Re: BUG: unable to handle kernel NULL pointer dereference at000000000000000e (reset_prng_context)

[Andrew Morton]

On Wed, 16 Jul 2008 13:34:59 +0800 Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:

> On Tue, Jul 15, 2008 at 09:51:53PM -0700, Andrew Morton wrote:
> >
> > And I agree with me.  I fail to see how someone who is familiar with
> > kernel code and who is reviewing a submission could let something like
> > 
> > 	for (i=DEFAULT_BLK_SZ-1;i>0;i--) {
> > 
> > pass without comment.
> 
> Well here is the code in question in its entirety:
> 
> 
>         /*
>          * Now update our DT value
>          */
>         for (i=DEFAULT_BLK_SZ-1;i>0;i--) {
>                 ctx->DT[i] = ctx->DT[i-1];
>         }
> 
> I'm afraid that I simply don't see your point.

How odd.

> > btw, I've searched my linux-kernel archives and netdev archives and the
> > linux-crypto web archives and can find no sign of any submission or
> > discussion of this patch.  Am I looking in the wrong places?
> 
> http://www.mail-archive.com/linux-crypto@xxxxxxxxxxxxxxx/msg02082.html

Ah.  Neil went and gave all the patches the same title, so you (I
assume) ad to rewrite those titles so my googling for the title
failed to turn up the patches.  There's a lesson.

> commit b8454eebe380677789735fd6bad368af2e6b2d1e
> Author: Neil Horman <nhorman@xxxxxxxxxxxxx>
> Date:   Mon Jul 7 22:41:31 2008 +0800
> 
>     crypto: prng - Deterministic CPRNG
>     
>     This patch adds a cryptographic pseudo-random number generator
>     based on CTR(AES-128).  It is meant to be used in cases where a
>     deterministic CPRNG is required.
>     
>     One of the first applications will be as an input in the IPsec IV
>     generation process.
>     
>     Signed-off-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
>     Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> 
> diff --git a/crypto/Kconfig b/crypto/Kconfig
> index 795e31c..43b7473 100644
> --- a/crypto/Kconfig
> +++ b/crypto/Kconfig
> @@ -666,6 +666,15 @@ config CRYPTO_LZO
>  	help
>  	  This is the LZO algorithm.
>  
> +comment "Random Number Generation"
> +
> +config CRYPTO_PRNG
> +	tristate "Pseudo Random Number Generation for Cryptographic modules"
> +	help
> +	  This option enables the generic pseudo random number generator
> +	  for cryptographic modules.  Uses the Algorithm specified in
> +	  ANSI X9.31 A.2.4
> +
>  source "drivers/crypto/Kconfig"
>  
>  endif	# if CRYPTO
> diff --git a/crypto/Makefile b/crypto/Makefile
> index d4f3ed8..ef61b3b 100644
> --- a/crypto/Makefile
> +++ b/crypto/Makefile
> @@ -69,7 +69,7 @@ obj-$(CONFIG_CRYPTO_MICHAEL_MIC) += michael_mic.o
>  obj-$(CONFIG_CRYPTO_CRC32C) += crc32c.o
>  obj-$(CONFIG_CRYPTO_AUTHENC) += authenc.o
>  obj-$(CONFIG_CRYPTO_LZO) += lzo.o
> -
> +obj-$(CONFIG_CRYPTO_PRNG) += prng.o
>  obj-$(CONFIG_CRYPTO_TEST) += tcrypt.o
>  
>  #
> diff --git a/crypto/prng.c b/crypto/prng.c
> new file mode 100644
> index 0000000..24e4f32
> --- /dev/null
> +++ b/crypto/prng.c
> @@ -0,0 +1,410 @@
> +/*
> + * PRNG: Pseudo Random Number Generator
> + *       Based on NIST Recommended PRNG From ANSI X9.31 Appendix A.2.4 using
> + *       AES 128 cipher in RFC3686 ctr mode
> + *
> + *  (C) Neil Horman <nhorman@xxxxxxxxxxxxx>
> + *
> + *  This program is free software; you can redistribute it and/or modify it
> + *  under the terms of the GNU General Public License as published by the
> + *  Free Software Foundation; either version 2 of the License, or (at your
> + *  any later version.
> + *
> + *
> + */
> +
> +#include <linux/err.h>
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/fs.h>
> +#include <linux/scatterlist.h>
> +#include <linux/string.h>
> +#include <linux/crypto.h>
> +#include <linux/highmem.h>
> +#include <linux/moduleparam.h>
> +#include <linux/jiffies.h>
> +#include <linux/timex.h>
> +#include <linux/interrupt.h>
> +#include <linux/miscdevice.h>

Is miscdevice.h needed?

