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From: Rick Edgecombe <rick.p.edgecombe@intel.com>
To: Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, hpa@zytor.com,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, Nadav Amit <nadav.amit@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux_dti@icloud.com, linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org, akpm@linux-foundation.org,
	kernel-hardening@lists.openwall.com, linux-mm@kvack.org,
	will.deacon@arm.com, ard.biesheuvel@linaro.org,
	kristen@linux.intel.com, deneen.t.dock@intel.com,
	Nadav Amit <namit@vmware.com>, Kees Cook <keescook@chromium.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>
Subject: [PATCH v3 06/20] x86/alternative: Use temporary mm for text poking
Date: Thu, 21 Feb 2019 15:44:37 -0800	[thread overview]
Message-ID: <20190221234451.17632-7-rick.p.edgecombe@intel.com> (raw)
In-Reply-To: <20190221234451.17632-1-rick.p.edgecombe@intel.com>

From: Nadav Amit <namit@vmware.com>

text_poke() can potentially compromise security as it sets temporary
PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
from other cores accidentally or maliciously, if an attacker gains the
ability to write onto kernel memory.

Moreover, since remote TLBs are not flushed after the temporary PTEs are
removed, the time-window in which the code is writable is not limited if
the fixmap PTEs - maliciously or accidentally - are cached in the TLB.
To address these potential security hazards, use a temporary mm for
patching the code.

Finally, text_poke() is also not conservative enough when mapping pages,
as it always tries to map 2 pages, even when a single one is sufficient.
So try to be more conservative, and do not map more than needed.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
 arch/x86/include/asm/fixmap.h |   2 -
 arch/x86/kernel/alternative.c | 108 +++++++++++++++++++++++++++-------
 arch/x86/xen/mmu_pv.c         |   2 -
 3 files changed, 86 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index 50ba74a34a37..9da8cccdf3fb 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -103,8 +103,6 @@ enum fixed_addresses {
 #ifdef CONFIG_PARAVIRT
 	FIX_PARAVIRT_BOOTMAP,
 #endif
-	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
-	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
 #ifdef	CONFIG_X86_INTEL_MID
 	FIX_LNW_VRTC,
 #endif
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ae05fbb50171..cfe5bfe06f9d 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -11,6 +11,7 @@
 #include <linux/stop_machine.h>
 #include <linux/slab.h>
 #include <linux/kdebug.h>
+#include <linux/mmu_context.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>
@@ -683,41 +684,104 @@ __ro_after_init unsigned long poking_addr;
 
 static void *__text_poke(void *addr, const void *opcode, size_t len)
 {
+	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
+	struct page *pages[2] = {NULL};
+	temp_mm_state_t prev;
 	unsigned long flags;
-	char *vaddr;
-	struct page *pages[2];
-	int i;
+	pte_t pte, *ptep;
+	spinlock_t *ptl;
+	pgprot_t pgprot;
 
