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

[Jarkko Sakkinen <jarkko@kernel.org>]

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