> +#include "prng.h"
> +
> +#define TEST_PRNG_ON_START 0
> +
> +#define DEFAULT_PRNG_KEY "0123456789abcdef1011"
> +#define DEFAULT_PRNG_KSZ 20
> +#define DEFAULT_PRNG_IV "defaultv"
> +#define DEFAULT_PRNG_IVSZ 8
> +#define DEFAULT_BLK_SZ 16
> +#define DEFAULT_V_SEED "zaybxcwdveuftgsh"
> +
> +/*
> + * Flags for the prng_context flags field
> + */
> +
> +#define PRNG_FIXED_SIZE 0x1
> +#define PRNG_NEED_RESET 0x2
> +
> +/*
> + * Note: DT is our counter value
> + * 	 I is our intermediate value
> + *	 V is our seed vector
> + * See http://csrc.nist.gov/groups/STM/cavp/documents/rng/931rngext.pdf
> + * for implementation details
> + */
> +
> +
> +struct prng_context {
> +	char *prng_key;
> +	char *prng_iv;
> +	spinlock_t prng_lock;
> +	unsigned char rand_data[DEFAULT_BLK_SZ];
> +	unsigned char last_rand_data[DEFAULT_BLK_SZ];
> +	unsigned char DT[DEFAULT_BLK_SZ];
> +	unsigned char I[DEFAULT_BLK_SZ];
> +	unsigned char V[DEFAULT_BLK_SZ];
> +	u32 rand_data_valid;
> +	struct crypto_blkcipher *tfm;
> +	u32 flags;
> +};
> +
> +static int dbg;
> +
> +static void hexdump(char *note, unsigned char *buf, unsigned int len)
> +{
> +	if (dbg) {
> +		printk(KERN_CRIT "%s", note);
> +		print_hex_dump(KERN_CONT, "", DUMP_PREFIX_OFFSET,
> +				16, 1,
> +				buf, len, false);
> +	}
> +}
> +
> +#define dbgprint(format, args...) do {if(dbg) printk(format, ##args);} while(0)
> +
> +static void xor_vectors(unsigned char *in1, unsigned char *in2,
> +		        unsigned char *out, unsigned int size)
> +{
> +	int i;
> +
> +	for (i=0;i<size;i++)
> +		out[i] = in1[i] ^ in2[i];
> +
> +}
> +/*
> + * Returns DEFAULT_BLK_SZ bytes of random data per call
> + * returns 0 if generation succeded, <0 if something went wrong
> + */
> +static int _get_more_prng_bytes(struct prng_context *ctx)
> +{
> +	int i;
> +	struct blkcipher_desc desc;
> +	struct scatterlist sg_in, sg_out;
> +	int ret;
> +	unsigned char tmp[DEFAULT_BLK_SZ];
> +
> +	desc.tfm = ctx->tfm;
> +	desc.flags = 0;
> +
> +
> +	dbgprint(KERN_CRIT "Calling _get_more_prng_bytes for context %p\n",ctx);
> +
> +	hexdump("Input DT: ", ctx->DT, DEFAULT_BLK_SZ);
> +	hexdump("Input I: ", ctx->I, DEFAULT_BLK_SZ);
> +	hexdump("Input V: ", ctx->V, DEFAULT_BLK_SZ);
> +
> +	/*
> +	 * This algorithm is a 3 stage state machine
> +	 */
> +	for (i=0;i<3;i++) {
> +
> +		desc.tfm = ctx->tfm;
> +		desc.flags = 0;
> +		switch (i) {
> +			case 0:
> +				/*
> +				 * Start by encrypting the counter value
> +				 * This gives us an intermediate value I
> +				 */
> +				memcpy(tmp, ctx->DT, DEFAULT_BLK_SZ);
> +				sg_init_one(&sg_out, &ctx->I[0], DEFAULT_BLK_SZ);
> +				hexdump("tmp stage 0: ", tmp, DEFAULT_BLK_SZ);
> +				break;
> +			case 1:
> +
> +				/*
> +				 * Next xor I with our secret vector V
> +				 * encrypt that result to obtain our
> +				 * pseudo random data which we output
> +				 */
> +				xor_vectors(ctx->I, ctx->V, tmp, DEFAULT_BLK_SZ);
> +				sg_init_one(&sg_out, &ctx->rand_data[0], DEFAULT_BLK_SZ);
> +				hexdump("tmp stage 1: ", tmp, DEFAULT_BLK_SZ);
> +				break;
> +			case 2:
> +				/*
> +				 * First check that we didn't produce the same random data
> +				 * that we did last time around through this
> +				 */
> +				if (!memcmp(ctx->rand_data, ctx->last_rand_data, DEFAULT_BLK_SZ)) {
> +					printk(KERN_ERR "ctx %p Failed repetition check!\n",
> +						ctx);
> +					ctx->flags |= PRNG_NEED_RESET;
> +					return -1;

