> On Aug 7, 2018, at 11:49 AM, Boris Ostrovsky <boris.ostrov...@oracle.com> > wrote: > >> On 08/07/2018 01:20 PM, George Dunlap wrote: >>> On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon <c...@prgmr.com> wrote: >>> I just got the following patch from a colleague. It's a backport of >>> the XSA 274 kernel patch to 4.9.x kernels. The kernel patch given in >>> the XSA would not apply cleanly. Would someone mind reviewing it? It >>> would be much appreciated. >>> >>> commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream. >>> >>> This version applies to v4.9. >>> >>> 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. >>> >>> 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: Sarah Newman <s...@prgmr.com> >> I think you need to retain Andy's SoB, and add your own underneath. >> >> This looks plausible to me -- Andy / Boris, any opinions? > > > LGTM. > > Note also that Andy's patch had slightly longer commit message > (including some of the tags that you are missing), with this suggestion: > > [ Note to stable maintainers: this should probably get applied to all > kernels. If you're nervous about that, a more conservative fix to > add xorl %ebx,%ebx; incl %ebx before the jump to error_exit should > also fix the problem. ] >
On further review, I don’t like that suggestion. What if the callback came from user code. It’s not supposed to happen on modern kernels, but I’m not sure I trust that. > > > > -boris > _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel