From ee46485b45de9c2008e3ef5f847041fc18743052 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Wed, 3 Jan 2018 19:52:04 +0100 Subject: [PATCH 225/231] x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit CVE-2017-5754 The preparation for PTI which added CR3 switching to the entry code misplaced the CR3 switch in entry_SYSCALL_compat(). With PTI enabled the entry code tries to access a per cpu variable after switching to kernel GS. This fails because that variable is not mapped to user space. This results in a double fault and in the worst case a kernel crash. Move the switch ahead of the access and clobber RSP which has been saved already. Fixes: 8a09317b895f ("x86/mm/pti: Prepare the x86/entry assembly code for entry/exit CR3 switching") Reported-by: Lars Wendler Reported-by: Laura Abbott Signed-off-by: Thomas Gleixner Cc: Borislav Betkov Cc: Andy Lutomirski , Cc: Dave Hansen , Cc: Peter Zijlstra , Cc: Greg KH , , Cc: Boris Ostrovsky , Cc: Juergen Gross Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801031949200.1957@nanos Signed-off-by: Andy Whitcroft Signed-off-by: Kleber Sacilotto de Souza (cherry picked from commit 2f45cd7a57da0a4d7f3a91a5f577c76b9ed9eb8a) Signed-off-by: Fabian Grünbichler --- arch/x86/entry/entry_64_compat.S | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 973527e34887..2b5e7685823c 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -189,8 +189,13 @@ ENTRY(entry_SYSCALL_compat) /* Interrupts are off on entry. */ swapgs - /* Stash user ESP and switch to the kernel stack. */ + /* Stash user ESP.*/ movl %esp, %r8d + + /* Use %rsp as scratch reg. User ESP is stashed in r8 */ + SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp + + /* Switch to the kernel stack */ movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp /* Construct struct pt_regs on stack */ @@ -218,12 +223,6 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe) pushq $0 /* pt_regs->r14 = 0 */ pushq $0 /* pt_regs->r15 = 0 */ - /* - * We just saved %rdi so it is safe to clobber. It is not - * preserved during the C calls inside TRACE_IRQS_OFF anyway. - */ - SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi - /* * User mode is traced as though IRQs are on, and SYSENTER * turned them off. -- 2.14.2