Re: [PATCH v4] x86/sgx: Free backing memory after faulting the enclave page

Jarkko Sakkinen
2022-02-22T12:23:48Z

On Tue, Feb 22, 2022 at 01:03:41PM +0100, Jarkko Sakkinen wrote:
> There is a limited amount of SGX memory (EPC) on each system. When that
> memory is used up, SGX has its own swapping mechanism which is similar
> in concept but totally separate from the core mm/* code. Instead of
> swapping to disk, SGX swaps from EPC to normal RAM. That normal RAM
> comes from a shared memory pseudo-file and can itself be swapped by the
> core mm code. There is a hierarchy like this:
>
> EPC <-> shmem <-> disk
>
> After data is swapped back in from shmem to EPC, the shmem backing
> storage needs to be freed. Currently, the backing shmem is not freed.
> This effectively wastes the shmem while the enclave is running. The
> memory is recovered when the enclave is destroyed and the backing
> storage freed.
>
> Sort this out by freeing memory with shmem_truncate_range(), as soon as
> a page is faulted back to the EPC. In addition, free the memory for
> PCMD pages as soon as all PCMD's in a page have been marked as unused
> by zeroing its contents.
>
> Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: stable@vger.kernel.org
> Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> ---
> v4:
> * Sanitized the offset calculations.
> v3:
> * Resend.
> v2:
> * Rewrite commit message as proposed by Dave.
> * Truncate PCMD pages (Dave).
> ---
> arch/x86/kernel/cpu/sgx/encl.c | 44 +++++++++++++++++++++++++++-------
> 1 file changed, 36 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 8be6f0592bdc..94319190efb1 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -12,6 +12,17 @@
> #include "encls.h"
> #include "sgx.h"
>
> +
> +/*
> + * Free a page from the backing storage in the given page index.
> + */
> +static inline void sgx_encl_truncate_backing_page(struct sgx_encl *encl, pgoff_t index)
> +{
> + struct inode *inode = file_inode(encl->backing);
> +
> + shmem_truncate_range(inode, PFN_PHYS(index), PFN_PHYS(index) + PAGE_SIZE - 1);
> +}
> +
> /*
> * ELDU: Load an EPC page as unblocked. For more info, see "OS Management of EPC
> * Pages" in the SDM.
> @@ -24,7 +35,9 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> struct sgx_encl *encl = encl_page->encl;
> struct sgx_pageinfo pginfo;
> struct sgx_backing b;
> + bool pcmd_page_empty;
> pgoff_t page_index;
> + u8 *pcmd_page;
> int ret;
>
> if (secs_page)
> @@ -38,8 +51,8 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
>
> pginfo.addr = encl_page->desc & PAGE_MASK;
> pginfo.contents = (unsigned long)kmap_atomic(b.contents);
> - pginfo.metadata = (unsigned long)kmap_atomic(b.pcmd) +
> - b.pcmd_offset;
> + pcmd_page = kmap_atomic(b.pcmd);
> + pginfo.metadata = (unsigned long)pcmd_page + b.pcmd_offset;
>
> if (secs_page)
> pginfo.secs = (u64)sgx_get_epc_virt_addr(secs_page);
> @@ -55,11 +68,28 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> ret = -EFAULT;
> }
>
> - kunmap_atomic((void *)(unsigned long)(pginfo.metadata - b.pcmd_offset));
> + memset(pcmd_page + b.pcmd_offset, 0, sizeof(struct sgx_pcmd));
> +
> + /*
> + * The area for the PCMD in the page was zeroed above. Check if the
> + * whole page is now empty meaning that all PCMD's have been zeroed:
> + */
> + pcmd_page_empty = !memchr_inv(pcmd_page, 0, PAGE_SIZE);
> +
> + kunmap_atomic(pcmd_page);
> kunmap_atomic((void *)(unsigned long)pginfo.contents);
>
> sgx_encl_put_backing(&b, false);
>
> + sgx_encl_truncate_backing_page(encl, page_index);
> +
> + if (pcmd_page_empty) {
> + pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ +
> + page_index * sizeof(struct sgx_pcmd);
> +
> + sgx_encl_truncate_backing_page(encl, PFN_DOWN(pcmd_off));
> + }
> +
> return ret;
> }
>
> @@ -583,7 +613,7 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
> static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
> struct sgx_backing *backing)
> {
> - pgoff_t pcmd_index = PFN_DOWN(encl->size) + 1 + (page_index >> 5);
> + pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ + page_index * sizeof(struct sgx_pcmd);
> struct page *contents;
> struct page *pcmd;
>
> @@ -591,7 +621,7 @@ static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
> if (IS_ERR(contents))
> return PTR_ERR(contents);
>
> - pcmd = sgx_encl_get_backing_page(encl, pcmd_index);
> + pcmd = sgx_encl_get_backing_page(encl, PFN_DOWN(pcmd_off));
> if (IS_ERR(pcmd)) {
> put_page(contents);
> return PTR_ERR(pcmd);
> @@ -600,9 +630,7 @@ static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
> backing->page_index = page_index;
> backing->contents = contents;
> backing->pcmd = pcmd;
> - backing->pcmd_offset =
> - (page_index & (PAGE_SIZE / sizeof(struct sgx_pcmd) - 1)) *
> - sizeof(struct sgx_pcmd);
> + backing->pcmd_offset = pcmd_off & (PAGE_SIZE - 1);
>
> return 0;
> }
> --
> 2.35.1
>

Tested this in my XPS13 2020 model: i7-1065G7

I did the testing in bare metal to minimize any possible side-effects.

$ ./test_sgx
TAP version 13
1..6
# Starting 6 tests from 2 test cases.
# RUN enclave.unclobbered_vdso ...
# OK enclave.unclobbered_vdso
ok 1 enclave.unclobbered_vdso
# RUN enclave.unclobbered_vdso_oversubscribed ...
# OK enclave.unclobbered_vdso_oversubscribed
ok 2 enclave.unclobbered_vdso_oversubscribed
# RUN enclave.clobbered_vdso ...
# OK enclave.clobbered_vdso
ok 3 enclave.clobbered_vdso
# RUN enclave.clobbered_vdso_and_user_function ...
# OK enclave.clobbered_vdso_and_user_function
ok 4 enclave.clobbered_vdso_and_user_function
# RUN enclave.tcs_entry ...
# OK enclave.tcs_entry
ok 5 enclave.tcs_entry
# RUN enclave.pte_permissions ...
# OK enclave.pte_permissions
ok 6 enclave.pte_permissions
# PASSED: 6 / 6 tests passed.
# Totals: pass:6 fail:0 xfail:0 xpass:0 skip:0 error:0

BR, Jarkko