* [PATCH v2] x86/sgx: Fix sgx_encl_may_map locking
@ 2020-10-05 3:19 Jarkko Sakkinen
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From: Jarkko Sakkinen @ 2020-10-05 3:19 UTC (permalink / raw)
To: linux-sgx
Cc: Jarkko Sakkinen, Haitao Huang, Matthew Wilcox,
Sean Christopherson, Jethro Beekman, Dave Hansen
Fix the issue further discussed in:
1. https://lore.kernel.org/linux-sgx/op.0rwbv916wjvjmi@mqcpg7oapc828.gar.corp.intel.com/
2. https://lore.kernel.org/linux-sgx/20201003195440.GD20115@casper.infradead.org/
Use the approach suggested by Matthew, and supported by the analysis
that I wrote:
https://lore.kernel.org/linux-sgx/20201005030619.GA126283@linux.intel.com/
Reported-by: Haitao Huang <haitao.huang@linux.intel.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Jethro Beekman <jethro@fortanix.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
---
arch/x86/kernel/cpu/sgx/encl.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 4c6407cd857a..2bb3ec6996e9 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -307,6 +307,7 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
unsigned long idx_start = PFN_DOWN(start);
unsigned long idx_end = PFN_DOWN(end - 1);
struct sgx_encl_page *page;
+ unsigned long count = 0;
XA_STATE(xas, &encl->page_array, idx_start);
@@ -317,10 +318,31 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
if (current->personality & READ_IMPLIES_EXEC)
return -EACCES;
- xas_for_each(&xas, page, idx_end)
+ /*
+ * No need to hold encl->lock:
+ * 1. None of the page->* get written.
+ * 2. page->vm_max_prot_bits is set in sgx_encl_page_alloc(). This
+ * is before calling xa_insert(). After that it is never modified.
+ */
+ xas_lock(&xas);
+ xas_for_each(&xas, page, idx_end) {
+ if (++count % XA_CHECK_SCHED)
+ continue;
+
+ xas_pause(&xas);
+ xas_unlock(&xas);
+
+ /*
+ * Attributes are not protected by the xa_lock, so I'm assuming
+ * that this is the legit place for the check.
+ */
if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
return -EACCES;
+ cond_resched();
+ xas_lock(&xas);
+ }
+
return 0;
}
--
2.25.1
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