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    Subject[PATCH V3 16/30] x86/sgx: Tighten accessible memory range after enclave initialization
    Date
    Before an enclave is initialized the enclave's memory range is unknown.
    The enclave's memory range is learned at the time it is created via the
    SGX_IOC_ENCLAVE_CREATE ioctl() where the provided memory range is
    obtained from an earlier mmap() of /dev/sgx_enclave. After an enclave
    is initialized its memory can be mapped into user space (mmap()) from
    where it can be entered at its defined entry points.

    With the enclave's memory range known after it is initialized there is
    no reason why it should be possible to map memory outside this range.

    Lock down access to the initialized enclave's memory range by denying
    any attempt to map memory outside its memory range.

    Locking down the memory range also makes adding pages to an initialized
    enclave more efficient. Pages are added to an initialized enclave by
    accessing memory that belongs to the enclave's memory range but not yet
    backed by an enclave page. If it is possible for user space to map
    memory that does not form part of the enclave then an access to this
    memory would eventually fail. Failures range from a prompt general
    protection fault if the access was an ENCLU[EACCEPT] from within the
    enclave, or a page fault via the vDSO if it was another access from
    within the enclave, or a SIGBUS (also resulting from a page fault) if
    the access was from outside the enclave.

    Disallowing invalid memory to be mapped in the first place avoids
    preventable failures.

    Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
    ---
    No changes since V2

    Changes since V1:
    - Add comment (Jarkko).

    arch/x86/kernel/cpu/sgx/encl.c | 5 +++++
    1 file changed, 5 insertions(+)

    diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
    index fa4f947f8496..7909570736a0 100644
    --- a/arch/x86/kernel/cpu/sgx/encl.c
    +++ b/arch/x86/kernel/cpu/sgx/encl.c
    @@ -409,6 +409,11 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,

    XA_STATE(xas, &encl->page_array, PFN_DOWN(start));

    + /* Disallow mapping outside enclave's address range. */
    + if (test_bit(SGX_ENCL_INITIALIZED, &encl->flags) &&
    + (start < encl->base || end > encl->base + encl->size))
    + return -EACCES;
    +
    /*
    * Disallow READ_IMPLIES_EXEC tasks as their VMA permissions might
    * conflict with the enclave page permissions.
    --
    2.25.1
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