[PATCH 0/6] x86/alternatives: text_poke() fixes

[PATCH 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit]

This patch-set addresses some issues that were raised in a recent
correspondence and might affect the security and the correctness of code
patching. (Note that patching performance is not addressed by this
patch-set).

The main issue that the patches deal with is the fact that the fixmap
PTEs that are used for patching are available for access from other
cores and might be exploited. They are not even flushed from the TLB in
remote cores, so the risk is even higher. Address this issue by
introducing a temporary mm that is only used during patching.
Unfortunately, due to init ordering, fixmap is still used during
boot-time patching. Future patches can eliminate the need for it.

The second issue is the missing lockdep assertion to ensure text_mutex
is taken. It is actually not always taken, so fix the instances that
were found not to take the lock (although they should be safe even
without taking the lock).

Finally, try to be more conservative and to map a single page, instead
of two, when possible. This helps both security and performance.

In addition, there is some cleanup of the patching code to make it more
readable.

RFC->v1:
- Added handling of error in get_locked_pte()
- Remove lockdep assertion, clarify text_mutex use instead [masami]
- Comment fix [peterz]
- Removed remainders of text_poke return value [masami]
- Use __weak for poking_init instead of macros [masami]
- Simplify error handling in poking_init [masami]  

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.org/lkml/2018/8/24/586

Andy Lutomirski (1):
  x86/mm: temporary mm struct

Nadav Amit (5):
  x86/alternatives: clarify text_mutex use in text_poke
  fork: provide a function for copying init_mm
  x86/alternatives: initializing temporary mm for patching
  x86/alternatives: use temporary mm for text poking
  x86/alternatives: remove text_poke() return value

 arch/x86/include/asm/mmu_context.h   |  20 ++++
 arch/x86/include/asm/pgtable.h       |   3 +
 arch/x86/include/asm/text-patching.h |   4 +-
 arch/x86/kernel/alternative.c        | 173 +++++++++++++++++++++++----
 arch/x86/mm/init_64.c                |  29 +++++
 include/linux/sched/task.h           |   1 +
 init/main.c                          |   3 +
 kernel/fork.c                        |  24 +++-
 8 files changed, 226 insertions(+), 31 deletions(-)

-- 
2.17.1

[PATCH 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit]

This patch-set addresses some issues that were raised in a recent
correspondence and might affect the security and the correctness of code
patching. (Note that patching performance is not addressed by this
patch-set).

The main issue that the patches deal with is the fact that the fixmap
PTEs that are used for patching are available for access from other
cores and might be exploited. They are not even flushed from the TLB in
remote cores, so the risk is even higher. Address this issue by
introducing a temporary mm that is only used during patching.
Unfortunately, due to init ordering, fixmap is still used during
boot-time patching. Future patches can eliminate the need for it.

The second issue is the missing lockdep assertion to ensure text_mutex
is taken. It is actually not always taken, so fix the instances that
were found not to take the lock (although they should be safe even
without taking the lock).

Finally, try to be more conservative and to map a single page, instead
of two, when possible. This helps both security and performance.

In addition, there is some cleanup of the patching code to make it more
readable.

RFC->v1:
- Added handling of error in get_locked_pte()
- Remove lockdep assertion, clarify text_mutex use instead [masami]
- Comment fix [peterz]
- Removed remainders of text_poke return value [masami]
- Use __weak for poking_init instead of macros [masami]
- Simplify error handling in poking_init [masami]  

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.org/lkml/2018/8/24/586

Andy Lutomirski (1):
  x86/mm: temporary mm struct

Nadav Amit (5):
  x86/alternatives: clarify text_mutex use in text_poke
  fork: provide a function for copying init_mm
  x86/alternatives: initializing temporary mm for patching
  x86/alternatives: use temporary mm for text poking
  x86/alternatives: remove text_poke() return value

 arch/x86/include/asm/mmu_context.h   |  20 ++++
 arch/x86/include/asm/pgtable.h       |   3 +
 arch/x86/include/asm/text-patching.h |   4 +-
 arch/x86/kernel/alternative.c        | 173 +++++++++++++++++++++++----
 arch/x86/mm/init_64.c                |  29 +++++
 include/linux/sched/task.h           |   1 +
 init/main.c                          |   3 +
 kernel/fork.c                        |  24 +++-
 8 files changed, 226 insertions(+), 31 deletions(-)

-- 
2.17.1

[PATCH 1/6] x86/alternatives: clarify text_mutex use in text_poke

[Nadav Amit]

text_mutex is expected to be held before text_poke() is called, but we
cannot add a lockdep assertion since kgdb does not take it, and instead
*supposedly* ensures the lock is not taken and will not be acquired by
any other core while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely clear
that this would be the case if gdb_do_roundup is zero.

Add a comment to clarify this behavior.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/alternative.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..d0c0d8b724e1 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -685,7 +685,10 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  * in a way that permits an atomic write. It also makes sure we fit on a single
  * page.
  *
- * Note: Must be called under text_mutex.
+ * Context: Must be called under text_mutex. kgdb is an exception: it does not
+ *	    hold the mutex, as it *supposedly* ensures that no other core is
+ *	    holding the mutex and ensures that none of them will acquire the
+ *	    mutex while the code runs.
  */
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
-- 
2.17.1

[PATCH 1/6] x86/alternatives: clarify text_mutex use in text_poke

[Nadav Amit]

text_mutex is expected to be held before text_poke() is called, but we
cannot add a lockdep assertion since kgdb does not take it, and instead
*supposedly* ensures the lock is not taken and will not be acquired by
any other core while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely clear
that this would be the case if gdb_do_roundup is zero.

