Hi Jarkko,
On 12/10/2021 11:42 PM, Jarkko Sakkinen wrote:
On Mon, 2021-12-06 at 13:20 -0800, Reinette Chatre wrote:
This is a valid question. Since EMODPE exists why not just make things for
EMODPE, and ignore EMODPR altogether?
I believe that we should support the best practice of principle of least
privilege - once a page no longer needs a particular permission there
should be a way to remove it (the unneeded permission).
What if EMODPR was not used at all, since EMODPE is there anyway?
EMODPR and EMODPE are not equivalent.
EMODPE can only be used to "extend"/relax permissions while EMODPR can
only be used to restrict permissions.
Notice in the EMODPE instruction reference of the SDM:
(* Update EPCM permissions *)
EPCM(DS:RCX).R := EPCM(DS:RCX).R | SCRATCH_SECINFO.FLAGS.R;
EPCM(DS:RCX).W := EPCM(DS:RCX).W | SCRATCH_SECINFO.FLAGS.W;
EPCM(DS:RCX).X := EPCM(DS:RCX).X | SCRATCH_SECINFO.FLAGS.X;
So, when using EMODPE it is only possible to add permissions, not remove
permissions.
If a user wants to remove permissions from an EPCM page it is only
possible when using EMODPR. Notice in its instruction reference found in
the SDM how it in turn can only be used to restrict permissions:
(* Update EPCM permissions *)
EPCM(DS:RCX).R := EPCM(DS:RCX).R & SCRATCH_SECINFO.FLAGS.R;
EPCM(DS:RCX).W := EPCM(DS:RCX).W & SCRATCH_SECINFO.FLAGS.W;
EPCM(DS:RCX).X := EPCM(DS:RCX).X & SCRATCH_SECINFO.FLAGS.X;
This could be achieved e.g. by having ioctl to change protection
bits in encl->page_tree.
This would simplify things a lot given that there would be only
two, instead of three, EACCEPT code paths.
Reinette
