[PATCH v6 15/76] x86/boot/compressed/64: Add page-fault handler

Joerg Roedel
2020-08-24T08:56:06Z

Install a page-fault handler to add an identity mapping to addresses
not yet mapped. Also do some checking whether the error code is sane.

This makes non SEV-ES machines use the exception handling
infrastructure in the pre-decompressions boot code too, making it less
likely to break in the future.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200724160336.5435-15-joro@8bytes.org
---
arch/x86/boot/compressed/ident_map_64.c | 39 ++++++++++++++++++++++
arch/x86/boot/compressed/idt_64.c | 2 ++
arch/x86/boot/compressed/idt_handlers_64.S | 2 ++
arch/x86/boot/compressed/misc.h | 6 ++++
4 files changed, 49 insertions(+)

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index d9932a133ac9..e3d980ae9c2b 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -19,10 +19,13 @@
/* No PAGE_TABLE_ISOLATION support needed either: */
#undef CONFIG_PAGE_TABLE_ISOLATION

+#include "error.h"
#include "misc.h"

/* These actually do the work of building the kernel identity maps. */
#include <linux/pgtable.h>
+#include <asm/trap_pf.h>
+#include <asm/trapnr.h>
#include <asm/init.h>
/* Use the static base for this part of the boot process */
#undef __PAGE_OFFSET
@@ -160,3 +163,39 @@ void finalize_identity_maps(void)
{
write_cr3(top_level_pgt);
}
+
+static void do_pf_error(const char *msg, unsigned long error_code,
+ unsigned long address, unsigned long ip)
+{
+ error_putstr(msg);
+
+ error_putstr("\nError Code: ");
+ error_puthex(error_code);
+ error_putstr("\nCR2: 0x");
+ error_puthex(address);
+ error_putstr("\nRIP relative to _head: 0x");
+ error_puthex(ip - (unsigned long)_head);
+ error_putstr("\n");
+
+ error("Stopping.\n");
+}
+
+void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
+{
+ unsigned long address = native_read_cr2();
+
+ /*
+ * 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);
+
+ /*
+ * Error code is sane - now identity map the 2M region around
+ * the faulting address.
+ */
+ add_identity_map(address & PMD_MASK, PMD_SIZE);
+}
diff --git a/arch/x86/boot/compressed/idt_64.c b/arch/x86/boot/compressed/idt_64.c
index 082cd6bca033..5f083092a86d 100644
--- a/arch/x86/boot/compressed/idt_64.c
+++ b/arch/x86/boot/compressed/idt_64.c
@@ -40,5 +40,7 @@ void load_stage2_idt(void)
{
boot_idt_desc.address = (unsigned long)boot_idt;

+ set_idt_entry(X86_TRAP_PF, boot_page_fault);
+
load_boot_idt(&boot_idt_desc);
}
diff --git a/arch/x86/boot/compressed/idt_handlers_64.S b/arch/x86/boot/compressed/idt_handlers_64.S
index 36dee2f40a8b..b20e57504a94 100644
--- a/arch/x86/boot/compressed/idt_handlers_64.S
+++ b/arch/x86/boot/compressed/idt_handlers_64.S
@@ -68,3 +68,5 @@ SYM_FUNC_END(\name)

.text
.code64
+
+EXCEPTION_HANDLER boot_page_fault do_boot_page_fault error_code=1
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 98b7a1df9c59..f0e199174c5f 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -37,6 +37,9 @@
#define memptr unsigned
#endif

+/* boot/compressed/vmlinux start and end markers */
+extern char _head[], _end[];
+
/* misc.c */
extern memptr free_mem_ptr;
extern memptr free_mem_end_ptr;
@@ -146,4 +149,7 @@ extern pteval_t __default_kernel_pte_mask;
extern gate_desc boot_idt[BOOT_IDT_ENTRIES];
extern struct desc_ptr boot_idt_desc;

+/* IDT Entry Points */
+void boot_page_fault(void);
+
#endif /* BOOT_COMPRESSED_MISC_H */