Add a comment to clarify this behavior.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/alternative.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..d0c0d8b724e1 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -685,7 +685,10 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  * in a way that permits an atomic write. It also makes sure we fit on a single
  * page.
  *
- * Note: Must be called under text_mutex.
+ * Context: Must be called under text_mutex. kgdb is an exception: it does not
+ *	    hold the mutex, as it *supposedly* ensures that no other core is
+ *	    holding the mutex and ensures that none of them will acquire the
+ *	    mutex while the code runs.
  */
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
-- 
2.17.1

[PATCH 2/6] x86/mm: temporary mm struct

[Nadav Amit]

From: Andy Lutomirski <luto@kernel.org>

Sometimes we want to set a temporary page-table entries (PTEs) in one of
the cores, without allowing other cores to use - even speculatively -
these mappings. There are two benefits for doing so:

(1) Security: if sensitive PTEs are set, temporary mm prevents their use
in other cores. This hardens the security as it prevents exploding a
dangling pointer to overwrite sensitive data using the sensitive PTE.

(2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
remote page-tables.

To do so a temporary mm_struct can be used. Mappings which are private
for this mm can be set in the userspace part of the address-space.
During the whole time in which the temporary mm is loaded, interrupts
must be disabled.

The first use-case for temporary PTEs, which will follow, is for poking
the kernel text.

[ Commit message was written by Nadav ]

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index eeeb9289c764..96afc8c0cf15 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
 	return cr3;
 }
 
+typedef struct {
+	struct mm_struct *prev;
+} temporary_mm_state_t;
+
+static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
+{
+	temporary_mm_state_t state;
+
+	lockdep_assert_irqs_disabled();
+	state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
+	switch_mm_irqs_off(NULL, mm, current);
+	return state;
+}
+
+static inline void unuse_temporary_mm(temporary_mm_state_t prev)
+{
+	lockdep_assert_irqs_disabled();
+	switch_mm_irqs_off(NULL, prev.prev, current);
+}
+
 #endif /* _ASM_X86_MMU_CONTEXT_H */
-- 
2.17.1

[PATCH 2/6] x86/mm: temporary mm struct

[Nadav Amit]

From: Andy Lutomirski <luto@kernel.org>

Sometimes we want to set a temporary page-table entries (PTEs) in one of
the cores, without allowing other cores to use - even speculatively -
these mappings. There are two benefits for doing so:

(1) Security: if sensitive PTEs are set, temporary mm prevents their use
in other cores. This hardens the security as it prevents exploding a
dangling pointer to overwrite sensitive data using the sensitive PTE.

(2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
remote page-tables.

To do so a temporary mm_struct can be used. Mappings which are private
for this mm can be set in the userspace part of the address-space.
During the whole time in which the temporary mm is loaded, interrupts
must be disabled.

The first use-case for temporary PTEs, which will follow, is for poking
the kernel text.

[ Commit message was written by Nadav ]

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index eeeb9289c764..96afc8c0cf15 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
 	return cr3;
 }
 
+typedef struct {
+	struct mm_struct *prev;
+} temporary_mm_state_t;
+
+static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
+{
+	temporary_mm_state_t state;
+
+	lockdep_assert_irqs_disabled();
+	state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
+	switch_mm_irqs_off(NULL, mm, current);
+	return state;
+}
+
+static inline void unuse_temporary_mm(temporary_mm_state_t prev)
+{
+	lockdep_assert_irqs_disabled();
+	switch_mm_irqs_off(NULL, prev.prev, current);
+}
+
 #endif /* _ASM_X86_MMU_CONTEXT_H */
-- 
2.17.1

[PATCH 3/6] fork: provide a function for copying init_mm

[Nadav Amit]

Provide a function for copying init_mm. This function will be later used
for setting a temporary mm.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 include/linux/sched/task.h |  1 +
 kernel/fork.c              | 24 ++++++++++++++++++------
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 108ede99e533..ac0a675678f5 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -74,6 +74,7 @@ extern void exit_itimers(struct signal_struct *);
 extern long _do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *, unsigned long);
 extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);
 struct task_struct *fork_idle(int);
+struct mm_struct *copy_init_mm(void);
 extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
 extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index d896e9ca38b0..a1c637b903c1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1254,13 +1254,20 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 		complete_vfork_done(tsk);
 }
 
-/*
- * Allocate a new mm structure and copy contents from the
- * mm structure of the passed in task structure.
+/**
+ * dup_mm() - duplicates an existing mm structure
+ * @tsk: the task_struct with which the new mm will be associated.
+ * @oldmm: the mm to duplicate.
+ *
+ * Allocates a new mm structure and copy contents from the provided
+ * @oldmm structure.
+ *
+ * Return: the duplicated mm or NULL on failure.
  */
-static struct mm_struct *dup_mm(struct task_struct *tsk)
+static struct mm_struct *dup_mm(struct task_struct *tsk,
+				struct mm_struct *oldmm)
 {
-	struct mm_struct *mm, *oldmm = current->mm;
+	struct mm_struct *mm;
 	int err;
 
 	mm = allocate_mm();
@@ -1327,7 +1334,7 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
 	}
 
 	retval = -ENOMEM;
-	mm = dup_mm(tsk);
+	mm = dup_mm(tsk, current->mm);
 	if (!mm)
 		goto fail_nomem;
 
@@ -2127,6 +2134,11 @@ struct task_struct *fork_idle(int cpu)
 	return task;
 }
 
+struct mm_struct *copy_init_mm(void)
+{
+	return dup_mm(NULL, &init_mm);
+}
+
 /*
  *  Ok, this is the main fork-routine.
  *
-- 
2.17.1

