[PATCHv5 1/1] ima: re-introduce own integrity cache lock

[Roberto Sassu]

On 12/4/2017 1:06 PM, Mimi Zohar wrote:
> Hi Dmitry,
> 
> On Fri, 2017-12-01 at 20:40 +0200, Dmitry Kasatkin wrote:
>> The original design was discussed 3+ years ago, but was never completed/upstreamed.
>> Based on the recent discussions with Linus
>> https://patchwork.kernel.org/patch/9975919, I've rebased this patch.
>>
>> Before IMA appraisal was introduced, IMA was using own integrity cache
>> lock along with i_mutex. process_measurement and ima_file_free took
>> the iint->mutex first and then the i_mutex, while setxattr, chmod and
>> chown took the locks in reverse order. To resolve the potential deadlock,
>> i_mutex was moved to protect entire IMA functionality and the redundant
>> iint->mutex was eliminated.
>>
>> Solution was based on the assumption that filesystem code does not take
>> i_mutex further. But when file is opened with O_DIRECT flag, direct-io
>> implementation takes i_mutex and produces deadlock. Furthermore, certain
>> other filesystem operations, such as llseek, also take i_mutex.
>>
>> More recently some filesystems have replaced their filesystem specific
>> lock with the global i_rwsem to read a file.  As a result, when IMA
>> attempts to calculate the file hash, reading the file attempts to take
>> the i_rwsem again.
>>
>> To resolve O_DIRECT related deadlock problem, this patch re-introduces
>> iint->mutex. But to eliminate the original chmod() related deadlock
>> problem, this patch eliminates the requirement for chmod hooks to take
>> the iint->mutex by introducing additional atomic iint->attr_flags to
>> indicate calling of the hooks. The allowed locking order is to take
>> the iint->mutex first and then the i_rwsem.
>>
>> Original flags were cleared in chmod(), setxattr() or removwxattr() hooks
>> and tested when file was closed or opened again. New atomic flags are set
>> or cleared in those hooks and tested to clear iint->flags on close or on open.
>>
>> Atomic flags are following:
>> * IMA_CHANGE_ATTR - indicates that chATTR() was called (chmod, chown, chgrp)
>>    and file attributes have changed. On file open, it causes IMA to clear
>>    iint->flags to re-evaluate policy and perform IMA functions again.
>> * IMA_CHANGE_XATTR - indicates that setxattr or removexattr was called and
>>    extended attributes have changed. On file open, it causes IMA to clear
>>    iint->flags IMA_DONE_MASK to re-appraise.
>> * IMA_UPDATE_XATTR - indicates that security.ima needs to be updated.
>>    It is cleared if file policy changes and no update is needed.
>> * IMA_DIGSIG - indicates that file security.ima has signature and file
>>    security.ima must not update to file has on file close.
> 
> Nice!  I've been testing with this patch and all seems good.  Before
> queueing this patch to be upstreamed, I'd appreciate if others tested
> using it as well.  It applies cleanly to the next-queued branch.

If the inode lock is released before the IMA_MEASURE flag is set, the
ToMToU violation will not be detected when a writer accesses the same
inode. This issue was fixed with commit f7a859ff7395c.

Roberto


> A subsequent patch will remove the O_DIRECT check in
> ima_calc_file_hash().
> 
> Fixes: Commit 6552321831dc "xfs: remove i_iolock and use i_rwsem in
> the VFS inode instead"
> 
> thanks,
> 
> Mimi
> 
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