mode and ->i_uid before it's too late")
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
      8d7a5100
  3. 01 Feb, 2020 38 commits
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