[PATCH 3/6] fork: provide a function for copying init_mm

[Nadav Amit]

Provide a function for copying init_mm. This function will be later used
for setting a temporary mm.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 include/linux/sched/task.h |  1 +
 kernel/fork.c              | 24 ++++++++++++++++++------
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 108ede99e533..ac0a675678f5 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -74,6 +74,7 @@ extern void exit_itimers(struct signal_struct *);
 extern long _do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *, unsigned long);
 extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);
 struct task_struct *fork_idle(int);
+struct mm_struct *copy_init_mm(void);
 extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
 extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index d896e9ca38b0..a1c637b903c1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1254,13 +1254,20 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 		complete_vfork_done(tsk);
 }
 
-/*
- * Allocate a new mm structure and copy contents from the
- * mm structure of the passed in task structure.
+/**
+ * dup_mm() - duplicates an existing mm structure
+ * @tsk: the task_struct with which the new mm will be associated.
+ * @oldmm: the mm to duplicate.
+ *
+ * Allocates a new mm structure and copy contents from the provided
+ * @oldmm structure.
+ *
+ * Return: the duplicated mm or NULL on failure.
  */
-static struct mm_struct *dup_mm(struct task_struct *tsk)
+static struct mm_struct *dup_mm(struct task_struct *tsk,
+				struct mm_struct *oldmm)
 {
-	struct mm_struct *mm, *oldmm = current->mm;
+	struct mm_struct *mm;
 	int err;
 
 	mm = allocate_mm();
@@ -1327,7 +1334,7 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
 	}
 
 	retval = -ENOMEM;
-	mm = dup_mm(tsk);
+	mm = dup_mm(tsk, current->mm);
 	if (!mm)
 		goto fail_nomem;
 
@@ -2127,6 +2134,11 @@ struct task_struct *fork_idle(int cpu)
 	return task;
 }
 
+struct mm_struct *copy_init_mm(void)
+{
+	return dup_mm(NULL, &init_mm);
+}
+
 /*
  *  Ok, this is the main fork-routine.
  *
-- 
2.17.1

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

[Nadav Amit]

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 <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/pgtable.h       |  3 +++
 arch/x86/include/asm/text-patching.h |  2 ++
 arch/x86/mm/init_64.c                | 29 ++++++++++++++++++++++++++++
 init/main.c                          |  3 +++
 4 files changed, 37 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index e4ffa565a69f..3de9a1fb7a9a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1022,6 +1022,9 @@ 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);
+
 # 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..db33a724a054 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -54,6 +54,7 @@
 #include <asm/init.h>
 #include <asm/uv/uv.h>
 #include <asm/setup.h>
+#include <asm/text-patching.h>
 
 #include "mm_internal.h"
 
@@ -1389,6 +1390,34 @@ 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) {
+		pr_err("x86/mm: error setting a separate poking address space");
+		return;
+	}
+
+	/*
+	 * 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;
+}
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 /*
  * Initialise the sparsemem vmemmap using huge-pages at the PMD level.
diff --git a/init/main.c b/init/main.c
index 18f8f0140fa0..8c6dd8d88fca 100644
--- a/init/main.c
+++ b/init/main.c
@@ -496,6 +496,8 @@ void __init __weak thread_stack_cache_init(void)
 
 void __init __weak mem_encrypt_init(void) { }
 
+void __init __weak poking_init(void) { }
+
 bool initcall_debug;
 core_param(initcall_debug, initcall_debug, bool, 0644);
 
@@ -725,6 +727,7 @@ asmlinkage __visible void __init start_kernel(void)
 	taskstats_init_early();
 	delayacct_init();
 
+	poking_init();
 	check_bugs();
 
 	acpi_subsystem_init();
-- 
2.17.1

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

[Nadav Amit]

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 <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/pgtable.h       |  3 +++
 arch/x86/include/asm/text-patching.h |  2 ++
 arch/x86/mm/init_64.c                | 29 ++++++++++++++++++++++++++++
 init/main.c                          |  3 +++
 4 files changed, 37 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index e4ffa565a69f..3de9a1fb7a9a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1022,6 +1022,9 @@ 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);
+
 # 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..db33a724a054 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -54,6 +54,7 @@
 #include <asm/init.h>
 #include <asm/uv/uv.h>
 #include <asm/setup.h>
+#include <asm/text-patching.h>
 
 #include "mm_internal.h"
 
@@ -1389,6 +1390,34 @@ 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) {
+		pr_err("x86/mm: error setting a separate poking address space");
+		return;
+	}
+
+	/*
+	 * 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;
+}
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 /*
  * Initialise the sparsemem vmemmap using huge-pages at the PMD level.
diff --git a/init/main.c b/init/main.c
index 18f8f0140fa0..8c6dd8d88fca 100644
--- a/init/main.c
+++ b/init/main.c
@@ -496,6 +496,8 @@ void __init __weak thread_stack_cache_init(void)
 
 void __init __weak mem_encrypt_init(void) { }
 
+void __init __weak poking_init(void) { }
+
 bool initcall_debug;
 core_param(initcall_debug, initcall_debug, bool, 0644);
 
@@ -725,6 +727,7 @@ asmlinkage __visible void __init start_kernel(void)
 	taskstats_init_early();
 	delayacct_init();
 
+	poking_init();
 	check_bugs();
 
 	acpi_subsystem_init();
-- 
2.17.1

[PATCH 5/6] x86/alternatives: use temporary mm for text poking

[Nadav Amit]

text_poke() can potentially compromise the security as it sets temporary
PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
from other cores accidentally or maliciously, if an attacker gains the
ability to write onto kernel memory.

