Re: [PATCH] uprobes: __replace_page() avoid BUG in munlock_vma_page()

[Srikar Dronamraju]

* Hugh Dickins <hughd@xxxxxxxxxx> [2020-08-16 13:44:25]:

> syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(),
> when called from uprobes __replace_page().  Which of many ways to fix it?
> Settled on not calling when PageCompound (since Head and Tail are equals
> in this context, PageCompound the usual check in uprobes.c, and the prior
> use of FOLL_SPLIT_PMD will have cleared PageMlocked already).
> 
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.4+
> ---
> This one is not a 5.9-rc regression, but still good to fix.
> 
>  kernel/events/uprobes.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- v5.9-rc/kernel/events/uprobes.c	2020-08-12 19:46:50.851196584 -0700
> +++ linux/kernel/events/uprobes.c	2020-08-16 13:18:35.292821674 -0700
> @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area
>  		try_to_free_swap(old_page);
>  	page_vma_mapped_walk_done(&pvmw);
> 
> -	if (vma->vm_flags & VM_LOCKED)
> +	if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page))
>  		munlock_vma_page(old_page);
>  	put_page(old_page);
> 

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>

-- 
Thanks and Regards
Srikar Dronamraju