RE: [RFC PATCH 6/9] x86/sgx: Require userspace to provide allowed prots to ADD_PAGES

[Xing, Cedric]

> From: Christopherson, Sean J
> Sent: Friday, May 31, 2019 4:32 PM
> 
> ...to support (the equivalent) of existing Linux Security Module functionality.
> 
> Because SGX manually manages EPC memory, all enclave VMAs are backed by the same vm_file,
> i.e. /dev/sgx/enclave, so that SGX can implement the necessary hooks to move pages in/out
> of the EPC.  And because EPC pages for any given enclave are fundamentally shared between
> processes, i.e.
> CoW semantics are not possible with EPC pages, /dev/sgx/enclave must always be MAP_SHARED.
> Lastly, all real world enclaves will need read, write and execute permissions to EPC pages.
> As a result, SGX does not play nice with existing LSM behavior as it is impossible to
> apply policies to enclaves with any reasonable granularity, e.g. an LSM can deny access to
> EPC altogether, but can't deny potentially dangerous behavior such as mapping pages RW->RW
> or RWX.
> 
> To give LSMs enough information to implement their policies without having to resort to
> ugly things, e.g. holding a reference to the vm_file of each enclave page, require
> userspace to explicitly state the allowed protections for each page (region), i.e. take
> ALLOW_{READ,WRITE,EXEC} in the ADD_PAGES ioctl.
> 
> The ALLOW_* flags will be passed to LSMs so that they can make informed decisions when the
> enclave is being built, i.e. when the source vm_file is available.  For example, SELinux's
> EXECMOD permission can be required if an enclave is requesting both ALLOW_WRITE and
> ALLOW_EXEC.
> 
> Update the mmap()/mprotect() hooks to enforce the ALLOW_* protections, a la the standard
> VM_MAY{READ,WRITE,EXEC} flags.
> 
> The ALLOW_EXEC flag also has a second (important) use in that it can be used to prevent
> loading an enclave from a noexec file system, on
> SGX2 hardware (regardless of kernel support for SGX2), userspace could EADD from a noexec
> path using read-only permissions and later mprotect() and ENCLU[EMODPE] the page to gain
> execute permissions.  By requiring ALLOW_EXEC up front, SGX will be able to enforce noexec
> paths when building the enclave.

ALLOW_* flags shall be kept internal to LSM.

This patch is completely unnecessary.