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x86/64/mm: Map all kernel memory into trampoline_pgd

PATCH In review Joerg Roedel 2 participants 3 messages Sep 13, 2021 – Sep 15, 2021

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Joerg Roedel

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PATCH 1/1 x86/64/mm: Map all kernel memory into trampoline_pgd

From: Joerg Roedel <[email protected]>

The trampoline_pgd only maps the 0xfffffff000000000-0xffffffffffffffff range of kernel memory (with 4-level paging). This range contains the kernels text+data+bss mappings and the module mapping space, but not the direct mapping and the vmalloc area.

This is enough to get an application processors out of real-mode, but for code that switches back to real-mode the trampoline_pgd is missing important parts of the address space. For example, consider this code from arch/x86/kernel/reboot.c, function machine_real_restart() for a 64-bit kernel:

	#ifdef CONFIG_X86_32
		load_cr3(initial_page_table);
	#else
		write_cr3(real_mode_header->trampoline_pgd);

		/* Exiting long mode will fail if CR4.PCIDE is set. */
		if (boot_cpu_has(X86_FEATURE_PCID))
			cr4_clear_bits(X86_CR4_PCIDE);
	#endif

		/* Jump to the identity-mapped low memory code */
	#ifdef CONFIG_X86_32
		asm volatile("jmpl *%0" : :
			     "rm" (real_mode_header->machine_real_restart_asm),
			     "a" (type));
	#else
		asm volatile("ljmpl *%0" : :
			     "m" (real_mode_header->machine_real_restart_asm),
			     "D" (type));
	#endif

The code switches to the trampoline_pgd, which unmaps the direct mapping and also the kernel stack. The call to cr4_clear_bits() will find no stack and crash the machine. The real_mode_header pointer below points into the direct mapping, and dereferencing it also causes a crash.

The reason this does not crash always is only that kernel mappings are global and the CR3 switch does not flush those mappings. But if theses mappings are not in the TLB already, the above code will crash before it can jump to the real-mode stub.

Extend the trampoline_pgd to contain all kernel mappings to prevent these crashes and to make code which runs on this page-table more robust.

Cc [email protected]
Signed-off-by Joerg Roedel <[email protected]>
---
 arch/x86/realmode/init.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
arch/x86/realmode/init.c +11 -1
index 31b5856010cb..7a08c96cb42a 100644
@@ -72,6 +72,7 @@ static void __init setup_real_mode(void)
 #ifdef CONFIG_X86_64
 	u64 *trampoline_pgd;
 	u64 efer;
+	int i;
 #endif
 
 	base = (unsigned char *)real_mode_header;

@@ -128,8 +129,17 @@ static void __init setup_real_mode(void)
 	trampoline_header->flags = 0;
 
 	trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd);
+
+	/*
+	 * Map all of kernel memory into the trampoline PGD so that it includes
+	 * the direct mapping and vmalloc space. This is needed to keep the
+	 * stack and real_mode_header mapped when switching to this page table.
+	 */
+	for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++)
+		trampoline_pgd[i] = init_top_pgt[i].pgd;
+
+	/* Map the real mode stub as virtual == physical */
 	trampoline_pgd[0] = trampoline_pgd_entry.pgd;
-	trampoline_pgd[511] = init_top_pgt[511].pgd;
 #endif
 
 	sme_sev_setup_real_mode(trampoline_header);
Signature · 1 line
2.33.0
#1 lore raw
Quoted 68 lines · Joerg Roedel

From: Joerg Roedel <[email protected]>

The trampoline_pgd only maps the 0xfffffff000000000-0xffffffffffffffff range of kernel memory (with 4-level paging). This range contains the kernels text+data+bss mappings and the module mapping space, but not the direct mapping and the vmalloc area.

This is enough to get an application processors out of real-mode, but for code that switches back to real-mode the trampoline_pgd is missing important parts of the address space. For example, consider this code from arch/x86/kernel/reboot.c, function machine_real_restart() for a 64-bit kernel:

	#ifdef CONFIG_X86_32
		load_cr3(initial_page_table);
	#else
		write_cr3(real_mode_header->trampoline_pgd);

		/* Exiting long mode will fail if CR4.PCIDE is set. */
		if (boot_cpu_has(X86_FEATURE_PCID))
			cr4_clear_bits(X86_CR4_PCIDE);
	#endif

		/* Jump to the identity-mapped low memory code */
	#ifdef CONFIG_X86_32
		asm volatile("jmpl *%0" : :
			     "rm" (real_mode_header->machine_real_restart_asm),
			     "a" (type));
	#else
		asm volatile("ljmpl *%0" : :
			     "m" (real_mode_header->machine_real_restart_asm),
			     "D" (type));
	#endif

The code switches to the trampoline_pgd, which unmaps the direct mapping and also the kernel stack. The call to cr4_clear_bits() will find no stack and crash the machine. The real_mode_header pointer below points into the direct mapping, and dereferencing it also causes a crash.

The reason this does not crash always is only that kernel mappings are global and the CR3 switch does not flush those mappings. But if theses mappings are not in the TLB already, the above code will crash before it can jump to the real-mode stub.

Extend the trampoline_pgd to contain all kernel mappings to prevent these crashes and to make code which runs on this page-table more robust.

Cc [email protected]
Signed-off-by Joerg Roedel <[email protected]>
---
 arch/x86/realmode/init.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
arch/x86/realmode/init.c +1 -0
index 31b5856010cb..7a08c96cb42a 100644
@@ -72,6 +72,7 @@ static void __init setup_real_mode(void)
 #ifdef CONFIG_X86_64
 	u64 *trampoline_pgd;
 	u64 efer;
+	int i;
 #endif
 
 	base = (unsigned char *)real_mode_header;

@@ -128,8 +129,17 @@ static void __init setup_real_mode(void)
 	trampoline_header->flags = 0;

arch/x86/realmode/init.c static void __init setup_real_mode(void) · line 139

Quoted 10 lines
arch/x86/realmode/init.c +8 -0
 
 	trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd);
+
+	/*
+	 * Map all of kernel memory into the trampoline PGD so that it includes
+	 * the direct mapping and vmalloc space. This is needed to keep the
+	 * stack and real_mode_header mapped when switching to this page table.
+	 */
+	for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++)
+		trampoline_pgd[i] = init_top_pgt[i].pgd;

Don't we need to update the trampoline_pgd in sync_global_pgds() as well?

Quoted 10 lines
arch/x86/realmode/init.c +2 -1
+
+	/* Map the real mode stub as virtual == physical */
 	trampoline_pgd[0] = trampoline_pgd_entry.pgd;
-	trampoline_pgd[511] = init_top_pgt[511].pgd;
 #endif
 
 	sme_sev_setup_real_mode(trampoline_header);
Signature · 1 line
2.33.0
Signature · 2 lines
Sincerely yours,
Mike.
author
#2 lore raw

Hi Mike,

Quoted 5 lines · Mike Rapoport

arch/x86/realmode/init.c static void __init setup_real_mode(void) · line 139

Quoted 2 lines · Joerg Roedel
arch/x86/realmode/init.c +2 -0
+	for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++)
+		trampoline_pgd[i] = init_top_pgt[i].pgd;

Don't we need to update the trampoline_pgd in sync_global_pgds() as well?

No, the trampoline_pgd is setup after preallocate_vmalloc_pages(), so everything that would need synchronization is already in the reference page-table.

Regards,

Joerg