| Message ID | dab948690a74db1bb75d95aa7e0362deeca6dbf4.1678785672.git.baskov@ispras.ru |
|---|---|
| State | New |
| Headers | show |
| Series | x86_64: Improvements at compressed kernel stage | expand |
On 2023-03-14 23:33, Andy Lutomirski wrote: > On Tue, Mar 14, 2023, at 3:13 AM, Evgeniy Baskov wrote: >> After every implicit mapping is removed, this code is no longer >> needed. >> >> Remove memory mapping from page fault handler to ensure that there are >> no hidden invalid memory accesses. > > This patch is *by far* the scariest of the bunch in my boot. And it > violates a basic principle of kernel development: it's better to run > in degraded mode than to fail outright unless running in degraded mode > is dangerous for some reason. > > And this boot code is not actually meaningfully exposed to attack. > Anyone who can get the boot code to consume garbage likely *already* > controls the system, including anything that we might write to TPM or > any other verification mechanism. > > So I think this should log an error, set a flag to make sure we print > an even louder error after full boot, but still add the mapping and > keep trying. Good point. This patch can be dropped and replaced by the loud warning, since it is not required for the functioning of the rest of the series it is here mainly to indicate bugs in the kernel rather than for the increased protection. But I would not expect anything in the working systems, I made my best to map all the things explicitly. And since no code but the extraction code is supposed to be run (interrupts are disabled and we are not using any UEFI services there), this should be practically save to remove the implicit mapping. And at least this allowed me to find out about the insufficient size of the boot page tables which did not account for the ACPI and UEFI mappings. (patch 4 "x86/boot: Increase boot page table size") If this patch is dropped now, I can send the follow up patch later adding the warning. Thanks, Evgeniy Baskov > > --Andy > >> >> Tested-by: Mario Limonciello <mario.limonciello@amd.com> >> Signed-off-by: Evgeniy Baskov <baskov@ispras.ru> >> --- >> arch/x86/boot/compressed/ident_map_64.c | 26 >> ++++++++++--------------- >> 1 file changed, 10 insertions(+), 16 deletions(-) >> ...
diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c index eb28ce9812c5..378f99b1d7e8 100644 --- a/arch/x86/boot/compressed/ident_map_64.c +++ b/arch/x86/boot/compressed/ident_map_64.c @@ -393,27 +393,21 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code) { unsigned long address = native_read_cr2(); unsigned long end; - bool ghcb_fault; + char *msg; - ghcb_fault = sev_es_check_ghcb_fault(address); + if (sev_es_check_ghcb_fault(address)) + msg = "Page-fault on GHCB page:"; + else + msg = "Unexpected page-fault:"; address &= PMD_MASK; end = address + PMD_SIZE; /* - * Check for unexpected error codes. Unexpected are: - * - Faults on present pages - * - User faults - * - Reserved bits set - */ - if (error_code & (X86_PF_PROT | X86_PF_USER | X86_PF_RSVD)) - do_pf_error("Unexpected page-fault:", error_code, address, regs->ip); - else if (ghcb_fault) - do_pf_error("Page-fault on GHCB page:", error_code, address, regs->ip); - - /* - * Error code is sane - now identity map the 2M region around - * the faulting address. + * Since all memory allocations are made explicit + * now, every page fault at this stage is an + * error and the error handler is there only + * for debug purposes. */ - kernel_add_identity_map(address, end, MAP_WRITE); + do_pf_error(msg, error_code, address, regs->ip); }