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 >
