Re: [PATCH v2] proc: use untagged_addr() for pagemap_read addresses

[Vincenzo Frascino]

Hi Miles,

On 11/27/20 5:07 AM, Miles Chen wrote:
> When we try to visit the pagemap of a tagged userspace pointer, we find
> that the start_vaddr is not correct because of the tag.
> To fix it, we should untag the usespace pointers in pagemap_read().
> 

Nit: s/usespace/userspace/ (please search and replace all occurrences :) )

> I tested with 5.10-rc4 and the issue remains.
> 
> Explaination from Catalin in [1]:
>

Nit: s/Explaination/Explanation/ (please search and replace all occurrences :) )

> "
> Arguably, that's a user-space bug since tagged file offsets were never
> supported. In this case it's not even a tag at bit 56 as per the arm64
> tagged address ABI but rather down to bit 47. You could say that the
> problem is caused by the C library (malloc()) or whoever created the
> tagged vaddr and passed it to this function. It's not a kernel
> regression as we've never supported it.
> 
> Now, pagemap is a special case where the offset is usually not generated
> as a classic file offset but rather derived by shifting a user virtual
> address. I guess we can make a concession for pagemap (only) and allow
> such offset with the tag at bit (56 - PAGE_SHIFT + 3).
> "
> 
> My test code is baed on [2]:

Nit: s/baed/based/ (please search and replace all occurrences :) )

[...]

> ---
>  fs/proc/task_mmu.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 217aa2705d5d..92b277388f05 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>  
>  	src = *ppos;
>  	svpfn = src / PM_ENTRY_BYTES;
> -	start_vaddr = svpfn << PAGE_SHIFT;
>  	end_vaddr = mm->task_size;
>  
>  	/* watch out for wraparound */
> -	if (svpfn > mm->task_size >> PAGE_SHIFT)
> +	start_vaddr = end_vaddr;
> +	if (svpfn < (ULONG_MAX >> PAGE_SHIFT))

It seems that 'svpfn' should be less-then-equal (<=) '(ULONG_MAX >>
PAGE_SHIFT)'. Is there any specific reason why this is not the case?

> +		start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> +
> +	/* Ensure the address is inside the task */
> +	if (start_vaddr > mm->task_size)
>  		start_vaddr = end_vaddr;
>  
>  	/*
> 

Otherwise:

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>

-- 
Regards,
Vincenzo