Re: [PATCH v2] powerpc: Avoid code patching freed init sections

Le 13/09/2018 à 02:36, Michael Neuling a écrit :

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--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c

@@ -23,11 +23,33 @@
   #include <asm/code-patching.h>
   #include <asm/setup.h>
   
+

This blank line is not needed

Ack

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+static inline bool in_init_section(unsigned int *patch_addr)
+{
+	if (patch_addr < (unsigned int *)__init_begin)
+		return false;
+	if (patch_addr >= (unsigned int *)__init_end)
+		return false;
+	return true;
+}

Can we use the existing function init_section_contains() instead of this
new function ?

Nice, I was looking for something like that...

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+
+static inline bool init_freed(void)
+{
+	return (system_state >= SYSTEM_RUNNING);
+}
+

I would call this function differently, for instance init_is_finished(),
because as you mentionned it doesn't exactly mean that init memory is freed.

Talking to Nick and mpe offline I think we are going to have to add a flag when
we free init mem rather than doing what we have now since what we have now has a
potential race. That change will eliminate the function entirely.

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   static int __patch_instruction(unsigned int *exec_addr, unsigned int
instr,
   			       unsigned int *patch_addr)
   {
   	int err;
   
+	/* Make sure we aren't patching a freed init section */
+	if (in_init_section(patch_addr) && init_freed()) {

The test must be done on exec_addr, not on patch_addr, as patch_addr is
the address where the instruction as been remapped RW for allowing its
modification.

Thanks for the catch

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Also I think it should be tested the other way round, because the
init_freed() is a simpler test which will be false most of the time once
the system is running so it should be checked first.

ok, I'll change.

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+		printk(KERN_DEBUG "Skipping init section patching addr:
0x%lx\n",

Maybe use pr_debug() instead.

Sure.

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+			(unsigned long)patch_addr);

Please align second line as per Codying style.

Sorry I can't see what's wrong. You're (or Cody :-P) going to have to spell it
this out for me...

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+		return 0;
+	}
+
   	__put_user_size(instr, patch_addr, 4, err);
   	if (err)
   		return err;

I think it would be better to put this verification in
patch_instruction() instead, to avoid RW mapping/unmapping the
instruction to patch when we are not going to do the patching.

If we do it there then we miss the raw_patch_intruction case.

raw_patch_instruction() can only be used during init. Once kernel memory 
has been marked readonly, raw_patch_instruction() cannot be used 
anymore. And mark_readonly() is called immediately after free_initmem()

Christophe

IMHO I don't think we need to optimise this rare and non-critical path.

Mikey