Note the `return -1's.... (see below)

> +				}
> +				memcpy(ctx->last_rand_data, ctx->rand_data, DEFAULT_BLK_SZ);
> +
> +				/*
> +				 * Lastly xor the random data with I
> +				 * and encrypt that to obtain a new secret vector V
> +				 */
> +				xor_vectors(ctx->rand_data, ctx->I, tmp, DEFAULT_BLK_SZ);
> +				sg_init_one(&sg_out, &ctx->V[0], DEFAULT_BLK_SZ);
> +				hexdump("tmp stage 2: ", tmp, DEFAULT_BLK_SZ);
> +				break;
> +		}
> +
> +		/* Initialize our input buffer */
> +		sg_init_one(&sg_in, &tmp[0], DEFAULT_BLK_SZ);
> +
> +		/* do the encryption */
> +		ret = crypto_blkcipher_encrypt(&desc, &sg_out, &sg_in, DEFAULT_BLK_SZ);
> +
> +		/* And check the result */
> +		if (ret) {
> +			dbgprint(KERN_CRIT "Encryption of new block failed for context %p\n",ctx);
> +			ctx->rand_data_valid = DEFAULT_BLK_SZ;
> +			return -1;
> +		}
> +
> +	}
> +
> +	/*
> +	 * Now update our DT value
> +	 */
> +	for (i=DEFAULT_BLK_SZ-1;i>0;i--) {
> +		ctx->DT[i] = ctx->DT[i-1];
> +	}
> +	ctx->DT[0] += 1;
> +
> +	dbgprint("Returning new block for context %p\n",ctx);
> +	ctx->rand_data_valid = 0;
> +
> +	hexdump("Output DT: ", ctx->DT, DEFAULT_BLK_SZ);
> +	hexdump("Output I: ", ctx->I, DEFAULT_BLK_SZ);
> +	hexdump("Output V: ", ctx->V, DEFAULT_BLK_SZ);
> +	hexdump("New Random Data: ", ctx->rand_data, DEFAULT_BLK_SZ);

There will be some overhead in evaluating all the args, stacking them,
calling hexdump(), testing `dbg' then just returning.

This:

	if (dbg) {
		hexdump(...);
		hexdump(...);
		hexdump(...);
	}

would save rather a lot of instructions.

> +	return 0;
> +}
> +
> +/* Our exported functions */

It's good to document the exported interface (at least).

> +int get_prng_bytes(char *buf, int nbytes, struct prng_context *ctx)
> +{
> +	unsigned long flags;
> +	unsigned char *ptr = buf;
> +	unsigned int byte_count = (unsigned int)nbytes;
> +	int err;
> +
> +
> +	if (nbytes < 0)
> +		return -EINVAL;
> +
> +	spin_lock_irqsave(&ctx->prng_lock, flags);
> +
> +	err = -EFAULT;

Why -EFAULT?

> +	if (ctx->flags & PRNG_NEED_RESET)
> +		goto done;
> +
> +	/*
> +	 * If the FIXED_SIZE flag is on, only return whole blocks of
> +	 * pseudo random data
> +	 */
> +	err = -EINVAL;
> +	if (ctx->flags & PRNG_FIXED_SIZE) {
> +		if (nbytes < DEFAULT_BLK_SZ)
> +			goto done;
> +		byte_count = DEFAULT_BLK_SZ;
> +	}
> +
> +	err = byte_count;

So it returns the number of bytes copied on success, and -ve on error.

