From af39d6494fc6f8334cf1dc27c361c0d199bbd1cc Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Thu, 21 Oct 2021 15:09:32 +1100 Subject: [PATCH] x86/xen: update xen_oldmem_pfn_is_ram() documentation After removing /dev/kmem, sanitizing /proc/kcore and handling /dev/mem, this series tackles the last sane way how a VM could accidentially access logically unplugged memory managed by a virtio-mem device: /proc/vmcore When dumping memory via "makedumpfile", PG_offline pages, used by virtio-mem to flag logically unplugged memory, are already properly excluded; however, especially when accessing/copying /proc/vmcore "the usual way", we can still end up reading logically unplugged memory part of a virtio-mem device. Patch #1-#3 are cleanups. Patch #4 extends the existing oldmem_pfn_is_ram mechanism. Patch #5-#7 are virtio-mem refactorings for patch #8, which implements the virtio-mem logic to query the state of device blocks. Patch #8: " Although virtio-mem currently supports reading unplugged memory in the hypervisor, this will change in the future, indicated to the device via a new feature flag. We similarly sanitized /proc/kcore access recently. [...] Distributions that support virtio-mem+kdump have to make sure that the virtio_mem module will be part of the kdump kernel or the kdump initrd; dracut was recently [2] extended to include virtio-mem in the generated initrd. As long as no special kdump kernels are used, this will automatically make sure that virtio-mem will be around in the kdump initrd and sanitize /proc/vmcore access -- with dracut. " This is the last remaining bit to support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE [3] in the Linux implementation of virtio-mem. Note: this is best-effort. We'll never be able to control what runs inside the second kernel, really, but we also don't have to care: we only care about sane setups where we don't want our VM getting zapped once we touch the wrong memory location while dumping. While we usually expect sane setups to use "makedumfile", nothing really speaks against just copying /proc/vmcore, especially in environments where HWpoisioning isn't typically expected. Also, we really don't want to put all our trust completely on the memmap, so sanitizing also makes sense when just using "makedumpfile". [1] https://lkml.kernel.org/r/20210526093041.8800-1-david@redhat.com [2] https://github.com/dracutdevs/dracut/pull/1157 [3] https://lists.oasis-open.org/archives/virtio-comment/202109/msg00021.html This patch (of 9): The callback is only used for the vmcore nowadays. Link: https://lkml.kernel.org/r/20211005121430.30136-1-david@redhat.com Link: https://lkml.kernel.org/r/20211005121430.30136-2-david@redhat.com Signed-off-by: David Hildenbrand Reviewed-by: Boris Ostrovsky Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: "H. Peter Anvin" Cc: Juergen Gross Cc: Stefano Stabellini Cc: "Michael S. Tsirkin" Cc: Jason Wang Cc: Dave Young Cc: Baoquan He Cc: Vivek Goyal Cc: Michal Hocko Cc: Oscar Salvador Cc: Mike Rapoport Cc: "Rafael J. Wysocki" Signed-off-by: Andrew Morton Signed-off-by: Stephen Rothwell --- arch/x86/xen/mmu_hvm.c | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-) diff --git a/arch/x86/xen/mmu_hvm.c b/arch/x86/xen/mmu_hvm.c index 57409373750f0..b242d1f4b4269 100644 --- a/arch/x86/xen/mmu_hvm.c +++ b/arch/x86/xen/mmu_hvm.c @@ -9,12 +9,9 @@ #ifdef CONFIG_PROC_VMCORE /* - * This function is used in two contexts: - * - the kdump kernel has to check whether a pfn of the crashed kernel - * was a ballooned page. vmcore is using this function to decide - * whether to access a pfn of the crashed kernel. - * - the kexec kernel has to check whether a pfn was ballooned by the - * previous kernel. If the pfn is ballooned, handle it properly. + * The kdump kernel has to check whether a pfn of the crashed kernel + * was a ballooned page. vmcore is using this function to decide + * whether to access a pfn of the crashed kernel. * Returns 0 if the pfn is not backed by a RAM page, the caller may * handle the pfn special in this case. */