Message ID | 20200527191847.17207-12-andrew.cooper3@citrix.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | x86: Support for CET Supervisor Shadow Stacks | expand |
On 27.05.2020 21:18, Andrew Cooper wrote: > @@ -398,6 +399,19 @@ static void __init _alternative_instructions(bool force) > panic("Timed out waiting for alternatives self-NMI to hit\n"); > > set_nmi_callback(saved_nmi_callback); > + > + /* > + * When Xen is using shadow stacks, the alternatives clearing CR0.WP and > + * writing into the mappings set dirty bits, turning the mappings into > + * shadow stack mappings. > + * > + * While we can execute from them, this would also permit them to be the > + * target of WRSS instructions, so reset the dirty after patching. > + */ > + if ( cpu_has_xen_shstk ) > + modify_xen_mappings(XEN_VIRT_START + MB(2), > + (unsigned long)&__2M_text_end, > + PAGE_HYPERVISOR_RX); Am I misremembering, or did you post a patch before that should be part of this series, as being a prereq to this change, making modify_xen_mappings() also respect _PAGE_DIRTY as requested by the caller? Additionally I notice this if ( desc->Attribute & (efi_bs_revision < EFI_REVISION(2, 5) ? EFI_MEMORY_WP : EFI_MEMORY_RO) ) prot &= ~_PAGE_RW; in efi_init_memory(). Afaict we will need to clear _PAGE_DIRTY there as well, with prot starting out as PAGE_HYPERVISOR_RWX. Jan
On 29/05/2020 13:23, Jan Beulich wrote: > On 27.05.2020 21:18, Andrew Cooper wrote: >> @@ -398,6 +399,19 @@ static void __init _alternative_instructions(bool force) >> panic("Timed out waiting for alternatives self-NMI to hit\n"); >> >> set_nmi_callback(saved_nmi_callback); >> + >> + /* >> + * When Xen is using shadow stacks, the alternatives clearing CR0.WP and >> + * writing into the mappings set dirty bits, turning the mappings into >> + * shadow stack mappings. >> + * >> + * While we can execute from them, this would also permit them to be the >> + * target of WRSS instructions, so reset the dirty after patching. >> + */ >> + if ( cpu_has_xen_shstk ) >> + modify_xen_mappings(XEN_VIRT_START + MB(2), >> + (unsigned long)&__2M_text_end, >> + PAGE_HYPERVISOR_RX); > Am I misremembering, or did you post a patch before that should > be part of this series, as being a prereq to this change, > making modify_xen_mappings() also respect _PAGE_DIRTY as > requested by the caller? No. Its the hunk you deleted from lower in this patch. > Additionally I notice this > > if ( desc->Attribute & (efi_bs_revision < EFI_REVISION(2, 5) > ? EFI_MEMORY_WP : EFI_MEMORY_RO) ) > prot &= ~_PAGE_RW; > > in efi_init_memory(). Afaict we will need to clear _PAGE_DIRTY > there as well, with prot starting out as PAGE_HYPERVISOR_RWX. Ok. I'll pull that out into a separate patch. ~Andrew
diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c index ce2b4302e6..004e9ede25 100644 --- a/xen/arch/x86/alternative.c +++ b/xen/arch/x86/alternative.c @@ -21,6 +21,7 @@ #include <asm/processor.h> #include <asm/alternative.h> #include <xen/init.h> +#include <asm/setup.h> #include <asm/system.h> #include <asm/traps.h> #include <asm/nmi.h> @@ -398,6 +399,19 @@ static void __init _alternative_instructions(bool force) panic("Timed out waiting for alternatives self-NMI to hit\n"); set_nmi_callback(saved_nmi_callback); + + /* + * When Xen is using shadow stacks, the alternatives clearing CR0.WP and + * writing into the mappings set dirty bits, turning the mappings into + * shadow stack mappings. + * + * While we can execute from them, this would also permit them to be the + * target of WRSS instructions, so reset the dirty after patching. + */ + if ( cpu_has_xen_shstk ) + modify_xen_mappings(XEN_VIRT_START + MB(2), + (unsigned long)&__2M_text_end, + PAGE_HYPERVISOR_RX); } void __init alternative_instructions(void) diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c index 4d6d22cc41..711b12828f 100644 --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -5442,8 +5442,8 @@ int populate_pt_range(unsigned long virt, unsigned long nr_mfns) * mappings, but will shatter superpages if necessary, and will destroy * mappings if not passed _PAGE_PRESENT. * - * The only flags considered are NX, RW and PRESENT. All other input flags - * are ignored. + * The only flags considered are NX, D, A, RW and PRESENT. All other input + * flags are ignored. * * It is an error to call with present flags over an unpopulated range. */ @@ -5456,7 +5456,7 @@ int modify_xen_mappings(unsigned long s, unsigned long e, unsigned int nf) unsigned long v = s; /* Set of valid PTE bits which may be altered. */ -#define FLAGS_MASK (_PAGE_NX|_PAGE_RW|_PAGE_PRESENT) +#define FLAGS_MASK (_PAGE_NX|_PAGE_DIRTY|_PAGE_ACCESSED|_PAGE_RW|_PAGE_PRESENT) nf &= FLAGS_MASK; ASSERT(IS_ALIGNED(s, PAGE_SIZE));