Moreover, since remote TLBs are not flushed after the temporary PTEs are
removed, the time-window in which the code is writable is not limited if
the fixmap PTEs - maliciously or accidentally - are cached in the TLB.

To address these potential security hazards, we use a temporary mm for
patching the code. Unfortunately, the temporary-mm cannot be initialized
early enough during the init, and as a result x86_late_time_init() needs
to use text_poke() before it can be initialized. text_poke() therefore
keeps the two poking versions - using fixmap and using temporary mm -
and uses them accordingly.

More adventurous developers can try to reorder the init sequence or use
text_poke_early() instead of text_poke() to remove the use of fixmap for
patching completely.

Finally, text_poke() is also not conservative enough when mapping pages,
as it always tries to map 2 pages, even when a single one is sufficient.
So try to be more conservative, and do not map more than needed.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/alternative.c | 165 +++++++++++++++++++++++++++++-----
 1 file changed, 144 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d0c0d8b724e1..2b976296c9f3 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -11,6 +11,7 @@
 #include <linux/stop_machine.h>
 #include <linux/slab.h>
 #include <linux/kdebug.h>
+#include <linux/mmu_context.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>
@@ -674,6 +675,124 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
 	return addr;
 }
 
+/**
+ * text_poke_fixmap - poke using the fixmap.
+ *
+ * Fallback function for poking the text using the fixmap. It is used during
+ * early boot and in the rare case in which initialization of safe poking fails.
+ *
+ * Poking in this manner should be avoided, since it allows other cores to use
+ * the fixmap entries, and can be exploited by an attacker to overwrite the code
+ * (assuming he gained the write access through another bug).
+ */
+static void text_poke_fixmap(void *addr, const void *opcode, size_t len,
+			     struct page *pages[2])
+{
+	u8 *vaddr;
+
+	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
+	if (pages[1])
+		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
+	vaddr = (u8 *)fix_to_virt(FIX_TEXT_POKE0);
+	memcpy(vaddr + offset_in_page(addr), opcode, len);
+
+	/*
+	 * clear_fixmap() performs a TLB flush, so no additional TLB
+	 * flush is needed.
+	 */
+	clear_fixmap(FIX_TEXT_POKE0);
+	if (pages[1])
+		clear_fixmap(FIX_TEXT_POKE1);
+	sync_core();
+
+	/*
+	 * Could also do a CLFLUSH here to speed up CPU recovery; but
+	 * that causes hangs on some VIA CPUs.
+	 */
+}
+
+__ro_after_init struct mm_struct *poking_mm;
+__ro_after_init unsigned long poking_addr;
+
+/**
+ * text_poke_safe() - Pokes the text using a separate address space.
+ *
+ * This is the preferable way for patching the kernel after boot, as it does not
+ * allow other cores to accidentally or maliciously modify the code using the
+ * temporary PTEs.
+ */
+static void text_poke_safe(void *addr, const void *opcode, size_t len,
+			   struct page *pages[2])
+{
+	temporary_mm_state_t prev;
+	pte_t pte, *ptep;
+	spinlock_t *ptl;
+
+	/*
+	 * The lock is not really needed, but this allows to avoid open-coding.
+	 */
+	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
+
+	/*
+	 * If we failed to allocate a PTE, fail silently. The caller (text_poke)
+	 * will detect that the write failed when it compares the memory with
+	 * the new opcode.
+	 */
+	if (unlikely(!ptep))
+		return;
+
+	pte = mk_pte(pages[0], PAGE_KERNEL);
+	set_pte_at(poking_mm, poking_addr, ptep, pte);
+
+	if (pages[1]) {
+		pte = mk_pte(pages[1], PAGE_KERNEL);
+		set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte);
+	}
+
+	/*
+	 * Loading the temporary mm behaves as a compiler barrier, which
+	 * guarantees that the PTE will be set at the time memcpy() is done.
+	 */
+	prev = use_temporary_mm(poking_mm);
+
+	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
+
+	/*
+	 * Ensure that the PTE is only cleared after the instructions of memcpy
+	 * were issued by using a compiler barrier.
+	 */
+	barrier();
+
+	pte_clear(poking_mm, poking_addr, ptep);
+
+	/*
+	 * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
+	 * as it also flushes the corresponding "user" address spaces, which
+	 * does not exist.
+	 *
+	 * Poking, however, is already very inefficient since it does not try to
+	 * batch updates, so we ignore this problem for the time being.
+	 *
+	 * Since the PTEs do not exist in other kernel address-spaces, we do
+	 * not use __flush_tlb_one_kernel(), which when PTI is on would cause
+	 * more unwarranted TLB flushes.
+	 */
+	__flush_tlb_one_user(poking_addr);
+	if (pages[1]) {
+		pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
+		__flush_tlb_one_user(poking_addr + PAGE_SIZE);
+	}
+
+	/*
+	 * Loading the previous page-table hierarchy requires a serializing
+	 * instruction that already allows the core to see the updated version.
+	 * Xen-PV is assumed to serialize execution in a similar manner.
+	 */
+	unuse_temporary_mm(prev);
+
+	pte_unmap_unlock(ptep, ptl);
+}
+
 /**
  * text_poke - Update instructions on a live kernel
  * @addr: address to modify
@@ -692,41 +811,45 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  */
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
+	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
+	struct page *pages[2] = {0};
 	unsigned long flags;
-	char *vaddr;
-	struct page *pages[2];
-	int i;
 