 	/*
-	 * While boot memory allocator is runnig we cannot use struct
-	 * pages as they are not yet initialized.
+	 * While boot memory allocator is running we cannot use struct pages as
+	 * they are not yet initialized. There is no way to recover.
 	 */
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
 		pages[0] = vmalloc_to_page(addr);
-		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));
-		pages[1] = virt_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
-	BUG_ON(!pages[0]);
+	/*
+	 * If something went wrong, crash and burn since recovery paths are not
+	 * implemented.
+	 */
+	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
+
 	local_irq_save(flags);
-	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
-	if (pages[1])
-		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
-	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	clear_fixmap(FIX_TEXT_POKE0);
-	if (pages[1])
-		clear_fixmap(FIX_TEXT_POKE1);
-	local_flush_tlb();
-	sync_core();
-	/* Could also do a CLFLUSH here to speed up CPU recovery; but
-	   that causes hangs on some VIA CPUs. */
-	for (i = 0; i < len; i++)
-		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
+
+	/*
+	 * Map the page without the global bit, as TLB flushing is done with
+	 * flush_tlb_mm_range(), which is intended for non-global PTEs.
+	 */
+	pgprot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);
+
+	/*
+	 * The lock is not really needed, but this allows to avoid open-coding.
+	 */
+	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
+
+	/*
+	 * This must not fail; preallocated in poking_init().
+	 */
+	VM_BUG_ON(!ptep);
+
+	pte = mk_pte(pages[0], pgprot);
+	set_pte_at(poking_mm, poking_addr, ptep, pte);
+
+	if (cross_page_boundary) {
+		pte = mk_pte(pages[1], pgprot);
+		set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte);
+	}
+
+	/*
+	 * Loading the temporary mm behaves as a compiler barrier, which
+	 * guarantees that the PTE will be set at the time memcpy() is done.
+	 */
+	prev = use_temporary_mm(poking_mm);
+
+	kasan_disable_current();
+	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
+	kasan_enable_current();
+
+	/*
+	 * Ensure that the PTE is only cleared after the instructions of memcpy
+	 * were issued by using a compiler barrier.
+	 */
+	barrier();
+
+	pte_clear(poking_mm, poking_addr, ptep);
+	if (cross_page_boundary)
+		pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
+
+	/*
+	 * Loading the previous page-table hierarchy requires a serializing
+	 * instruction that already allows the core to see the updated version.
+	 * Xen-PV is assumed to serialize execution in a similar manner.
+	 */
+	unuse_temporary_mm(prev);
+
+	/*
+	 * Flushing the TLB might involve IPIs, which would require enabled
+	 * IRQs, but not if the mm is not used, as it is in this point.
+	 */
+	flush_tlb_mm_range(poking_mm, poking_addr, poking_addr +
+			   (cross_page_boundary ? 2 : 1) * PAGE_SIZE,
+			   PAGE_SHIFT, false);
+
+	/*
+	 * If the text does not match what we just wrote then something is
+	 * fundamentally screwy; there's nothing we can really do about that.
+	 */
+	BUG_ON(memcmp(addr, opcode, len));
+
+	pte_unmap_unlock(ptep, ptl);
 	local_irq_restore(flags);
 	return addr;
 }
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 0f4fe206dcc2..82b181fcefe5 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2319,8 +2319,6 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
 #elif defined(CONFIG_X86_VSYSCALL_EMULATION)
 	case VSYSCALL_PAGE:
 #endif
-	case FIX_TEXT_POKE0:
-	case FIX_TEXT_POKE1:
 		/* All local page mappings */
 		pte = pfn_pte(phys, prot);
 		break;
-- 
2.17.1


  parent reply	other threads:[~2019-02-21 23:52 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-21 23:44 [PATCH v3 00/20] Merge text_poke fixes and executable lockdowns Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 01/20] x86/jump_label: Use text_poke_early() during early init Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 02/20] x86/mm: Introduce temporary mm structs Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm Rick Edgecombe
2019-02-22  0:07   ` Sean Christopherson
2019-02-22  0:17     ` Nadav Amit
2019-02-21 23:44 ` [PATCH v3 04/20] fork: Provide a function for copying init_mm Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 05/20] x86/alternative: Initialize temporary mm for patching Rick Edgecombe
2019-02-21 23:44 ` Rick Edgecombe [this message]
2019-02-21 23:44 ` [PATCH v3 07/20] x86/kgdb: Avoid redundant comparison of patched code Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 08/20] x86/ftrace: Set trampoline pages as executable Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 09/20] x86/kprobes: Set instruction page " Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 10/20] x86/module: Avoid breaking W^X while loading modules Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 11/20] x86/jump-label: Remove support for custom poker Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 12/20] x86/alternative: Remove the return value of text_poke_*() Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 13/20] x86/mm/cpa: Add set_direct_map_ functions Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 14/20] mm: Make hibernate handle unmapped pages Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 15/20] vmalloc: Add flag for free of special permsissions Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 16/20] modules: Use vmalloc special flag Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 17/20] bpf: " Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 18/20] x86/ftrace: " Rick Edgecombe
2019-02-22  0:22   ` Steven Rostedt
2019-02-22  0:55     ` Edgecombe, Rick P
2019-02-21 23:44 ` [PATCH v3 19/20] x86/kprobes: " Rick Edgecombe
2019-02-21 23:44 ` [PATCH v3 20/20] x86/alternative: Comment about module removal races Rick Edgecombe
2019-02-22 16:14 ` [PATCH v3 00/20] Merge text_poke fixes and executable lockdowns Borislav Petkov
2019-02-22 18:32   ` Edgecombe, Rick P

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