> +	dbgprint(KERN_CRIT "getting %d random bytes for context %p\n",byte_count, ctx);
> +
> +
> +remainder:
> +	if (ctx->rand_data_valid == DEFAULT_BLK_SZ) {
> +		if (_get_more_prng_bytes(ctx) < 0) {
> +			memset(buf, 0, nbytes);
> +			err = -EFAULT;
> +			goto done;
> +		}
> +	}
> +
> +	/*
> +	 * Copy up to the next whole block size
> +	 */
> +	if (byte_count < DEFAULT_BLK_SZ) {
> +		for (;ctx->rand_data_valid < DEFAULT_BLK_SZ; ctx->rand_data_valid++) {
> +			*ptr = ctx->rand_data[ctx->rand_data_valid];
> +			ptr++;
> +			byte_count--;
> +			if (byte_count == 0)
> +				goto done;
> +		}
> +	}
> +
> +	/*
> +	 * Now copy whole blocks
> +	 */
> +	for(;byte_count >= DEFAULT_BLK_SZ; byte_count -= DEFAULT_BLK_SZ) {
> +		if (_get_more_prng_bytes(ctx) < 0) {
> +			memset(buf, 0, nbytes);
> +			err = -1;
> +			goto done;
> +		}
> +		memcpy(ptr, ctx->rand_data, DEFAULT_BLK_SZ);
> +		ctx->rand_data_valid += DEFAULT_BLK_SZ;
> +		ptr += DEFAULT_BLK_SZ;
> +	}
> +
> +	/*
> +	 * Now copy any extra partial data
> +	 */
> +	if (byte_count)
> +		goto remainder;
> +
> +done:
> +	spin_unlock_irqrestore(&ctx->prng_lock, flags);
> +	dbgprint(KERN_CRIT "returning %d from get_prng_bytes in context %p\n",err, ctx);
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(get_prng_bytes);

On error the function will sometimes return -EFOO and other times will
just return -1.  This is inconsistent and makes the error returns
rather less useful than they could be.  I'd suggest that it be switched
to -1 throughout or, better, to consistently use appropriate error codes.

> +struct prng_context *alloc_prng_context(void)
> +{
> +	struct prng_context *ctx=kzalloc(sizeof(struct prng_context), GFP_KERNEL);
> +
> +	spin_lock_init(&ctx->prng_lock);
> +
> +	if (reset_prng_context(ctx, NULL, NULL, NULL, NULL)) {
> +		kfree(ctx);
> +		ctx = NULL;
> +	}
> +
> +	dbgprint(KERN_CRIT "returning context %p\n",ctx);
> +	return ctx;
> +}
> +
> +EXPORT_SYMBOL_GPL(alloc_prng_context);
> +
> +void free_prng_context(struct prng_context *ctx)
> +{
> +	crypto_free_blkcipher(ctx->tfm);
> +	kfree(ctx);
> +}
> +EXPORT_SYMBOL_GPL(free_prng_context);
> +
> +int reset_prng_context(struct prng_context *ctx,
> +		       unsigned char *key, unsigned char *iv,
> +		       unsigned char *V, unsigned char *DT)
> +{
> +	int ret;
> +	int iv_len;
> +	int rc = -EFAULT;
> +
> +	spin_lock(&ctx->prng_lock);
> +	ctx->flags |= PRNG_NEED_RESET;
> +
> +	if (key)
> +		memcpy(ctx->prng_key,key,strlen(ctx->prng_key));
> +	else
> +		ctx->prng_key = DEFAULT_PRNG_KEY;

This is very strange.  In one case we'll copy a string onto existing
storage at ctx->prng_key.  In the other case we'll *modify*
ctx->prng_key directly.

This is quite unusual, and it makes it rather tricky for the owner of
ctx to clean it up correctly.  It also introduces risks of memory leaks.

There is only one caller of this function, and that caller uses NULL
for all its arguments, so it looks like the first leg of all these `if'
statements has never been used.

I'd suggest that this interface needs a rethink.  And documenting.

> +	if (iv)
> +		memcpy(ctx->prng_iv,iv, strlen(ctx->prng_iv));

Also, it's odd that we'll take a copy of a variable-length string
without checking its length and without in any way taking a copy of its
length.  How can it ever be used?

If we choose to change this by taking a copy of the length and if we
decide that the way to do that is by sticking a \0 at the end of the
string then to above memcpy() could become a strcpy() (or similar).

We don't support \0's in the key or the IV.  I assume that's OK?

