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SubjectRe: [PATCH 08/17] x86/ftrace: set trampoline pages as executable
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> On Feb 6, 2019, at 8:22 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 16 Jan 2019 16:32:50 -0800
> Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
>
>> From: Nadav Amit <namit@vmware.com>
>>
>> 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@goodmis.org>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>> ---
>> 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@goodmis.org>

Thanks!

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