 	/*
-	 * While boot memory allocator is runnig we cannot use struct
-	 * pages as they are not yet initialized.
+	 * While boot memory allocator is running we cannot use struct pages as
+	 * they are not yet initialized.
 	 */
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
 		pages[0] = vmalloc_to_page(addr);
-		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));
-		pages[1] = virt_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
 	BUG_ON(!pages[0]);
 	local_irq_save(flags);
-	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
-	if (pages[1])
-		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
-	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	clear_fixmap(FIX_TEXT_POKE0);
-	if (pages[1])
-		clear_fixmap(FIX_TEXT_POKE1);
-	local_flush_tlb();
-	sync_core();
-	/* Could also do a CLFLUSH here to speed up CPU recovery; but
-	   that causes hangs on some VIA CPUs. */
-	for (i = 0; i < len; i++)
-		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
+
+	/*
+	 * During initial boot, it is hard to initialize poking_mm due to
+	 * dependencies in boot order.
+	 */
+	if (poking_mm)
+		text_poke_safe(addr, opcode, len, pages);
+	else
+		text_poke_fixmap(addr, opcode, len, pages);
+
+	/*
+	 * To be on the safe side, do the comparison before enabling IRQs, as it
+	 * was done before. However, it makes more sense to allow the callers to
+	 * deal with potential failures and not to panic so easily.
+	 */
+	BUG_ON(memcmp(addr, opcode, len));
+
 	local_irq_restore(flags);
 	return addr;
 }
-- 
2.17.1

[PATCH 5/6] x86/alternatives: use temporary mm for text poking

[Nadav Amit]

text_poke() can potentially compromise the security as it sets temporary
PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
from other cores accidentally or maliciously, if an attacker gains the
ability to write onto kernel memory.

Moreover, since remote TLBs are not flushed after the temporary PTEs are
removed, the time-window in which the code is writable is not limited if
the fixmap PTEs - maliciously or accidentally - are cached in the TLB.

To address these potential security hazards, we use a temporary mm for
patching the code. Unfortunately, the temporary-mm cannot be initialized
early enough during the init, and as a result x86_late_time_init() needs
to use text_poke() before it can be initialized. text_poke() therefore
keeps the two poking versions - using fixmap and using temporary mm -
and uses them accordingly.

More adventurous developers can try to reorder the init sequence or use
text_poke_early() instead of text_poke() to remove the use of fixmap for
patching completely.

Finally, text_poke() is also not conservative enough when mapping pages,
as it always tries to map 2 pages, even when a single one is sufficient.
So try to be more conservative, and do not map more than needed.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/alternative.c | 165 +++++++++++++++++++++++++++++-----
 1 file changed, 144 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d0c0d8b724e1..2b976296c9f3 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -11,6 +11,7 @@
 #include <linux/stop_machine.h>
 #include <linux/slab.h>
 #include <linux/kdebug.h>
+#include <linux/mmu_context.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>
@@ -674,6 +675,124 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
 	return addr;
 }
 
+/**
+ * text_poke_fixmap - poke using the fixmap.
+ *
+ * Fallback function for poking the text using the fixmap. It is used during
+ * early boot and in the rare case in which initialization of safe poking fails.
+ *
+ * Poking in this manner should be avoided, since it allows other cores to use
+ * the fixmap entries, and can be exploited by an attacker to overwrite the code
+ * (assuming he gained the write access through another bug).
+ */
+static void text_poke_fixmap(void *addr, const void *opcode, size_t len,
+			     struct page *pages[2])
+{
+	u8 *vaddr;
+
+	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
+	if (pages[1])
+		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
+	vaddr = (u8 *)fix_to_virt(FIX_TEXT_POKE0);
+	memcpy(vaddr + offset_in_page(addr), opcode, len);
+
+	/*
+	 * clear_fixmap() performs a TLB flush, so no additional TLB
+	 * flush is needed.
+	 */
+	clear_fixmap(FIX_TEXT_POKE0);
+	if (pages[1])
+		clear_fixmap(FIX_TEXT_POKE1);
+	sync_core();
+
+	/*
+	 * Could also do a CLFLUSH here to speed up CPU recovery; but
+	 * that causes hangs on some VIA CPUs.
+	 */
+}
+
+__ro_after_init struct mm_struct *poking_mm;
+__ro_after_init unsigned long poking_addr;
+
+/**
+ * text_poke_safe() - Pokes the text using a separate address space.
+ *
+ * This is the preferable way for patching the kernel after boot, as it does not
+ * allow other cores to accidentally or maliciously modify the code using the
+ * temporary PTEs.
+ */
+static void text_poke_safe(void *addr, const void *opcode, size_t len,
+			   struct page *pages[2])
+{
+	temporary_mm_state_t prev;
+	pte_t pte, *ptep;
+	spinlock_t *ptl;
+
+	/*
+	 * The lock is not really needed, but this allows to avoid open-coding.
+	 */
+	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
+
+	/*
+	 * If we failed to allocate a PTE, fail silently. The caller (text_poke)
+	 * will detect that the write failed when it compares the memory with
+	 * the new opcode.
+	 */
+	if (unlikely(!ptep))
+		return;
+
+	pte = mk_pte(pages[0], PAGE_KERNEL);
+	set_pte_at(poking_mm, poking_addr, ptep, pte);
+
+	if (pages[1]) {
+		pte = mk_pte(pages[1], PAGE_KERNEL);
+		set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte);
+	}
+
+	/*
+	 * Loading the temporary mm behaves as a compiler barrier, which
+	 * guarantees that the PTE will be set at the time memcpy() is done.
+	 */
+	prev = use_temporary_mm(poking_mm);
+
+	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
+
+	/*
+	 * Ensure that the PTE is only cleared after the instructions of memcpy
+	 * were issued by using a compiler barrier.
+	 */
+	barrier();
+
+	pte_clear(poking_mm, poking_addr, ptep);
+
+	/*
+	 * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
+	 * as it also flushes the corresponding "user" address spaces, which
+	 * does not exist.
+	 *
+	 * Poking, however, is already very inefficient since it does not try to
+	 * batch updates, so we ignore this problem for the time being.
+	 *
+	 * Since the PTEs do not exist in other kernel address-spaces, we do
+	 * not use __flush_tlb_one_kernel(), which when PTI is on would cause
+	 * more unwarranted TLB flushes.
+	 */
+	__flush_tlb_one_user(poking_addr);
+	if (pages[1]) {
+		pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
+		__flush_tlb_one_user(poking_addr + PAGE_SIZE);
+	}
+
+	/*
+	 * Loading the previous page-table hierarchy requires a serializing
+	 * instruction that already allows the core to see the updated version.
+	 * Xen-PV is assumed to serialize execution in a similar manner.
+	 */
+	unuse_temporary_mm(prev);
+
+	pte_unmap_unlock(ptep, ptl);
+}
+
 /**
  * text_poke - Update instructions on a live kernel
  * @addr: address to modify
@@ -692,41 +811,45 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  */
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
+	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
+	struct page *pages[2] = {0};
 	unsigned long flags;
-	char *vaddr;
-	struct page *pages[2];
-	int i;
 