> +	else
> +		ctx->prng_iv = DEFAULT_PRNG_IV;
> +
> +	if (V)
> +		memcpy(ctx->V,V,DEFAULT_BLK_SZ);
> +	else
> +		memcpy(ctx->V,DEFAULT_V_SEED,DEFAULT_BLK_SZ);
> +
> +	if (DT)
> +		memcpy(ctx->DT, DT, DEFAULT_BLK_SZ);
> +	else
> +		memset(ctx->DT, 0, DEFAULT_BLK_SZ);
> +
> +	memset(ctx->rand_data,0,DEFAULT_BLK_SZ);
> +	memset(ctx->last_rand_data,0,DEFAULT_BLK_SZ);
> +
> +	if (ctx->tfm)
> +		crypto_free_blkcipher(ctx->tfm);
> +
> +	ctx->tfm = crypto_alloc_blkcipher("rfc3686(ctr(aes))",0,0);
> +	if (!ctx->tfm) {
> +		dbgprint(KERN_CRIT "Failed to alloc crypto tfm for context %p\n",ctx->tfm);
> +		goto out;
> +	}
> +
> +	ctx->rand_data_valid = DEFAULT_BLK_SZ;
> +
> +	ret = crypto_blkcipher_setkey(ctx->tfm, ctx->prng_key, strlen(ctx->prng_key));
> +	if (ret) {
> +		dbgprint(KERN_CRIT "PRNG: setkey() failed flags=%x\n",
> +			crypto_blkcipher_get_flags(ctx->tfm));
> +		crypto_free_blkcipher(ctx->tfm);
> +		goto out;
> +	}
> +
> +	iv_len = crypto_blkcipher_ivsize(ctx->tfm);
> +	if (iv_len) {
> +		crypto_blkcipher_set_iv(ctx->tfm, ctx->prng_iv, iv_len);
> +	}
> +	rc = 0;
> +	ctx->flags &= ~PRNG_NEED_RESET;
> +out:
> +	spin_unlock(&ctx->prng_lock);
> +
> +	return rc;
> +
> +}
> +EXPORT_SYMBOL_GPL(reset_prng_context);
> +
> +/* Module initalization */
> +static int __init prng_mod_init(void)
> +{
> +
> +#ifdef TEST_PRNG_ON_START

Might be nice to make this a module parameter.

> +	int i;
> +	unsigned char tmpbuf[DEFAULT_BLK_SZ];
> +
> +	struct prng_context *ctx = alloc_prng_context();
> +	if (ctx == NULL)
> +		return -EFAULT;
> +	for (i=0;i<16;i++) {
> +		if (get_prng_bytes(tmpbuf, DEFAULT_BLK_SZ, ctx) < 0) {
> +			free_prng_context(ctx);
> +			return -EFAULT;
> +		}
> +	}
> +	free_prng_context(ctx);
> +#endif
> +
> +	return 0;
> +}
> +
> +static void __exit prng_mod_fini(void)
> +{
> +	return;
> +}
> +
> +MODULE_LICENSE("GPL");
> +MODULE_DESCRIPTION("Software Pseudo Random Number Generator");
> +MODULE_AUTHOR("Neil Horman <nhorman@xxxxxxxxxxxxx>");
> +module_param(dbg, int, 0);
> +MODULE_PARM_DESC(dbg, "Boolean to enable debugging (0/1 == off/on)");
> +module_init(prng_mod_init);
> +module_exit(prng_mod_fini);
> diff --git a/crypto/prng.h b/crypto/prng.h
> new file mode 100644
> index 0000000..1ac9be5
> --- /dev/null
> +++ b/crypto/prng.h
> @@ -0,0 +1,27 @@
> +/*
> + * PRNG: Pseudo Random Number Generator
> + *
> + *  (C) Neil Horman <nhorman@xxxxxxxxxxxxx>
> + *
> + *  This program is free software; you can redistribute it and/or modify it
> + *  under the terms of the GNU General Public License as published by the
> + *  Free Software Foundation; either version 2 of the License, or (at your
> + *  any later version.
> + *
> + *
> + */
> +
> +#ifndef _PRNG_H_
> +#define _PRNG_H_
> +struct prng_context;
> +
> +int get_prng_bytes(char *buf, int nbytes, struct prng_context *ctx);
> +struct prng_context *alloc_prng_context(void);
> +int reset_prng_context(struct prng_context *ctx,
> +			unsigned char *key, unsigned char *iv,
> +			unsigned char *V,
> +			unsigned char *DT);
> +void free_prng_context(struct prng_context *ctx);
> +

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