On January 17, 2019 2:39:15 PM PST, Nadav Amit [off-list ref] wrote:

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On Jan 17, 2019, at 1:15 PM, hpa@zytor.com wrote:

On January 16, 2019 10:47:01 PM PST, Masami Hiramatsu

[off-list ref] wrote:

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On Wed, 16 Jan 2019 16:32:43 -0800
Rick Edgecombe [off-list ref] wrote:

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From: Nadav Amit <redacted>

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

*supposedly*

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ensures the lock is not taken and will not be acquired by any other

core

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while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely

clear

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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

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meaningful. One day, callers might actually respect it and the

existing

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BUG_ON() when patching fails could be removed. For kgdb, the return
value can actually be used.

Looks good to me.

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

Thank you,

quoted

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <redacted>
Cc: Dave Hansen <redacted>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex

in

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text_poke*()")

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Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Jiri Kosina <redacted>
Signed-off-by: Nadav Amit <redacted>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
arch/x86/include/asm/text-patching.h |  1 +
arch/x86/kernel/alternative.c        | 52

++++++++++++++++++++--------

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arch/x86/kernel/kgdb.c               | 11 +++---
3 files changed, 45 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/text-patching.h

b/arch/x86/include/asm/text-patching.h

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index e85ff65c43c3..f8fc8e86cf01 100644

--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h

@@ -35,6 +35,7 @@ extern void *text_poke_early(void *addr, const

void

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*opcode, size_t len);

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 * inconsistent instruction while you patch.
 */
extern void *text_poke(void *addr, const void *opcode, size_t len);
+extern void *text_poke_kgdb(void *addr, const void *opcode, size_t

len);

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extern int poke_int3_handler(struct pt_regs *regs);
extern void *text_poke_bp(void *addr, const void *opcode, size_t

len, void *handler);

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extern int after_bootmem;

diff --git a/arch/x86/kernel/alternative.c

b/arch/x86/kernel/alternative.c

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index ebeac487a20c..c6a3a10a2fd5 100644

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c

@@ -678,18 +678,7 @@ void *__init_or_module text_poke_early(void

*addr, const void *opcode,

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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

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patching.

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- * It means the size must be writable atomically and the address

must be aligned

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- * in a way that permits an atomic write. It also makes sure we

fit

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on a single

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- * page.
- */
-void *text_poke(void *addr, const void *opcode, size_t len)
+static void *__text_poke(void *addr, const void *opcode, size_t

len)

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quoted

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{
	unsigned long flags;
	char *vaddr;

@@ -702,8 +691,6 @@ void *text_poke(void *addr, const void *opcode,

size_t len)

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 */
	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);

@@ -732,6 +719,43 @@ void *text_poke(void *addr, const void

*opcode,

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size_t len)

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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

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patching.

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+ * It means the size must be writable atomically and the address

must be aligned

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+ * in a way that permits an atomic write. It also makes sure we

fit

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on a single

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+ * page.
+ */
+void *text_poke(void *addr, const void *opcode, size_t len)
+{
+	lockdep_assert_held(&text_mutex);
+
+	return __text_poke(addr, opcode, len);
+}
+
+/**
+ * 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

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patching.

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+ * It means the size must be writable atomically and the address

must be aligned

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+ * in a way that permits an atomic write. It also makes sure we

fit

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on a single

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+ * page.
+ *
+ * Context: should only be used by kgdb, which ensures no other

core

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is running,

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+ *	    despite the fact it does not hold the text_mutex.
+ */
+void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
+{
+	return __text_poke(addr, opcode, len);
+}
+
static void do_sync_core(void *info)
{
	sync_core();

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 5db08425063e..1461544cba8b 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c

@@ -758,13 +758,13 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt

*bpt)

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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

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 * is stopped on all cores, so long as the text_mutex is not

locked.

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 */
	if (mutex_is_locked(&text_mutex))
		return -EBUSY;
-	text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
-		  BREAK_INSTR_SIZE);
+	text_poke_kgdb((void *)bpt->bpt_addr,

arch_kgdb_ops.gdb_bpt_instr,

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+		       BREAK_INSTR_SIZE);
	err = probe_kernel_read(opc, (char *)bpt->bpt_addr,

BREAK_INSTR_SIZE);

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if (err)
		return err;

@@ -783,12 +783,13 @@ int kgdb_arch_remove_breakpoint(struct

kgdb_bkpt *bpt)

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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

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 * is stopped on all cores, so long as the text_mutex is not

locked.

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 */
	if (mutex_is_locked(&text_mutex))
		goto knl_write;
-	text_poke((void *)bpt->bpt_addr, bpt->saved_instr,

BREAK_INSTR_SIZE);

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+	text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
+		       BREAK_INSTR_SIZE);
	err = probe_kernel_read(opc, (char *)bpt->bpt_addr,

BREAK_INSTR_SIZE);

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if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
		goto knl_write;
-- 
2.17.1

If you are reorganizing this code, please do so so that the caller

doesn’t

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have to worry about if it should call text_poke_bp() or

text_poke_early().

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Right now the caller had to know that, which makes no sense.

Did you look at "[11/17] x86/jump-label: remove support for custom
poker”?

https://lore.kernel.org/patchwork/patch/1032857/

If this is not what you regard, please be more concrete.
text_poke_early()
is still used directly on init and while modules are loaded, which
might not
be great, but is outside of the scope of this patch-set.

I don't think it is out of scope, although that patch is a huge step in the right direction.

text_poke_{early,bp,...}, however, should be fully internal, that is, static functions, and we should present a single interface, preferably called text_poke(), to the outside world.

I think we have three subcases:

1. Early, UP, or under stop_machine();
2. Atomic and aligned;
3. Breakpoint.

My proposed algorithm should remove the need for a fixup which should help this interface, too.

The specific alignment needed for #2 is started by the hardware people to be not crossing 16 bytes (NOT a cache line) on any CPU we support SMP on and, of course, being possible to do atomically do on the specific CPU (note that we *can* do a redundantly large store of existing bytes, which adds flexibility.)

To the best of my knowledge any CPU supporting SSE can do an atomic (for our purposes) aligned 16-byte store via MOVAPS; of course any CPU with cx16 can do it without SSE registers. For older CPUs we may be limited to 8-byte stores (cx8) or even 4-byte stores before we need to use the breakpoint algorithm.

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.