x86/sgx: Maintain encl->refcount for each encl->mm_list entry
This has been shown in tests:
[ +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100
This is essentially a use-after free, although SRCU notices it as
an SRCU cleanup in an invalid context.
== Background ==
SGX has a data structure (struct sgx_encl_mm) which keeps per-mm SGX
metadata. This is separate from struct sgx_encl because, in theory,
an enclave can be mapped from more than one mm. sgx_encl_mm includes
a pointer back to the sgx_encl.
This means that sgx_encl must have a longer lifetime than all of the
sgx_encl_mm's that point to it. That's usually the case: sgx_encl_mm
is freed only after the mmu_notifier is unregistered in sgx_release().
However, there's a race. If the process is exiting,
sgx_mmu_notifier_release() can be called in parallel with sgx_release()
instead of being called *by* it. The mmu_notifier path keeps encl_mm
alive past when sgx_encl can be freed. This inverts the lifetime rules
and means that sgx_mmu_notifier_release() can access a freed sgx_encl.
== Fix ==
Increase encl->refcount when encl_mm->encl is established. Release
this reference when encl_mm is freed. This ensures that encl outlives
encl_mm.
[ bp: Massage commit message. ]
Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Reported-by: Haitao Huang <haitao.huang@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20210207221401.29933-1-jarkko@kernel.org
2 files changed