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

[Beau Belgrave]

On Thu, Sep 14, 2023 at 03:11:02PM +0200, Clément Léger 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>

Thanks for fixing! I have a few comments on this.

I unfortunately do not have RISC-V hardware to validate this on.

> ---
>  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;

I think for future readers of this code it would be more clear to use
BITS_PER_BYTE instead of the hardcoded 8. Given we always align on a
"natural" boundary, I believe the bit_offset will always be 32 bits.

A comment here might help clarify why we do this as well in case folks
don't see the change description.

> +		uaddr &= ~(sizeof(unsigned long) - 1);

Shouldn't this uaddr change be done before calling pin_user_pages_remote()
to ensure things cannot go bad? (I don't think they can, but it looks a
little odd).

Thanks,
-Beau

> +	}
>  	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);
>  
>  	kunmap_local(kaddr);
>  	unpin_user_pages_dirty_lock(&page, 1, true);
> -- 
> 2.40.1