From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Fri, 15 Dec 2017 20:35:11 +0100
Subject: [PATCH] x86/ldt: Make the LDT mapping RO
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CVE-2017-5754

Now that the LDT mapping is in a known area when PAGE_TABLE_ISOLATION is
enabled its a primary target for attacks, if a user space interface fails
to validate a write address correctly. That can never happen, right?

The SDM states:

    If the segment descriptors in the GDT or an LDT are placed in ROM, the
    processor can enter an indefinite loop if software or the processor
    attempts to update (write to) the ROM-based segment descriptors. To
    prevent this problem, set the accessed bits for all segment descriptors
    placed in a ROM. Also, remove operating-system or executive code that
    attempts to modify segment descriptors located in ROM.

So its a valid approach to set the ACCESS bit when setting up the LDT entry
and to map the table RO. Fixup the selftest so it can handle that new mode.

Remove the manual ACCESS bit setter in set_tls_desc() as this is now
pointless. Folded the patch from Peter Ziljstra.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 9f5cb6b32d9e0a3a7453222baaf15664d92adbf2)
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
(cherry picked from commit f4b13d6f67b3a89d878094901a9ca834b39415c1)
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
---
 arch/x86/include/asm/desc.h           |  2 ++
 arch/x86/kernel/ldt.c                 |  7 ++++++-
 arch/x86/kernel/tls.c                 | 11 ++---------
 tools/testing/selftests/x86/ldt_gdt.c |  3 +--
 4 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index de40c514ba25..c765bc294a9d 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -20,6 +20,8 @@ static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *in
 
 	desc->type		= (info->read_exec_only ^ 1) << 1;
 	desc->type	       |= info->contents << 2;
+	/* Set the ACCESS bit so it can be mapped RO */
+	desc->type	       |= 1;
 
 	desc->s			= 1;
 	desc->dpl		= 0x3;
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index eceaada581ff..2260eb6e2de7 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -157,7 +157,12 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
 		ptep = get_locked_pte(mm, va, &ptl);
 		if (!ptep)
 			return -ENOMEM;
-		pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL));
+		/*
+		 * Map it RO so the easy to find address is not a primary
+		 * target via some kernel interface which misses a
+		 * permission check.
+		 */
+		pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));
 		set_pte_at(mm, va, ptep, pte);
 		pte_unmap_unlock(ptep, ptl);
 	}
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index a106b9719c58..41880a2421ea 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -92,17 +92,10 @@ static void set_tls_desc(struct task_struct *p, int idx,
 	cpu = get_cpu();
 
 	while (n-- > 0) {
-		if (LDT_empty(info) || LDT_zero(info)) {
+		if (LDT_empty(info) || LDT_zero(info))
 			memset(desc, 0, sizeof(*desc));
-		} else {
+		else
 			fill_ldt(desc, info);
-
-			/*
-			 * Always set the accessed bit so that the CPU
-			 * doesn't try to write to the (read-only) GDT.
-			 */
-			desc->type |= 1;
-		}
 		++info;
 		++desc;
 	}
diff --git a/tools/testing/selftests/x86/ldt_gdt.c b/tools/testing/selftests/x86/ldt_gdt.c
index 783e1a754b78..bbd1d0e4d683 100644
--- a/tools/testing/selftests/x86/ldt_gdt.c
+++ b/tools/testing/selftests/x86/ldt_gdt.c
@@ -121,8 +121,7 @@ static void check_valid_segment(uint16_t index, int ldt,
 	 * NB: Different Linux versions do different things with the
 	 * accessed bit in set_thread_area().
 	 */
-	if (ar != expected_ar &&
-	    (ldt || ar != (expected_ar | AR_ACCESSED))) {
+	if (ar != expected_ar && ar != (expected_ar | AR_ACCESSED)) {
 		printf("[FAIL]\t%s entry %hu has AR 0x%08X but expected 0x%08X\n",
 		       (ldt ? "LDT" : "GDT"), index, ar, expected_ar);
 		nerrs++;
-- 
2.14.2

