On Wed, 2018-08-08 at 10:35 -0700, Sarah Newman wrote: > commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream. > > This version applies to v4.9.
I think you can kill the 'xorl %ebx,%ebx' from error_entry too but yes, this does want to go to 4.9 and earlier because the 'Fixes:' tag is a bit of a lie — the problem existed before that, at least in theory. > From Andy Lutomirski, original author: > > error_entry and error_exit communicate the user vs kernel status of > the frame using %ebx. This is unnecessary -- the information is in > regs->cs. Just use regs->cs. > > This makes error_entry simpler and makes error_exit more robust. > > It also fixes a nasty bug. Before all the Spectre nonsense, The > xen_failsafe_callback entry point returned like this: > > ALLOC_PT_GPREGS_ON_STACK > SAVE_C_REGS > SAVE_EXTRA_REGS > ENCODE_FRAME_POINTER > jmp error_exit > > And it did not go through error_entry. This was bogus: RBX > contained garbage, and error_exit expected a flag in RBX. > Fortunately, it generally contained *nonzero* garbage, so the > correct code path was used. As part of the Spectre fixes, code was > added to clear RBX to mitigate certain speculation attacks. Now, > depending on kernel configuration, RBX got zeroed and, when running > some Wine workloads, the kernel crashes. This was introduced by: > > commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for > exceptions/interrupts, to reduce speculation attack surface") > > With this patch applied, RBX is no longer needed as a flag, and the > problem goes away. > > I suspect that malicious userspace could use this bug to crash the > kernel even without the offending patch applied, though. > > [Historical note: I wrote this patch as a cleanup before I was aware > of the bug it fixed.] > > [Note to stable maintainers: this should probably get applied to all > kernels.] > > Cc: Brian Gerst <brge...@gmail.com> > Cc: Borislav Petkov <b...@alien8.de> > Cc: Dominik Brodowski <li...@dominikbrodowski.net> > Cc: Ingo Molnar <mi...@redhat.com> > Cc: "H. Peter Anvin" <h...@zytor.com> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Boris Ostrovsky <boris.ostrov...@oracle.com> > Cc: Juergen Gross <jgr...@suse.com> > Cc: xen-devel@lists.xenproject.org > Cc: x...@kernel.org > Cc: sta...@vger.kernel.org > Cc: Andy Lutomirski <l...@kernel.org> > Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for > exceptions/interrupts, to reduce speculation attack surface") > Reported-and-tested-by: "M. Vefa Bicakci" <m....@runbox.com> > Signed-off-by: Andy Lutomirski <l...@kernel.org> > Signed-off-by: Sarah Newman <s...@prgmr.com> > --- > arch/x86/entry/entry_64.S | 19 ++++--------------- > 1 file changed, 4 insertions(+), 15 deletions(-) > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index d58d8dc..0dab47a 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -774,7 +774,7 @@ ENTRY(\sym) > > call \do_sym > > - jmp error_exit /* %ebx: no > swapgs flag */ > + jmp error_exit > .endif > END(\sym) > .endm > @@ -1043,7 +1043,6 @@ END(paranoid_exit) > > /* > * Save all registers in pt_regs, and switch gs if needed. > - * Return: EBX=0: came from user mode; EBX=1: otherwise > */ > ENTRY(error_entry) > cld > @@ -1087,7 +1086,6 @@ ENTRY(error_entry) > * for these here too. > */ > .Lerror_kernelspace: > - incl %ebx > leaq native_irq_return_iret(%rip), %rcx > cmpq %rcx, RIP+8(%rsp) > je .Lerror_bad_iret > @@ -1119,28 +1117,19 @@ ENTRY(error_entry) > > /* > * Pretend that the exception came from user mode: set up > pt_regs > - * as if we faulted immediately after IRET and clear EBX so > that > - * error_exit knows that we will be returning to user mode. > + * as if we faulted immediately after IRET. > */ > mov %rsp, %rdi > call fixup_bad_iret > mov %rax, %rsp > - decl %ebx > jmp .Lerror_entry_from_usermode_after_swapgs > END(error_entry) > > - > -/* > - * On entry, EBX is a "return to kernel mode" flag: > - * 1: already in kernel mode, don't need SWAPGS > - * 0: user gsbase is loaded, we need SWAPGS and standard > preparation for return to usermode > - */ > ENTRY(error_exit) > - movl %ebx, %eax > DISABLE_INTERRUPTS(CLBR_NONE) > TRACE_IRQS_OFF > - testl %eax, %eax > - jnz retint_kernel > + testb $3, CS(%rsp) > + jz retint_kernel > jmp retint_user > END(error_exit) >
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