linux-kernel - Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl

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Date:   Mon, 18 Jan 2021 21:46:36 +0100
From:   Bodo Stroesser <bostroesser@...il.com>
To:     dgilbert@...erlog.com, Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        target-devel@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, martin.petersen@...cle.com,
        jejb@...ux.vnet.ibm.com, ddiss@...e.de, bvanassche@....org
Subject: Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free()
 warning

On 18.01.21 21:08, 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).
> 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.

I added my Reviewed-by after you added a working check of nent overflow.
I did not oppose to the usage of ilog() there. But now I think Jason is
right that indeed ilog usage is a bit 'indirect'.

Anyway I still think, there should be a check for nent overflow.

> 
>> 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
>