On 07/21/2018 05:45 PM, Andy Lutomirski wrote:
On Sat, Jul 21, 2018 at 12:49 PM, M. Vefa Bicakci <m.v.b@xxxxxxxxxx> wrote:Commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface") unintendedly broke Xen PV virtual machines by clearing the %rbx register at the end of xen_failsafe_callback before the latter jumps to error_exit. error_exit expects the %rbx register to be a flag indicating whether there should be a return to kernel mode. This commit makes sure that the %rbx register is not cleared by the PUSH_AND_CLEAR_REGS macro, when the macro in question is instantiated by xen_failsafe_callback, to avoid the issue.Seems like a genuine problem, but:diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index c7449f377a77..96e8ff34129e 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1129,7 +1129,7 @@ ENTRY(xen_failsafe_callback) addq $0x30, %rsp UNWIND_HINT_IRET_REGS pushq $-1 /* orig_ax = -1 => not a system call */ - PUSH_AND_CLEAR_REGS + PUSH_AND_CLEAR_REGS clear_rbx=0 ENCODE_FRAME_POINTER jmp error_exitThe old code first set RBX to zero then, if frame pointers are on, sets it to some special non-zero value, then crosses its fingers and hopes for the best. Your patched code just skips the zeroing part, so RBX either contains the ENCODE_FRAME_POINTER result or is uninitialized. How about actually initializing rbx to something sensible like, say, 1?
Hello Andy, Thank you for the review! Apparently, I have not done my homework fully. I will test your suggestion and report back, most likely in a few hours. I have been testing with the next/linux-next tree's master branch (dated 20180720), and I noticed that ENCODE_FRAME_POINTER changes the frame pointer (i.e., RBP) register, as opposed to the RBX register, which the patch aims to avoid changing before jumping to error_exit. It is possible that I am missing something though -- I am not sure about the connection between the RBP and RBX registers. The change introduced by commit 3ac6d8c787b8 is in the excerpt below. Would it be valid to state that the original code had the same issue that you referred to (i.e., leaving the RBX register uninitialized)? [I also realized that I forgot to include Andi Kleen and Dan Williams, authors of 3ac6d8c787b8, in the discussion; I am copying this e-mail to them as well.] Thank you, Vefa $ git show -W 3ac6d8c787b8 -- arch/x86/entry/entry_64.S ... ENTRY(xen_failsafe_callback) UNWIND_HINT_EMPTY movl %ds, %ecx cmpw %cx, 0x10(%rsp) jne 1f movl %es, %ecx cmpw %cx, 0x18(%rsp) jne 1f movl %fs, %ecx cmpw %cx, 0x20(%rsp) jne 1f movl %gs, %ecx cmpw %cx, 0x28(%rsp) jne 1f /* All segments match their saved values => Category 2 (Bad IRET). */ movq (%rsp), %rcx movq 8(%rsp), %r11 addq $0x30, %rsp pushq $0 /* RIP */ UNWIND_HINT_IRET_REGS offset=8 jmp general_protection 1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */ movq (%rsp), %rcx movq 8(%rsp), %r11 addq $0x30, %rsp UNWIND_HINT_IRET_REGS pushq $-1 /* orig_ax = -1 => not a system call */ ALLOC_PT_GPREGS_ON_STACK SAVE_C_REGS SAVE_EXTRA_REGS + CLEAR_REGS_NOSPEC ENCODE_FRAME_POINTER jmp error_exit END(xen_failsafe_callback) ...
