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Subject: Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free()
 warning
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Douglas Gilbert <dgilbert@interlog.com>
Cc:     linux-scsi@vger.kernel.org, linux-block@vger.kernel.org,
        target-devel@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org, martin.petersen@oracle.com,
        jejb@linux.vnet.ibm.com, ddiss@suse.de, bvanassche@acm.org
References: <20210118163006.61659-1-dgilbert@interlog.com>
 <20210118163006.61659-2-dgilbert@interlog.com>
 <20210118182854.GJ4605@ziepe.ca>
 <59707b66-0b6c-b397-82fe-5ad6a6f99ba1@interlog.com>
 <20210118202431.GO4605@ziepe.ca>
From:   Bodo Stroesser <bostroesser@gmail.com>
Message-ID: <7f443666-b210-6f99-7b50-6c26d87fa7ca@gmail.com>
Date:   Mon, 18 Jan 2021 22:22:56 +0100
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On 18.01.21 21:24, Jason Gunthorpe wrote:
> On Mon, Jan 18, 2021 at 03:08:51PM -0500, Douglas Gilbert wrote:
>> On 2021-01-18 1:28 p.m., Jason Gunthorpe wrote:
>>> On Mon, Jan 18, 2021 at 11:30:03AM -0500, Douglas Gilbert wrote:
>>>
>>>> After several flawed attempts to detect overflow, take the fastest
>>>> route by stating as a pre-condition that the 'order' function argument
>>>> cannot exceed 16 (2^16 * 4k = 256 MiB).
>>>
>>> That doesn't help, the point of the overflow check is similar to
>>> overflow checks in kcalloc: to prevent the routine from allocating
>>> less memory than the caller might assume.
>>>
>>> For instance ipr_store_update_fw() uses request_firmware() (which is
>>> controlled by userspace) to drive the length argument to
>>> sgl_alloc_order(). If userpace gives too large a value this will
>>> corrupt kernel memory.
>>>
>>> So this math:
>>>
>>>     	nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);
>>
>> But that check itself overflows if order is too large (e.g. 65).
> 
> I don't reall care about order. It is always controlled by the kernel
> and it is fine to just require it be low enough to not
> overflow. length is the data under userspace control so math on it
> must be checked for overflow.
> 
>> Also note there is another pre-condition statement in that function's
>> definition, namely that length cannot be 0.
> 
> I don't see callers checking for that either, if it is true length 0
> can't be allowed it should be blocked in the function
> 
> Jason
> 

A already said, I also think there should be a check for length or
rather nent overflow.

I like the easy to understand check in your proposed code:

	if (length >> (PAGE_SHIFT + order) >= UINT_MAX)
		return NULL;


But I don't understand, why you open-coded the nent calculation:

	nent = length >> (PAGE_SHIFT + order);
	if (length & ((1ULL << (PAGE_SHIFT + order)) - 1))
		nent++;

Wouldn't it be better to keep the original line instead:

	nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);

Or maybe even better:

	nent = DIV_ROUND_UP(length, PAGE_SIZE << order);


I think, combining the above lines results in short and easily readable code:


	u32 elem_len;

	if (length >> (PAGE_SHIFT + order) >= UINT_MAX)
		return NULL;
	nent = DIV_ROUND_UP(length, PAGE_SIZE << order);

	if (chainable) {
		if (check_add_overflow(nent, 1, &nalloc))
			return NULL;
	}
	else
		nalloc = nent;


Thank you,
Bodo