Re: [PATCH] uprobes: ensure that uprobe->offset and ->ref_ctr_offset are properly aligned

[Christian Borntraeger]

On 04.05.20 18:47, Oleg Nesterov wrote:
> uprobe_write_opcode() must not cross page boundary; prepare_uprobe()
> relies on arch_uprobe_analyze_insn() which should validate "vaddr" but
> some architectures (csky, s390, and sparc) don't do this.

I think the idea was that the uprobe instruction is 2 bytes and instructions
are always aligned to 2 bytes on s390.  (we can have 2,4 or 6 bytes).

> 
> We can remove the BUG_ON() check in prepare_uprobe() and validate the
> offset early in __uprobe_register(). The new IS_ALIGNED() check matches
> the alignment check in arch_prepare_kprobe() on supported architectures,
> so I think that all insns must be aligned to UPROBE_SWBP_INSN_SIZE.

Not sure if it would have been possible to try to create a uprobe on an 
odd address. If yes, then the new IS_ALIGNED check certainly makes this
better for s390, so the patch looks sane. Adding Vasily and Sven to double
check.


> 
> Another problem is __update_ref_ctr() which was wrong from the very
> beginning, it can read/write outside of kmap'ed page unless "vaddr" is
> aligned to sizeof(short), __uprobe_register() should check this too.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/events/uprobes.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index ece7e13f6e4a..cc2095607c74 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -867,10 +867,6 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
>  	if (ret)
>  		goto out;
>  
> -	/* uprobe_write_opcode() assumes we don't cross page boundary */
> -	BUG_ON((uprobe->offset & ~PAGE_MASK) +
> -			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
> -
>  	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
>  	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
>  
> @@ -1166,6 +1162,15 @@ static int __uprobe_register(struct inode *inode, loff_t offset,
>  	if (offset > i_size_read(inode))
>  		return -EINVAL;
>  
> +	/*
> +	 * This ensures that copy_from_page(), copy_to_page() and
> +	 * __update_ref_ctr() can't cross page boundary.
> +	 */
> +	if (!IS_ALIGNED(offset, UPROBE_SWBP_INSN_SIZE))
> +		return -EINVAL;
> +	if (!IS_ALIGNED(ref_ctr_offset, sizeof(short)))
> +		return -EINVAL;
> +
>   retry:
>  	uprobe = alloc_uprobe(inode, offset, ref_ctr_offset);
>  	if (!uprobe)
> @@ -2014,6 +2019,9 @@ static int is_trap_at_addr(struct mm_struct *mm, unsigned long vaddr)
>  	uprobe_opcode_t opcode;
>  	int result;
>  
> +	if (WARN_ON_ONCE(!IS_ALIGNED(vaddr, UPROBE_SWBP_INSN_SIZE)))
> +		return -EINVAL;
> +
>  	pagefault_disable();
>  	result = __get_user(opcode, (uprobe_opcode_t __user *)vaddr);
>  	pagefault_enable();
>