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 <cle...@rivosinc.com>
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
