124 lines
4.6 KiB
Diff
124 lines
4.6 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Thomas Gleixner <tglx@linutronix.de>
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Date: Fri, 15 Dec 2017 20:35:11 +0100
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Subject: [PATCH] x86/ldt: Make the LDT mapping RO
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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CVE-2017-5754
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Now that the LDT mapping is in a known area when PAGE_TABLE_ISOLATION is
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enabled its a primary target for attacks, if a user space interface fails
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to validate a write address correctly. That can never happen, right?
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The SDM states:
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If the segment descriptors in the GDT or an LDT are placed in ROM, the
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processor can enter an indefinite loop if software or the processor
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attempts to update (write to) the ROM-based segment descriptors. To
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prevent this problem, set the accessed bits for all segment descriptors
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placed in a ROM. Also, remove operating-system or executive code that
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attempts to modify segment descriptors located in ROM.
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So its a valid approach to set the ACCESS bit when setting up the LDT entry
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and to map the table RO. Fixup the selftest so it can handle that new mode.
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Remove the manual ACCESS bit setter in set_tls_desc() as this is now
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pointless. Folded the patch from Peter Ziljstra.
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Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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Cc: Andy Lutomirski <luto@kernel.org>
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Cc: Borislav Petkov <bp@alien8.de>
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Cc: Dave Hansen <dave.hansen@linux.intel.com>
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Cc: H. Peter Anvin <hpa@zytor.com>
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Cc: Josh Poimboeuf <jpoimboe@redhat.com>
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Cc: Juergen Gross <jgross@suse.com>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Peter Zijlstra <peterz@infradead.org>
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Signed-off-by: Ingo Molnar <mingo@kernel.org>
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(cherry picked from commit 9f5cb6b32d9e0a3a7453222baaf15664d92adbf2)
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Signed-off-by: Andy Whitcroft <apw@canonical.com>
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Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
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(cherry picked from commit f4b13d6f67b3a89d878094901a9ca834b39415c1)
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Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
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---
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arch/x86/include/asm/desc.h | 2 ++
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arch/x86/kernel/ldt.c | 7 ++++++-
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arch/x86/kernel/tls.c | 11 ++---------
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tools/testing/selftests/x86/ldt_gdt.c | 3 +--
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4 files changed, 11 insertions(+), 12 deletions(-)
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diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
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index de40c514ba25..c765bc294a9d 100644
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--- a/arch/x86/include/asm/desc.h
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+++ b/arch/x86/include/asm/desc.h
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@@ -20,6 +20,8 @@ static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *in
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desc->type = (info->read_exec_only ^ 1) << 1;
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desc->type |= info->contents << 2;
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+ /* Set the ACCESS bit so it can be mapped RO */
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+ desc->type |= 1;
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desc->s = 1;
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desc->dpl = 0x3;
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diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
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index eceaada581ff..2260eb6e2de7 100644
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--- a/arch/x86/kernel/ldt.c
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+++ b/arch/x86/kernel/ldt.c
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@@ -157,7 +157,12 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
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ptep = get_locked_pte(mm, va, &ptl);
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if (!ptep)
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return -ENOMEM;
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- pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL));
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+ /*
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+ * Map it RO so the easy to find address is not a primary
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+ * target via some kernel interface which misses a
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+ * permission check.
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+ */
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+ pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));
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set_pte_at(mm, va, ptep, pte);
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pte_unmap_unlock(ptep, ptl);
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}
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diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
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index a106b9719c58..41880a2421ea 100644
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--- a/arch/x86/kernel/tls.c
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+++ b/arch/x86/kernel/tls.c
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@@ -92,17 +92,10 @@ static void set_tls_desc(struct task_struct *p, int idx,
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cpu = get_cpu();
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while (n-- > 0) {
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- if (LDT_empty(info) || LDT_zero(info)) {
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+ if (LDT_empty(info) || LDT_zero(info))
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memset(desc, 0, sizeof(*desc));
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- } else {
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+ else
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fill_ldt(desc, info);
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-
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- /*
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- * Always set the accessed bit so that the CPU
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- * doesn't try to write to the (read-only) GDT.
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- */
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- desc->type |= 1;
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- }
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++info;
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++desc;
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}
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diff --git a/tools/testing/selftests/x86/ldt_gdt.c b/tools/testing/selftests/x86/ldt_gdt.c
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index 783e1a754b78..bbd1d0e4d683 100644
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--- a/tools/testing/selftests/x86/ldt_gdt.c
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+++ b/tools/testing/selftests/x86/ldt_gdt.c
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@@ -121,8 +121,7 @@ static void check_valid_segment(uint16_t index, int ldt,
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* NB: Different Linux versions do different things with the
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* accessed bit in set_thread_area().
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*/
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- if (ar != expected_ar &&
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- (ldt || ar != (expected_ar | AR_ACCESSED))) {
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+ if (ar != expected_ar && ar != (expected_ar | AR_ACCESSED)) {
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printf("[FAIL]\t%s entry %hu has AR 0x%08X but expected 0x%08X\n",
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(ldt ? "LDT" : "GDT"), index, ar, expected_ar);
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nerrs++;
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--
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2.14.2
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