Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning

From: Douglas Gilbert <dgilbert@interlog.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
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, bostroesser@gmail.com, ddiss@suse.de,
	bvanassche@acm.org
Subject: Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning
Date: Mon, 18 Jan 2021 15:08:51 -0500	[thread overview]
Message-ID: <59707b66-0b6c-b397-82fe-5ad6a6f99ba1@interlog.com> (raw)
In-Reply-To: <20210118182854.GJ4605@ziepe.ca>

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).
A pre-condition says that the caller must know or check a value
is sane, and if the user space can have a hand in the value passed
the caller _must_ check pre-conditions IMO. A pre-condition also
implies that the function's implementation will not have code to
check the pre-condition.

My "log of both sides" proposal at least got around the overflowing
left shift problem. And one reviewer, Bodo Stroesser, liked it.

> Needs to be checked, add a precondition to order does not help. I
> already proposed a straightforward algorithm you can use.

It does help, it stops your proposed check from being flawed :-)

Giving a false sense of security seems more dangerous than a
pre-condition statement IMO. Bart's original overflow check (in
the mainline) limits length to 4GB (due to wrapping inside a 32
bit unsigned).

Also note there is another pre-condition statement in that function's
definition, namely that length cannot be 0.

So perhaps you, Bart Van Assche and Bodo Stroesser, should compare
notes and come up with a solution that you are _all_ happy with.
The pre-condition works for me and is the fastest. The 'length'
argument might be large, say > 1 GB [I use 1 GB in testing but
did try 4GB and found the bug I'm trying to fix] but having
individual elements greater than say 32 MB each does not
seem very practical (and fails on the systems that I test with).
In my testing the largest element size is 4 MB.

Doug Gilbert