Re: [PATCH] tracing/user_events: align uaddr on unsigned long alignment

[Clément Léger]

On 15/09/2023 04:54, Masami Hiramatsu (Google) wrote:
> On Thu, 14 Sep 2023 15:11:02 +0200
> Clément Léger <cleger@xxxxxxxxxxxx> wrote:
> 
>> enabler->uaddr can be aligned on 32 or 64 bits. If aligned on 32 bits,
>> this will result in a misaligned access on 64 bits architectures since
>> set_bit()/clear_bit() are expecting an unsigned long (aligned) pointer.
>> On architecture that do not support misaligned access, this will crash
>> the kernel. Align uaddr on unsigned long size to avoid such behavior.
>> This bug was found while running kselftests on RISC-V.
>>
>> Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event enablement")
>> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
>> ---
>>  kernel/trace/trace_events_user.c | 12 +++++++++---
>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
>> index 6f046650e527..580c0fe4b23e 100644
>> --- a/kernel/trace/trace_events_user.c
>> +++ b/kernel/trace/trace_events_user.c
>> @@ -479,7 +479,7 @@ static int user_event_enabler_write(struct user_event_mm *mm,
>>  				    bool fixup_fault, int *attempt)
>>  {
>>  	unsigned long uaddr = enabler->addr;
>> -	unsigned long *ptr;
>> +	unsigned long *ptr, bit_offset;
>>  	struct page *page;
>>  	void *kaddr;
>>  	int ret;
>> @@ -511,13 +511,19 @@ static int user_event_enabler_write(struct user_event_mm *mm,
>>  	}
>>  
>>  	kaddr = kmap_local_page(page);
>> +
>> +	bit_offset = uaddr & (sizeof(unsigned long) - 1);
>> +	if (bit_offset) {
>> +		bit_offset *= 8;
>> +		uaddr &= ~(sizeof(unsigned long) - 1);
>> +	}
>>  	ptr = kaddr + (uaddr & ~PAGE_MASK);
>>  
>>  	/* Update bit atomically, user tracers must be atomic as well */
>>  	if (enabler->event && enabler->event->status)
>> -		set_bit(ENABLE_BIT(enabler), ptr);
>> +		set_bit(ENABLE_BIT(enabler) + bit_offset, ptr);
>>  	else
>> -		clear_bit(ENABLE_BIT(enabler), ptr);
>> +		clear_bit(ENABLE_BIT(enabler) + bit_offset, ptr);
> 
> What we need are generic set_bit_aligned() and clear_bit_aligned(), which align the ptr
> by unsigned long. (I think it should be done in set_bit/clear_bit, for architecture
> which requires aligned access...)
> 
> #define LONG_ALIGN_DIFF(p)	(p) & (sizeof(long) -1)
> #define LONG_ALINGNED(p)		(p) & ~(sizeof(long) - 1)
> 
> static inline void set_bit_aligned(int bit, unsigned long *ptr)
> {
> 	int offs = LONG_ALIGN_DIFF(ptr) * 8;
> 
> #ifdef __BIGENDIAN
> 	if (bit >= offs) {
> 		set_bit(bit - offs, LONG_ALIGNED(ptr));
> 	} else {
> 		set_bit(bit + BITS_PER_LONG - offs, LONG_ALIGNED(ptr) + 1);
> 	}
> #else
> 	if (bit < BITS_PER_LONG - offs) {
> 		set_bit(bit + offs, LONG_ALIGNED(ptr));
> 	} else {
> 		set_bit(bit - BITS_PER_LONG + offs, LONG_ALIGNED(ptr) + 1);
> 	}
> #endif
> }

Hi Masami,

Indeed, that is a more elegant version, thanks for the snippet.

Clément

> 
> And use it.
> 
> Thank you,
> 
>>  
>>  	kunmap_local(kaddr);
>>  	unpin_user_pages_dirty_lock(&page, 1, true);
>> -- 
>> 2.40.1
>>
> 
>