 	/*
-	 * While boot memory allocator is runnig we cannot use struct
-	 * pages as they are not yet initialized.
+	 * While boot memory allocator is running we cannot use struct pages as
+	 * they are not yet initialized.
 	 */
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
 		pages[0] = vmalloc_to_page(addr);
-		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));
-		pages[1] = virt_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
 	BUG_ON(!pages[0]);
 	local_irq_save(flags);
-	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
-	if (pages[1])
-		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
-	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	clear_fixmap(FIX_TEXT_POKE0);
-	if (pages[1])
-		clear_fixmap(FIX_TEXT_POKE1);
-	local_flush_tlb();
-	sync_core();
-	/* Could also do a CLFLUSH here to speed up CPU recovery; but
-	   that causes hangs on some VIA CPUs. */
-	for (i = 0; i < len; i++)
-		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
+
+	/*
+	 * During initial boot, it is hard to initialize poking_mm due to
+	 * dependencies in boot order.
+	 */
+	if (poking_mm)
+		text_poke_safe(addr, opcode, len, pages);
+	else
+		text_poke_fixmap(addr, opcode, len, pages);
+
+	/*
+	 * To be on the safe side, do the comparison before enabling IRQs, as it
+	 * was done before. However, it makes more sense to allow the callers to
+	 * deal with potential failures and not to panic so easily.
+	 */
+	BUG_ON(memcmp(addr, opcode, len));
+
 	local_irq_restore(flags);
 	return addr;
 }
-- 
2.17.1

[PATCH 6/6] x86/alternatives: remove text_poke() return value

[Nadav Amit]

The return value of text_poke() is meaningless - it is one of the
function inputs. One day someone may allow the callers to deal with
text_poke() failures, if those actually happen.

In the meanwhile, remove the return value.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/text-patching.h | 2 +-
 arch/x86/kernel/alternative.c        | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index ffe7902cc326..1f73f71b4de2 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -34,7 +34,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
  * On the local CPU you need to be protected again NMI or MCE handlers seeing an
  * inconsistent instruction while you patch.
  */
-extern void *text_poke(void *addr, const void *opcode, size_t len);
+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;
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2b976296c9f3..840448ca357e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -809,7 +809,7 @@ static void text_poke_safe(void *addr, const void *opcode, size_t len,
  *	    holding the mutex and ensures that none of them will acquire the
  *	    mutex while the code runs.
  */
-void *text_poke(void *addr, const void *opcode, size_t len)
+void text_poke(void *addr, const void *opcode, size_t len)
 {
 	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
 	struct page *pages[2] = {0};
@@ -851,7 +851,6 @@ void *text_poke(void *addr, const void *opcode, size_t len)
 	BUG_ON(memcmp(addr, opcode, len));
 
 	local_irq_restore(flags);
-	return addr;
 }
 
 static void do_sync_core(void *info)
-- 
2.17.1

[PATCH 6/6] x86/alternatives: remove text_poke() return value

[Nadav Amit]

The return value of text_poke() is meaningless - it is one of the
function inputs. One day someone may allow the callers to deal with
text_poke() failures, if those actually happen.

In the meanwhile, remove the return value.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/text-patching.h | 2 +-
 arch/x86/kernel/alternative.c        | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index ffe7902cc326..1f73f71b4de2 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -34,7 +34,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
  * On the local CPU you need to be protected again NMI or MCE handlers seeing an
  * inconsistent instruction while you patch.
  */
-extern void *text_poke(void *addr, const void *opcode, size_t len);
+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;
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2b976296c9f3..840448ca357e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -809,7 +809,7 @@ static void text_poke_safe(void *addr, const void *opcode, size_t len,
  *	    holding the mutex and ensures that none of them will acquire the
  *	    mutex while the code runs.
  */
-void *text_poke(void *addr, const void *opcode, size_t len)
+void text_poke(void *addr, const void *opcode, size_t len)
 {
 	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
 	struct page *pages[2] = {0};
@@ -851,7 +851,6 @@ void *text_poke(void *addr, const void *opcode, size_t len)
 	BUG_ON(memcmp(addr, opcode, len));
 
 	local_irq_restore(flags);
-	return addr;
 }
 
 static void do_sync_core(void *info)
-- 
2.17.1

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Masami Hiramatsu]

