Re: [PATCH AUTOSEL 6.12 1/5] uprobes: sanitiize xol_free_insn_slot()

[Oleg Nesterov <oleg@redhat.com>]

Hi Sasha,

but why do you think it makes sense to backport these "uprobes" cleanups?

Oleg.

On 11/24, Sasha Levin wrote:
>
> From: Oleg Nesterov <oleg@redhat.com>
>
> [ Upstream commit c7b4133c48445dde789ed30b19ccb0448c7593f7 ]
>
> 1. Clear utask->xol_vaddr unconditionally, even if this addr is not valid,
>    xol_free_insn_slot() should never return with utask->xol_vaddr != NULL.
>
> 2. Add a comment to explain why do we need to validate slot_addr.
>
> 3. Simplify the validation above. We can simply check offset < PAGE_SIZE,
>    unsigned underflows are fine, it should work if slot_addr < area->vaddr.
>
> 4. Kill the unnecessary "slot_nr >= UINSNS_PER_PAGE" check, slot_nr must
>    be valid if offset < PAGE_SIZE.
>
> The next patches will cleanup this function even more.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lore.kernel.org/r/20240929144235.GA9471@redhat.com
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  kernel/events/uprobes.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 4b52cb2ae6d62..cc605df73d72f 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1683,8 +1683,8 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
>  static void xol_free_insn_slot(struct task_struct *tsk)
>  {
>  	struct xol_area *area;
> -	unsigned long vma_end;
>  	unsigned long slot_addr;
> +	unsigned long offset;
>
>  	if (!tsk->mm || !tsk->mm->uprobes_state.xol_area || !tsk->utask)
>  		return;
> @@ -1693,24 +1693,21 @@ static void xol_free_insn_slot(struct task_struct *tsk)
>  	if (unlikely(!slot_addr))
>  		return;
>
> +	tsk->utask->xol_vaddr = 0;
>  	area = tsk->mm->uprobes_state.xol_area;
> -	vma_end = area->vaddr + PAGE_SIZE;
> -	if (area->vaddr <= slot_addr && slot_addr < vma_end) {
> -		unsigned long offset;
> -		int slot_nr;
> -
> -		offset = slot_addr - area->vaddr;
> -		slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
> -		if (slot_nr >= UINSNS_PER_PAGE)
> -			return;
> +	offset = slot_addr - area->vaddr;
> +	/*
> +	 * slot_addr must fit into [area->vaddr, area->vaddr + PAGE_SIZE).
> +	 * This check can only fail if the "[uprobes]" vma was mremap'ed.
> +	 */
> +	if (offset < PAGE_SIZE) {
> +		int slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
>
>  		clear_bit(slot_nr, area->bitmap);
>  		atomic_dec(&area->slot_count);
>  		smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
>  		if (waitqueue_active(&area->wq))
>  			wake_up(&area->wq);
> -
> -		tsk->utask->xol_vaddr = 0;
>  	}
>  }
>
> --
> 2.43.0
>