Re: [PATCH] x86/sgx: fix a NULL pointer
From: Haitao Huang
Date: Tue Jul 18 2023 - 14:11:45 EST
On Tue, 18 Jul 2023 09:27:49 -0500, Dave Hansen <dave.hansen@xxxxxxxxx>
wrote:
On 7/17/23 13:29, Haitao Huang wrote:
Under heavy load, the SGX EPC reclaimers (current ksgxd or future EPC
cgroup worker) may reclaim the SECS EPC page for an enclave and set
encl->secs.epc_page to NULL. But the SECS EPC page is used for EAUG in
the SGX #PF handler without checking for NULL and reloading.
Fix this by checking if SECS is loaded before EAUG and load it if it was
reclaimed.
It would be nice to see a _bit_ more theory of the bug in here.
What is an SECS page and why is it special in a reclaim context? Why is
this so hard to hit? What led you to discover this issue now? What is
EAUG?
Let me know if this clarify things.
The SECS page holds global states of an enclave, and all reclaimable pages
tracked by the SGX EPC reclaimer (ksgxd) are considered 'child' pages of
the SECS page corresponding to that enclave. The reclaimer only reclaims
the SECS page when all its children are reclaimed. That can happen on
system under high EPC pressure where multiple large enclaves demanding
much more EPC page than physically available. In a rare case, the
reclaimer may reclaim all EPC pages of an enclave and it SECS page,
setting encl->secs.epc_page to NULL, right before the #PF handler get the
chance to handle a #PF for that enclave. In that case, if that #PF happens
to require kernel to invoke the EAUG instruction to add a new EPC page for
the enclave, then a NULL pointer results as current code does not check if
encl->secs.epc_page is NULL before using it.
The bug is easier to reproduce with the EPC cgroup implementation when a
low EPC limit is set for a group of enclave hosting processes. Without the
EPC cgroup it's hard to trigger the reclaimer to reclaim all child pages
of an SECS page. And it'd also require a machine configured with large RAM
relative to EPC so no OOM killer triggered before this happens.
Thanks
Haitao