Re: [PATCH 08/17] x86/ftrace: set trampoline pages as executable

[Nadav Amit]

> On Feb 6, 2019, at 8:22 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> On Wed, 16 Jan 2019 16:32:50 -0800
> Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> wrote:
> 
>> From: Nadav Amit <namit@xxxxxxxxxx>
>> 
>> Since alloc_module() will not set the pages as executable soon, we need
>> to do so for ftrace trampoline pages after they are allocated.
>> 
>> For the time being, we do not change ftrace to use the text_poke()
>> interface. As a result, ftrace breaks still breaks W^X.
>> 
>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>> ---
>> arch/x86/kernel/ftrace.c | 9 +++++++++
>> 1 file changed, 9 insertions(+)
>> 
>> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
>> index 8257a59704ae..eb4a1937e72c 100644
>> --- a/arch/x86/kernel/ftrace.c
>> +++ b/arch/x86/kernel/ftrace.c
>> @@ -742,6 +742,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>> 	unsigned long end_offset;
>> 	unsigned long op_offset;
>> 	unsigned long offset;
>> +	unsigned long npages;
>> 	unsigned long size;
>> 	unsigned long retq;
>> 	unsigned long *ptr;
>> @@ -774,6 +775,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>> 		return 0;
>> 
>> 	*tramp_size = size + RET_SIZE + sizeof(void *);
>> +	npages = DIV_ROUND_UP(*tramp_size, PAGE_SIZE);
>> 
>> 	/* Copy ftrace_caller onto the trampoline memory */
>> 	ret = probe_kernel_read(trampoline, (void *)start_offset, size);
>> @@ -818,6 +820,13 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>> 	/* ALLOC_TRAMP flags lets us know we created it */
>> 	ops->flags |= FTRACE_OPS_FL_ALLOC_TRAMP;
>> 
>> +	/*
>> +	 * Module allocation needs to be completed by making the page
>> +	 * executable. The page is still writable, which is a security hazard,
>> +	 * but anyhow ftrace breaks W^X completely.
>> +	 */
> 
> Perhaps we should set the page to non writable after the page is
> updated? And set it to writable only when we need to update it.

You remember that I sent you a patch that changed all these writes into
text_poke() and you said that I should defer it until this series is merged?

> As for this patch:
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Thanks!