linux-kernel - [PATCH] x86/sgx: Drop racy follow

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Date:   Thu,  4 Feb 2021 19:45:19 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Sean Christopherson <seanjc@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Borislav Petkov <bp@...e.de>, linux-sgx@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: [PATCH] x86/sgx: Drop racy follow_pfn check

PTE insertion is fundamentally racy, and this check doesn't do
anything useful. Quoting Sean:

"Yeah, it can be whacked.  The original, never-upstreamed code asserted that the
resolved PFN matched the PFN being installed by the fault handler as a sanity
check on the SGX driver's EPC management.  The WARN assertion got dropped for
whatever reason, leaving that useless chunk."

Jason stumbled over this as a new user of follow_pfn, and I'm trying
to get rid of unsafe callers of that function so it can be locked down
further.

This is independent prep work for the referenced patch series.

References: https://lore.kernel.org/dri-devel/20201127164131.2244124-1-daniel.vetter@ffwll.ch/
Reported-by: Jason Gunthorpe <jgg@...pe.ca>
Cc: Jason Gunthorpe <jgg@...pe.ca>
Cc: Sean Christopherson <seanjc@...gle.com>
Fixes: 947c6e11fa43 ("x86/sgx: Add ptrace() support for the SGX driver")
Cc: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Borislav Petkov <bp@...e.de>
Cc: linux-sgx@...r.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
---
 arch/x86/kernel/cpu/sgx/encl.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index ee50a5010277..20a2dd5ba2b4 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -141,7 +141,6 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
 	struct sgx_encl_page *entry;
 	unsigned long phys_addr;
 	struct sgx_encl *encl;
-	unsigned long pfn;
 	vm_fault_t ret;
 
 	encl = vma->vm_private_data;
@@ -168,13 +167,6 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
 
 	phys_addr = sgx_get_epc_phys_addr(entry->epc_page);
 
-	/* Check if another thread got here first to insert the PTE. */
-	if (!follow_pfn(vma, addr, &pfn)) {
-		mutex_unlock(&encl->lock);
-
-		return VM_FAULT_NOPAGE;
-	}
-
 	ret = vmf_insert_pfn(vma, addr, PFN_DOWN(phys_addr));
 	if (ret != VM_FAULT_NOPAGE) {
 		mutex_unlock(&encl->lock);
-- 
2.30.0