Re: [PATCHv5 06/12] x86/boot/compressed: Handle unaccepted memory

[Kirill A. Shutemov]

On Tue, Apr 26, 2022 at 07:17:56PM -0500, Michael Roth wrote:
> On Mon, Apr 25, 2022 at 06:39:28AM +0300, Kirill A. Shutemov wrote:
> > The firmware will pre-accept the memory used to run the stub. But, the
> > stub is responsible for accepting the memory into which it decompresses
> > the main kernel. Accept memory just before decompression starts.
> > 
> > The stub is also responsible for choosing a physical address in which to
> > place the decompressed kernel image. The KASLR mechanism will randomize
> > this physical address. Since the unaccepted memory region is relatively
> > small, KASLR would be quite ineffective if it only used the pre-accepted
> > area (EFI_CONVENTIONAL_MEMORY). Ensure that KASLR randomizes among the
> > entire physical address space by also including EFI_UNACCEPTED_MEMOR
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/boot/compressed/Makefile        |  2 +-
> >  arch/x86/boot/compressed/kaslr.c         | 14 ++++++++++++--
> >  arch/x86/boot/compressed/mem.c           | 21 +++++++++++++++++++++
> >  arch/x86/boot/compressed/misc.c          |  9 +++++++++
> >  arch/x86/include/asm/unaccepted_memory.h |  2 ++
> >  5 files changed, 45 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > index 7f672f7e2fea..b59007e57cbf 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -102,7 +102,7 @@ endif
> >  
> >  vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
> >  vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdx.o $(obj)/tdcall.o
> > -vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/bitmap.o $(obj)/mem.o
> > +vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/bitmap.o $(obj)/find.o $(obj)/mem.o
> 
> Since it's possible to have CONFIG_UNACCEPTED_MEMORY=y while
> CONFIG_INTEL_TDX_GUEST=n (e.g. for SNP-only guest kernels), this can
> result in mem.o reporting linker errors due to tdx_accept_memory() not
> being defined. I think it needs a stub for !CONFIG_INTEL_TDX_GUEST, or
> something along that line.

Fair enough. This would do the trick:

diff --git a/arch/x86/boot/compressed/mem.c b/arch/x86/boot/compressed/mem.c
index 539fff27de49..4a49a2438180 100644
--- a/arch/x86/boot/compressed/mem.c
+++ b/arch/x86/boot/compressed/mem.c
@@ -19,6 +19,9 @@ static bool is_tdx_guest(void)
 	static bool once;
 	static bool is_tdx;

+	if (!IS_ENABLED(CONFIG_INTEL_TDX_GUEST))
+	    return false;
+
 	if (!once) {
 		u32 eax, sig[3];

> >  vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
> >  efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 411b268bc0a2..59db90626042 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -725,10 +725,20 @@ process_efi_entries(unsigned long minimum, unsigned long image_size)
> >  		 * but in practice there's firmware where using that memory leads
> >  		 * to crashes.
> >  		 *
> > -		 * Only EFI_CONVENTIONAL_MEMORY is guaranteed to be free.
> > +		 * Only EFI_CONVENTIONAL_MEMORY and EFI_UNACCEPTED_MEMORY (if
> > +		 * supported) are guaranteed to be free.
> >  		 */
> > -		if (md->type != EFI_CONVENTIONAL_MEMORY)
> > +
> > +		switch (md->type) {
> > +		case EFI_CONVENTIONAL_MEMORY:
> > +			break;
> > +		case EFI_UNACCEPTED_MEMORY:
> 
> Just FYI, but with latest tip boot/compressed now relies on a separate header
> in arch/x86/boot/compressed/efi.h where this need to be defined again.

Right.

Borislav, how do you want to handle this? Do you want me to rebase the
tree to a specific branch?

-- 
 Kirill A. Shutemov