drm/drm_file: Fix pid refcounting race
<maarten.lankhorst@linux.intel.com>, Maxime Ripard
<mripard@kernel.org>, Thomas Zimmermann <tzimmermann@suse.de>
filp->pid is supposed to be a refcounted pointer; however, before this
patch, drm_file_update_pid() only increments the refcount of a struct
pid after storing a pointer to it in filp->pid and dropping the
dev->filelist_mutex, making the following race possible:
process A process B
========= =========
begin drm_file_update_pid
mutex_lock(&dev->filelist_mutex)
rcu_replace_pointer(filp->pid, <pid B>, 1)
mutex_unlock(&dev->filelist_mutex)
begin drm_file_update_pid
mutex_lock(&dev->filelist_mutex)
rcu_replace_pointer(filp->pid, <pid A>, 1)
mutex_unlock(&dev->filelist_mutex)
get_pid(<pid A>)
synchronize_rcu()
put_pid(<pid B>) *** pid B reaches refcount 0 and is freed here ***
get_pid(<pid B>) *** UAF ***
synchronize_rcu()
put_pid(<pid A>)
As far as I know, this race can only occur with CONFIG_PREEMPT_RCU=y
because it requires RCU to detect a quiescent state in code that is not
explicitly calling into the scheduler.
This race leads to use-after-free of a "struct pid".
It is probably somewhat hard to hit because process A has to pass
through a synchronize_rcu() operation while process B is between
mutex_unlock() and get_pid().
Fix it by ensuring that by the time a pointer to the current task's pid
is stored in the file, an extra reference to the pid has been taken.
This fix also removes the condition for synchronize_rcu(); I think
that optimization is unnecessary complexity, since in that case we
would usually have bailed out on the lockless check above.
Fixes: 1c7a387ffe
("drm: Update file owner during use")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
This commit is contained in:
parent
3e6d5e1199
commit
4f2a129b33
@ -469,14 +469,12 @@ void drm_file_update_pid(struct drm_file *filp)
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dev = filp->minor->dev;
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mutex_lock(&dev->filelist_mutex);
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get_pid(pid);
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old = rcu_replace_pointer(filp->pid, pid, 1);
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mutex_unlock(&dev->filelist_mutex);
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if (pid != old) {
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get_pid(pid);
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synchronize_rcu();
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put_pid(old);
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}
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}
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/**
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