linux-kernel - [PATCH 02/10] x86/mm/cpa: Reject incorrect encryption change requests

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Message-Id: <20231017202505.340906-3-rick.p.edgecombe@intel.com>
Date:   Tue, 17 Oct 2023 13:24:57 -0700
From:   Rick Edgecombe <rick.p.edgecombe@...el.com>
To:     x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, luto@...nel.org,
        peterz@...radead.org, kirill.shutemov@...ux.intel.com,
        elena.reshetova@...el.com, isaku.yamahata@...el.com,
        seanjc@...gle.com, Michael Kelley <mikelley@...rosoft.com>,
        thomas.lendacky@....com, decui@...rosoft.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Cc:     rick.p.edgecombe@...el.com
Subject: [PATCH 02/10] x86/mm/cpa: Reject incorrect encryption change requests

Kernel memory is "encrypted" by default.  Some callers may "decrypt" it
in order to share it with things outside the kernel like a device or an
untrusted VMM.

There is nothing to stop set_memory_encrypted() from being passed memory
that is already "encrypted" (aka. "private" on TDX).  In fact, some
callers do this because ... $REASONS.  Unfortunately, part of the TDX
decrypted=>encrypted transition is truly one way*.  It can't handle
being asked to encrypt an already encrypted page

Allow __set_memory_enc_pgtable() to detect already-encrypted memory
before it hits the TDX code.

* The one way part is "page acceptance"

[commit log written by Dave Hansen]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
 arch/x86/mm/pat/set_memory.c | 41 +++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index bda9f129835e..1238b0db3e33 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -2122,6 +2122,21 @@ int set_memory_global(unsigned long addr, int numpages)
 				    __pgprot(_PAGE_GLOBAL), 0);
 }
 
+static bool kernel_vaddr_encryped(unsigned long addr, bool enc)
+{
+	unsigned int level;
+	pte_t *pte;
+
+	pte = lookup_address(addr, &level);
+	if (!pte)
+		return false;
+
+	if (enc)
+		return pte_val(*pte) == cc_mkenc(pte_val(*pte));
+
+	return pte_val(*pte) == cc_mkdec(pte_val(*pte));
+}
+
 /*
  * __set_memory_enc_pgtable() is used for the hypervisors that get
  * informed about "encryption" status via page tables.
@@ -2130,7 +2145,7 @@ static int __set_memory_enc_pgtable(unsigned long addr, int numpages, bool enc)
 {
 	pgprot_t empty = __pgprot(0);
 	struct cpa_data cpa;
-	int ret;
+	int ret, numpages_in_state = 0;
 
 	/* Should not be working on unaligned addresses */
 	if (WARN_ONCE(addr & ~PAGE_MASK, "misaligned address: %#lx\n", addr))
@@ -2143,6 +2158,30 @@ static int __set_memory_enc_pgtable(unsigned long addr, int numpages, bool enc)
 	cpa.mask_clr = enc ? pgprot_decrypted(empty) : pgprot_encrypted(empty);
 	cpa.pgd = init_mm.pgd;
 
+	/*
+	 * If any page is already in the right state, bail with an error
+	 * because the code doesn't handled it. This is likely because
+	 * something has gone wrong and isn't worth optimizing for.
+	 *
+	 * If all the memory pages are already in the desired state return
+	 * success.
+	 *
+	 * kernel_vaddr_encryped() does not synchronize against huge page
+	 * splits so take pgd_lock. A caller doing strange things could
+	 * get a new PMD mid level PTE confused with a huge PMD entry. Just
+	 * lock to tie up loose ends.
+	 */
+	spin_lock(&pgd_lock);
+	for (int i = 0; i < numpages; i++) {
+		if (kernel_vaddr_encryped(addr + (PAGE_SIZE * i), enc))
+			numpages_in_state++;
+	}
+	spin_unlock(&pgd_lock);
+	if (numpages_in_state == numpages)
+		return 0;
+	else if (numpages_in_state)
+		return 1;
+
 	/* Must avoid aliasing mappings in the highmem code */
 	kmap_flush_unused();
 	vm_unmap_aliases();
-- 
2.34.1