Re: [PATCH v6 2/8] debugfs: prevent access to removed files' private data

[Sasha Levin]

On 03/22/2016 09:11 AM, Nicolai Stange wrote:
> Upon return of debugfs_remove()/debugfs_remove_recursive(), it might
> still be attempted to access associated private file data through
> previously opened struct file objects. If that data has been freed by
> the caller of debugfs_remove*() in the meanwhile, the reading/writing
> process would either encounter a fault or, if the memory address in
> question has been reassigned again, unrelated data structures could get
> overwritten.
> 
> However, since debugfs files are seldomly removed, usually from module
> exit handlers only, the impact is very low.
> 
> Currently, there are ~1000 call sites of debugfs_create_file() spread
> throughout the whole tree and touching all of those struct file_operations
> in order to make them file removal aware by means of checking the result of
> debugfs_use_file_start() from within their methods is unfeasible.
> 
> Instead, wrap the struct file_operations by a lifetime managing proxy at
> file open:
> - In debugfs_create_file(), the original fops handed in has got stashed
>   away in ->d_fsdata already.
> - In debugfs_create_file(), install a proxy file_operations factory,
>   debugfs_full_proxy_file_operations, at ->i_fop.
> 
> This proxy factory has got an ->open() method only. It carries out some
> lifetime checks and if successful, dynamically allocates and sets up a new
> struct file_operations proxy at ->f_op. Afterwards, it forwards to the
> ->open() of the original struct file_operations in ->d_fsdata, if any.
> 
> The dynamically set up proxy at ->f_op has got a lifetime managing wrapper
> set for each of the methods defined in the original struct file_operations
> in ->d_fsdata.
> 
> Its ->release()er frees the proxy again and forwards to the original
> ->release(), if any.
> 
> In order not to mislead the VFS layer, it is strictly necessary to leave
> those fields blank in the proxy that have been NULL in the original
> struct file_operations also, i.e. aren't supported. This is why there is a
> need for dynamically allocated proxies. The choice made not to allocate a
> proxy instance for every dentry at file creation, but for every
> struct file object instantiated thereof is justified by the expected usage
> pattern of debugfs, namely that in general very few files get opened more
> than once at a time.
> 
> The wrapper methods set in the struct file_operations implement lifetime
> managing by means of the SRCU protection facilities already in place for
> debugfs:
> They set up a SRCU read side critical section and check whether the dentry
> is still alive by means of debugfs_use_file_start(). If so, they forward
> the call to the original struct file_operation stored in ->d_fsdata, still
> under the protection of the SRCU read side critical section.
> This SRCU read side critical section prevents any pending debugfs_remove()
> and friends to return to their callers. Since a file's private data must
> only be freed after the return of debugfs_remove(), the ongoing proxied
> call is guarded against any file removal race.
> 
> If, on the other hand, the initial call to debugfs_use_file_start() detects
> that the dentry is dead, the wrapper simply returns -EIO and does not
> forward the call. Note that the ->poll() wrapper is special in that its
> signature does not allow for the return of arbitrary -EXXX values and thus,
> POLLHUP is returned here.
> 
> In order not to pollute debugfs with wrapper definitions that aren't ever
> needed, I chose not to define a wrapper for every struct file_operations
> method possible. Instead, a wrapper is defined only for the subset of
> methods which are actually set by any debugfs users.
> Currently, these are:
> 
>   ->llseek()
>   ->read()
>   ->write()
>   ->unlocked_ioctl()
>   ->poll()
> 
> The ->release() wrapper is special in that it does not protect the original
> ->release() in any way from dead files in order not to leak resources.
> Thus, any ->release() handed to debugfs must implement file lifetime
> management manually, if needed.
> For only 33 out of a total of 434 releasers handed in to debugfs, it could
> not be verified immediately whether they access data structures that might
> have been freed upon a debugfs_remove() return in the meanwhile.
> 
> Export debugfs_use_file_start() and debugfs_use_file_finish() in order to
> allow any ->release() to manually implement file lifetime management.
> 
> For a set of common cases of struct file_operations implemented by the
> debugfs_core itself, future patches will incorporate file lifetime
> management directly within those in order to allow for their unproxied
> operation. Rename the original, non-proxying "debugfs_create_file()" to
> "debugfs_create_file_unsafe()" and keep it for future internal use by
> debugfs itself. Factor out code common to both into the new
> __debugfs_create_file().
> 
> Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx>

Hey,

I'm seeing a hang on boot which was bisected (twice) to this commit. I don't
see any lockup messages, but everything just seems to stop.


Thanks,
Sasha