Re: [RFC PATCH 4/6] x86/alternatives: initializing temporary mm for patching

To prevent improper use of the PTEs that are used for text patching, we
want to use a temporary mm struct. We initailize it by copying the init
mm.

The address that will be used for patching is taken from the lower area
that is usually used for the task memory. Doing so prevents the need to
frequently synchronize the temporary-mm (e.g., when BPF programs are
installed), since different PGDs are used for the task memory.

Finally, we randomize the address of the PTEs to harden against exploits
that use these PTEs.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <redacted>
---
 arch/x86/include/asm/pgtable.h       |  4 ++++
 arch/x86/include/asm/text-patching.h |  2 ++
 arch/x86/mm/init_64.c                | 35 ++++++++++++++++++++++++++++
 include/asm-generic/pgtable.h        |  4 ++++
 init/main.c                          |  1 +
 5 files changed, 46 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index e4ffa565a69f..c65d2b146ff6 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h

@@ -1022,6 +1022,10 @@ static inline void __meminit init_trampoline_default(void)
 	/* Default trampoline pgd value */
 	trampoline_pgd_entry = init_top_pgt[pgd_index(__PAGE_OFFSET)];
 }
+
+void __init poking_init(void);
+#define poking_init poking_init

+
 # ifdef CONFIG_RANDOMIZE_MEMORY
 void __meminit init_trampoline(void);
 # else

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index e85ff65c43c3..ffe7902cc326 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h

@@ -38,5 +38,7 @@ extern void *text_poke(void *addr, const void *opcode, size_t len);
 extern int poke_int3_handler(struct pt_regs *regs);
 extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
 extern int after_bootmem;
+extern __ro_after_init struct mm_struct *poking_mm;
+extern __ro_after_init unsigned long poking_addr;
 
 #endif /* _ASM_X86_TEXT_PATCHING_H */

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index dd519f372169..ed4a46a89946 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c

@@ -33,6 +33,7 @@
 #include <linux/nmi.h>
 #include <linux/gfp.h>
 #include <linux/kcore.h>
+#include <linux/sched/mm.h>
 
 #include <asm/processor.h>
 #include <asm/bios_ebda.h>

@@ -54,6 +55,7 @@
 #include <asm/init.h>
 #include <asm/uv/uv.h>
 #include <asm/setup.h>
+#include <asm/text-patching.h>
 
 #include "mm_internal.h"
 

@@ -1389,6 +1391,39 @@ unsigned long memory_block_size_bytes(void)
 	return memory_block_size_probed;
 }
 
+/*
+ * Initialize an mm_struct to be used during poking and a pointer to be used
+ * during patching. If anything fails during initialization, poking will be done
+ * using the fixmap, which is unsafe, so warn the user about it.
+ */
+void __init poking_init(void)
+{
+	unsigned long poking_addr;
+
+	poking_mm = copy_init_mm();
+	if (!poking_mm)
+		goto error;
+
+	/*
+	 * Randomize the poking address, but make sure that the following page
+	 * will be mapped at the same PMD. We need 2 pages, so find space for 3,
+	 * and adjust the address if the PMD ends after the first one.
+	 */
+	poking_addr = TASK_UNMAPPED_BASE +
+		(kaslr_get_random_long("Poking") & PAGE_MASK) %
+		(TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
+
+	if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
+		poking_addr += PAGE_SIZE;
+
+	return;
+error:
+	if (poking_mm)
+		mmput(poking_mm);
+	poking_mm = NULL;

+	pr_err("x86/mm: error setting a separate poking address space\n");
+}
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 /*
  * Initialise the sparsemem vmemmap using huge-pages at the PMD level.

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 88ebc6102c7c..c66579d0ee67 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h

@@ -1111,6 +1111,10 @@ static inline bool arch_has_pfn_modify_check(void)
 
 #ifndef PAGE_KERNEL_EXEC
 # define PAGE_KERNEL_EXEC PAGE_KERNEL
+
+#ifndef poking_init
+static inline void poking_init(void) { }
+#endif

 #endif
 
 #endif /* !__ASSEMBLY__ */

diff --git a/init/main.c b/init/main.c
index 18f8f0140fa0..6754ff2687c8 100644
--- a/init/main.c
+++ b/init/main.c

@@ -725,6 +725,7 @@ asmlinkage __visible void __init start_kernel(void)
 	taskstats_init_early();
 	delayacct_init();
 
+	poking_init();
 	check_bugs();
 
 	acpi_subsystem_init();

-- 
2.17.1