Re: [PATCH] Skip UV runtime services mapping in the efi_runtime_disabled case

From: Matt Fleming
Date: Mon Aug 01 2016 - 09:55:07 EST


On Tue, 26 Jul, at 05:38:33PM, Alex Thorlton wrote:
> This problem has actually been in the UV code for a while, but we didn't
> catch it until recently, because we had been relying on EFI_OLD_MEMMAP
> to allow our systems to boot for a period of time. We noticed the issue
> when trying to kexec a recent community kernel, where we hit this NULL
> pointer dereference in efi_sync_low_kernel_mappings:
>
> [ 0.337515] BUG: unable to handle kernel NULL pointer dereference at 0000000000000880
> [ 0.346276] IP: [<ffffffff8105df8d>] efi_sync_low_kernel_mappings+0x5d/0x1b0
>
> The problem doesn't show up with EFI_OLD_MEMMAP because we skip the
> chunk of setup_efi_state that sets the efi_loader_signature for the
> kexec'd kernel. When the kexec'd kernel boots, it won't set EFI_BOOT in
> setup_arch, so we completely avoid the bug.
>
> We always kexec with noefi on the command line, so this shouldn't be an
> issue, but since we're not actually checking for efi_runtime_disabled in
> uv_bios_init, we end up trying to do EFI runtime callbacks when we
> shouldn't be. This patch just adds a check for efi_runtime_disabled in
> uv_bios_init so that we don't map in uv_systab when runtime_disabled ==
> true.
>
> Signed-off-by: Alex Thorlton <athorlton@xxxxxxx>
> Cc: Russ Anderson <rja@xxxxxxx>
> Cc: Mike Travis <travis@xxxxxxx>
> Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
> arch/x86/platform/uv/bios_uv.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/platform/uv/bios_uv.c b/arch/x86/platform/uv/bios_uv.c
> index 66b2166..0df8a03 100644
> --- a/arch/x86/platform/uv/bios_uv.c
> +++ b/arch/x86/platform/uv/bios_uv.c
> @@ -187,7 +187,8 @@ EXPORT_SYMBOL_GPL(uv_bios_set_legacy_vga_target);
> void uv_bios_init(void)
> {
> uv_systab = NULL;
> - if ((efi.uv_systab == EFI_INVALID_TABLE_ADDR) || !efi.uv_systab) {
> + if ((efi.uv_systab == EFI_INVALID_TABLE_ADDR) ||
> + !efi.uv_systab || efi_runtime_disabled()) {
> pr_crit("UV: UVsystab: missing\n");
> return;
> }

The fix looks fine, but I'm losing track of which kernels this patch
should be applied to. Does it just need to be applied for v4.8 or
earlier kernels too?