On Thu, 30 Aug 2018 10:32:12 -0700
Nadav Amit <namit@vmware.com> wrote:

> This patch-set addresses some issues that were raised in a recent
> correspondence and might affect the security and the correctness of code
> patching. (Note that patching performance is not addressed by this
> patch-set).
> 
> The main issue that the patches deal with is the fact that the fixmap
> PTEs that are used for patching are available for access from other
> cores and might be exploited. They are not even flushed from the TLB in
> remote cores, so the risk is even higher. Address this issue by
> introducing a temporary mm that is only used during patching.
> Unfortunately, due to init ordering, fixmap is still used during
> boot-time patching. Future patches can eliminate the need for it.
> 
> The second issue is the missing lockdep assertion to ensure text_mutex
> is taken. It is actually not always taken, so fix the instances that
> were found not to take the lock (although they should be safe even
> without taking the lock).
> 
> Finally, try to be more conservative and to map a single page, instead
> of two, when possible. This helps both security and performance.
> 
> In addition, there is some cleanup of the patching code to make it more
> readable.

OK, this series looks good to me, and tested with ftracetest, kprobe testsets.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you!


> 
> RFC->v1:
> - Added handling of error in get_locked_pte()
> - Remove lockdep assertion, clarify text_mutex use instead [masami]
> - Comment fix [peterz]
> - Removed remainders of text_poke return value [masami]
> - Use __weak for poking_init instead of macros [masami]
> - Simplify error handling in poking_init [masami]  
> 
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Link: https://lkml.org/lkml/2018/8/24/586
> 
> Andy Lutomirski (1):
>   x86/mm: temporary mm struct
> 
> Nadav Amit (5):
>   x86/alternatives: clarify text_mutex use in text_poke
>   fork: provide a function for copying init_mm
>   x86/alternatives: initializing temporary mm for patching
>   x86/alternatives: use temporary mm for text poking
>   x86/alternatives: remove text_poke() return value
> 
>  arch/x86/include/asm/mmu_context.h   |  20 ++++
>  arch/x86/include/asm/pgtable.h       |   3 +
>  arch/x86/include/asm/text-patching.h |   4 +-
>  arch/x86/kernel/alternative.c        | 173 +++++++++++++++++++++++----
>  arch/x86/mm/init_64.c                |  29 +++++
>  include/linux/sched/task.h           |   1 +
>  init/main.c                          |   3 +
>  kernel/fork.c                        |  24 +++-
>  8 files changed, 226 insertions(+), 31 deletions(-)
> 
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Masami Hiramatsu]

On Thu, 30 Aug 2018 10:32:12 -0700
Nadav Amit <namit@vmware.com> wrote:

> This patch-set addresses some issues that were raised in a recent
> correspondence and might affect the security and the correctness of code
> patching. (Note that patching performance is not addressed by this
> patch-set).
> 
> The main issue that the patches deal with is the fact that the fixmap
> PTEs that are used for patching are available for access from other
> cores and might be exploited. They are not even flushed from the TLB in
> remote cores, so the risk is even higher. Address this issue by
> introducing a temporary mm that is only used during patching.
> Unfortunately, due to init ordering, fixmap is still used during
> boot-time patching. Future patches can eliminate the need for it.
> 
> The second issue is the missing lockdep assertion to ensure text_mutex
> is taken. It is actually not always taken, so fix the instances that
> were found not to take the lock (although they should be safe even
> without taking the lock).
> 
> Finally, try to be more conservative and to map a single page, instead
> of two, when possible. This helps both security and performance.
> 
> In addition, there is some cleanup of the patching code to make it more
> readable.

OK, this series looks good to me, and tested with ftracetest, kprobe testsets.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you!


> 
> RFC->v1:
> - Added handling of error in get_locked_pte()
> - Remove lockdep assertion, clarify text_mutex use instead [masami]
> - Comment fix [peterz]
> - Removed remainders of text_poke return value [masami]
> - Use __weak for poking_init instead of macros [masami]
> - Simplify error handling in poking_init [masami]  
> 
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Link: https://lkml.org/lkml/2018/8/24/586
> 
> Andy Lutomirski (1):
>   x86/mm: temporary mm struct
> 
> Nadav Amit (5):
>   x86/alternatives: clarify text_mutex use in text_poke
>   fork: provide a function for copying init_mm
>   x86/alternatives: initializing temporary mm for patching
>   x86/alternatives: use temporary mm for text poking
>   x86/alternatives: remove text_poke() return value
> 
>  arch/x86/include/asm/mmu_context.h   |  20 ++++
>  arch/x86/include/asm/pgtable.h       |   3 +
>  arch/x86/include/asm/text-patching.h |   4 +-
>  arch/x86/kernel/alternative.c        | 173 +++++++++++++++++++++++----
>  arch/x86/mm/init_64.c                |  29 +++++
>  include/linux/sched/task.h           |   1 +
>  init/main.c                          |   3 +
>  kernel/fork.c                        |  24 +++-
>  8 files changed, 226 insertions(+), 31 deletions(-)
> 
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Masami Hiramatsu]

On Fri, 31 Aug 2018 13:46:35 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 30 Aug 2018 10:32:12 -0700
> Nadav Amit <namit@vmware.com> wrote:
> 
> > This patch-set addresses some issues that were raised in a recent
> > correspondence and might affect the security and the correctness of code
> > patching. (Note that patching performance is not addressed by this
> > patch-set).
> > 
> > The main issue that the patches deal with is the fact that the fixmap
> > PTEs that are used for patching are available for access from other
> > cores and might be exploited. They are not even flushed from the TLB in
> > remote cores, so the risk is even higher. Address this issue by
> > introducing a temporary mm that is only used during patching.
> > Unfortunately, due to init ordering, fixmap is still used during
> > boot-time patching. Future patches can eliminate the need for it.
> > 
> > The second issue is the missing lockdep assertion to ensure text_mutex
> > is taken. It is actually not always taken, so fix the instances that
> > were found not to take the lock (although they should be safe even
> > without taking the lock).
> > 
> > Finally, try to be more conservative and to map a single page, instead
> > of two, when possible. This helps both security and performance.
> > 
> > In addition, there is some cleanup of the patching code to make it more
> > readable.
> 
> OK, this series looks good to me, and tested with ftracetest, kprobe testsets.
> 
> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Oh, I missed a note.

