Re: Possible bug or unintended behaviour using bpf_ima_file_hash

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2022-11-02 at 09:08 +0100, Roberto Sassu wrote:
> On Mon, 2022-10-31 at 16:25 +0000, Isaac Matthews wrote:
> > Using bpf_ima_file_hash() from kernel 6.0.
> > 
> > When using bpf_ima_file_hash() with the lsm.s/file_open hook, a hash
> > of the file is only sometimes returned.  This is because the
> > FMODE_CAN_READ flag is set after security_file_open() is already
> > called, and ima_calc_file_hash() only checks for FMODE_READ not
> > FMODE_CAN_READ in order to decide if a new instance needs to be
> > opened. Because of this, if a file passes the FMODE_READ check  it
> > will fail to be hashed as FMODE_CAN_READ has not yet been set.
> > 
> > To demonstrate: if the file is opened for write for example, when
> > ima_calc_file_hash() is called and the file->f_mode is checked
> > against
> > FMODE_READ, a new file instance is opened with the correct flags and
> > a
> > hash is returned. If the file is opened for read, a new file instance
> > is not returned in ima_calc_file_hash() as (!(file->f_mode &
> > FMODE_READ)) is now false. When __kernel_read() is called as part of
> > ima_calc_file_hash_tfm() it will fail on if (!(file->f_mode &
> > FMODE_CAN_READ)) and so no hash will be returned by
> > bpf_ima_file_hash().
> > 
> > If possible could someone please advise me as to whether this is
> > intended behaviour, and is it possible to either modify the flags
> > with
> > eBPF or to open a new instance with the correct flags set as IMA does
> > currently?
> 
> Hi Isaac
> 
> I think this is the intended behavior, as IMA is supposed to be called
> when the file descriptor is ready to use.
> 
> If we need to call ima_file_hash() from lsm.s/file_open, I think it
> should not be a problem to create a new fd just for eBPF, in
> __ima_inode_hash().
> 
> Mimi, what do you think?

Who/what is checking that this is a regular file and we have permission
to open the file?  Are we relying on eBPF to do this?  Will opening a
file circumvent all of the LSM checks?

> 
> > Alternatively, would a better solution be adding a check for
> > FMODE_CAN_READ to ima_calc_file_hash()? I noticed in the comment
> > above
> > the conditional in ima_calc_file_hash() that the conditional should
> > be
> > checking whether the file can be read, but only checks the FMODE_READ
> > flag which is not the only requirement for __kernel_read() to be able
> > to read a file.
> > 
> > Thanks for your help.
> > Isaac
>