linux-kernel - Re: on stack dynamic allocations

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Message-ID: <20120817111826.15226265ph07g70g@www.81.fi>
Date:	Fri, 17 Aug 2012 11:18:26 +0300
From:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>,
	herbert@...dor.apana.org.au,
	linux-crypto <linux-crypto@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: on stack dynamic allocations

Quoting David Daney <ddaney.cavm@...il.com>:

> On 08/16/2012 02:20 PM, Kasatkin, Dmitry wrote:
>> Hello,
>>
>> Some places in the code uses variable-size allocation on stack..
>> For example from hmac_setkey():
>>
>> 	struct {
>> 		struct shash_desc shash;
>> 		char ctx[crypto_shash_descsize(hash)];
>> 	} desc;
>>
>>
>> sparse complains
>>
>> CHECK   crypto/hmac.c
>> crypto/hmac.c:57:47: error: bad constant expression
>>
>> I like it instead of kmalloc..
>>
>> But what is position of kernel community about it?
>
> If you know that the range of crypto_shash_descsize(hash) is  
> bounded, just use the upper bound.
>
> If the range of crypto_shash_descsize(hash) is unbounded, then the  
> stack will overflow and ... BOOM!
>

Quick look shows that largest crypto_shash_descsize() would be with  
hmac+s390/sha512, 16 + 332 = 348. Crypto-api also prevents registering  
shash with descsize larger than (PAGE_SIZE / 8).

-Jussi

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