To apply this series on -tip tree, we have to revert Jiri's text_mutex
checker.

9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")

Thanks!



-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Masami Hiramatsu]

On Fri, 31 Aug 2018 13:46:35 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 30 Aug 2018 10:32:12 -0700
> Nadav Amit <namit@vmware.com> wrote:
> 
> > This patch-set addresses some issues that were raised in a recent
> > correspondence and might affect the security and the correctness of code
> > patching. (Note that patching performance is not addressed by this
> > patch-set).
> > 
> > The main issue that the patches deal with is the fact that the fixmap
> > PTEs that are used for patching are available for access from other
> > cores and might be exploited. They are not even flushed from the TLB in
> > remote cores, so the risk is even higher. Address this issue by
> > introducing a temporary mm that is only used during patching.
> > Unfortunately, due to init ordering, fixmap is still used during
> > boot-time patching. Future patches can eliminate the need for it.
> > 
> > The second issue is the missing lockdep assertion to ensure text_mutex
> > is taken. It is actually not always taken, so fix the instances that
> > were found not to take the lock (although they should be safe even
> > without taking the lock).
> > 
> > Finally, try to be more conservative and to map a single page, instead
> > of two, when possible. This helps both security and performance.
> > 
> > In addition, there is some cleanup of the patching code to make it more
> > readable.
> 
> OK, this series looks good to me, and tested with ftracetest, kprobe testsets.
> 
> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Oh, I missed a note.

To apply this series on -tip tree, we have to revert Jiri's text_mutex
checker.

9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")

Thanks!



-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit]

at 9:49 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Fri, 31 Aug 2018 13:46:35 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
>> On Thu, 30 Aug 2018 10:32:12 -0700
>> Nadav Amit <namit@vmware.com> wrote:
>> 
>>> This patch-set addresses some issues that were raised in a recent
>>> correspondence and might affect the security and the correctness of code
>>> patching. (Note that patching performance is not addressed by this
>>> patch-set).
>>> 
>>> The main issue that the patches deal with is the fact that the fixmap
>>> PTEs that are used for patching are available for access from other
>>> cores and might be exploited. They are not even flushed from the TLB in
>>> remote cores, so the risk is even higher. Address this issue by
>>> introducing a temporary mm that is only used during patching.
>>> Unfortunately, due to init ordering, fixmap is still used during
>>> boot-time patching. Future patches can eliminate the need for it.
>>> 
>>> The second issue is the missing lockdep assertion to ensure text_mutex
>>> is taken. It is actually not always taken, so fix the instances that
>>> were found not to take the lock (although they should be safe even
>>> without taking the lock).
>>> 
>>> Finally, try to be more conservative and to map a single page, instead
>>> of two, when possible. This helps both security and performance.
>>> 
>>> In addition, there is some cleanup of the patching code to make it more
>>> readable.
>> 
>> OK, this series looks good to me, and tested with ftracetest, kprobe testsets.
>> 
>> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
>> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
> 
> Oh, I missed a note.
> 
> To apply this series on -tip tree, we have to revert Jiri's text_mutex
> checker.
> 
> 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")

Thanks for the heads-up. I’ll add a revert for this patch and send v2.

Regards,
Nadav

Re: [PATCH 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit]

at 9:49 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Fri, 31 Aug 2018 13:46:35 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
>> On Thu, 30 Aug 2018 10:32:12 -0700
>> Nadav Amit <namit@vmware.com> wrote:
>> 
>>> This patch-set addresses some issues that were raised in a recent
>>> correspondence and might affect the security and the correctness of code
>>> patching. (Note that patching performance is not addressed by this
>>> patch-set).
>>> 
>>> The main issue that the patches deal with is the fact that the fixmap
>>> PTEs that are used for patching are available for access from other
>>> cores and might be exploited. They are not even flushed from the TLB in
>>> remote cores, so the risk is even higher. Address this issue by
>>> introducing a temporary mm that is only used during patching.
>>> Unfortunately, due to init ordering, fixmap is still used during
>>> boot-time patching. Future patches can eliminate the need for it.
>>> 
>>> The second issue is the missing lockdep assertion to ensure text_mutex
>>> is taken. It is actually not always taken, so fix the instances that
>>> were found not to take the lock (although they should be safe even
>>> without taking the lock).
>>> 
>>> Finally, try to be more conservative and to map a single page, instead
>>> of two, when possible. This helps both security and performance.
>>> 
>>> In addition, there is some cleanup of the patching code to make it more
>>> readable.
>> 
>> OK, this series looks good to me, and tested with ftracetest, kprobe testsets.
>> 
>> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
>> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
> 
> Oh, I missed a note.
> 
> To apply this series on -tip tree, we have to revert Jiri's text_mutex
> checker.
> 
> 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")

Thanks for the heads-up. I’ll add a revert for this patch and send v2.

Regards,
Nadav