linux-kernel - [RFC PATCH 2/9] x86/sgx: Do not naturally align MAP

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Date:   Fri, 31 May 2019 16:31:52 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Jethro Beekman <jethro@...tanix.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-sgx@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, nhorman@...hat.com,
        npmccallum@...hat.com, Serge Ayoun <serge.ayoun@...el.com>,
        Shay Katz-zamir <shay.katz-zamir@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kai Svahn <kai.svahn@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        Kai Huang <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        William Roberts <william.c.roberts@...el.com>,
        Philip Tricca <philip.b.tricca@...el.com>
Subject: [RFC PATCH 2/9] x86/sgx: Do not naturally align MAP_FIXED address

SGX enclaves have an associated Enclave Linear Range (ELRANGE) that is
tracked and enforced by the CPU using a base+mask approach, similar to
how hardware range registers such as the variable MTRRs.  As a result,
the ELRANGE must be naturally sized and aligned.

To reduce boilerplate code that would be needed in every userspace
enclave loader, the SGX driver naturally aligns the mmap() address and
also requires the range to be naturally sized.  Unfortunately, SGX fails
to grant a waiver to the MAP_FIXED case, e.g. incorrectly rejects mmap()
if userspace is attempting to map a small slice of an existing enclave.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kernel/cpu/sgx/driver/main.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/driver/main.c b/arch/x86/kernel/cpu/sgx/driver/main.c
index afe844aa81d6..129d356aff30 100644
--- a/arch/x86/kernel/cpu/sgx/driver/main.c
+++ b/arch/x86/kernel/cpu/sgx/driver/main.c
@@ -79,7 +79,13 @@ static unsigned long sgx_get_unmapped_area(struct file *file,
 					   unsigned long pgoff,
 					   unsigned long flags)
 {
-	if (len < 2 * PAGE_SIZE || len & (len - 1) || flags & MAP_PRIVATE)
+	if (flags & MAP_PRIVATE)
+		return -EINVAL;
+
+	if (flags & MAP_FIXED)
+		return addr;
+
+	if (len < 2 * PAGE_SIZE || len & (len - 1))
 		return -EINVAL;
 
 	addr = current->mm->get_unmapped_area(file, addr, 2 * len, pgoff,
-- 
2.21.0