[PATCH v5 01/10] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"

Nadav Amit
2018-11-13T20:26:23Z

text_mutex is currently expected to be held before text_poke() is
called, but we kgdb does not take the mutex, 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.

This patch creates two wrapper functions, text_poke() and
text_poke_kgdb() which do or do not run the lockdep assertion
respectively.

While we are at it, change the return code of text_poke() to something
meaningful. One day, callers might actually respect it and the existing
BUG_ON() when patching fails could be removed. For kgdb, the return
value can actually be used.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
arch/x86/include/asm/text-patching.h | 3 +-
arch/x86/kernel/alternative.c | 72 +++++++++++++++++++++-------
arch/x86/kernel/kgdb.c | 15 ++++--
3 files changed, 66 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index e85ff65c43c3..5a2600370763 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -34,7 +34,8 @@ 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 int text_poke(void *addr, const void *opcode, size_t len);
+extern int text_poke_kgdb(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 ebeac487a20c..ebe9210dc92e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -678,23 +678,12 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
return addr;
}

-/**
- * text_poke - Update instructions on a live kernel
- * @addr: address to modify
- * @opcode: source of the copy
- * @len: length to copy
- *
- * Only atomic text poke/set should be allowed when not doing early patching.
- * It means the size must be writable atomically and the address must be aligned
- * in a way that permits an atomic write. It also makes sure we fit on a single
- * page.
- */
-void *text_poke(void *addr, const void *opcode, size_t len)
+static int __text_poke(void *addr, const void *opcode, size_t len)
{
unsigned long flags;
char *vaddr;
struct page *pages[2];
- int i;
+ int i, r = 0;

/*
* While boot memory allocator is runnig we cannot use struct
@@ -702,8 +691,6 @@ void *text_poke(void *addr, const void *opcode, size_t len)
*/
BUG_ON(!after_bootmem);

- lockdep_assert_held(&text_mutex);
-
if (!core_kernel_text((unsigned long)addr)) {
pages[0] = vmalloc_to_page(addr);
pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
@@ -712,7 +699,8 @@ void *text_poke(void *addr, const void *opcode, size_t len)
WARN_ON(!PageReserved(pages[0]));
pages[1] = virt_to_page(addr + PAGE_SIZE);
}
- BUG_ON(!pages[0]);
+ if (!pages[0])
+ return -EFAULT;
local_irq_save(flags);
set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
if (pages[1])
@@ -727,9 +715,57 @@ void *text_poke(void *addr, const void *opcode, size_t len)
/* 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]);
+ if (((char *)addr)[i] != ((char *)opcode)[i])
+ r = -EFAULT;
local_irq_restore(flags);
- return addr;
+ return r;
+}
+
+/**
+ * text_poke - Update instructions on a live kernel
+ * @addr: address to modify
+ * @opcode: source of the copy
+ * @len: length to copy
+ *
+ * Only atomic text poke/set should be allowed when not doing early patching.
+ * It means the size must be writable atomically and the address must be aligned
+ * in a way that permits an atomic write. It also makes sure we fit on a single
+ * page.
+ */
+int text_poke(void *addr, const void *opcode, size_t len)
+{
+ int r;
+
+ lockdep_assert_held(&text_mutex);
+
+ r = __text_poke(addr, opcode, len);
+
+ /*
+ * TODO: change the callers to consider the return value and remove this
+ * historical assertion.
+ */
+ BUG_ON(r);
+
+ return r;
+}
+
+/**
+ * text_poke_kgdb - Update instructions on a live kernel by kgdb
+ * @addr: address to modify
+ * @opcode: source of the copy
+ * @len: length to copy
+ *
+ * Only atomic text poke/set should be allowed when not doing early patching.
+ * It means the size must be writable atomically and the address must be aligned
+ * in a way that permits an atomic write. It also makes sure we fit on a single
+ * page.
+ *
+ * Context: should only be used by kgdb, which ensures no other core is running,
+ * despite the fact it does not hold the text_mutex.
+ */
+int text_poke_kgdb(void *addr, const void *opcode, size_t len)
+{
+ return __text_poke(addr, opcode, len);
}

static void do_sync_core(void *info)
diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 8e36f249646e..8091b2e381d4 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -763,13 +763,15 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
if (!err)
return err;
/*
- * It is safe to call text_poke() because normal kernel execution
+ * It is safe to call text_poke_kgdb() because normal kernel execution
* is stopped on all cores, so long as the text_mutex is not locked.
*/
if (mutex_is_locked(&text_mutex))
return -EBUSY;
- text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
- BREAK_INSTR_SIZE);
+ err = text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
+ BREAK_INSTR_SIZE);
+ if (err)
+ return err;
err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
if (err)
return err;
@@ -788,12 +790,15 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
if (bpt->type != BP_POKE_BREAKPOINT)
goto knl_write;
/*
- * It is safe to call text_poke() because normal kernel execution
+ * It is safe to call text_poke_kgdb() because normal kernel execution
* is stopped on all cores, so long as the text_mutex is not locked.
*/
if (mutex_is_locked(&text_mutex))
goto knl_write;
- text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
+ err = text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
+ BREAK_INSTR_SIZE);
+ if (err)
+ return err;
err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
goto knl_write;
--
2.17.1