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Date:   Mon, 24 Aug 2020 03:24:27 +0200
From:   Joan Bruguera <joanbrugueram@...il.com>
To:     keescook@...omium.org
Cc:     linux-kernel@...r.kernel.org, haren@...ibm.com,
        linux-crypto@...r.kernel.org, ddstreet@...e.org,
        herbert@...dor.apana.org.au
Subject: Re: [PATCH] Add missing bound checks for software 842 decompressor



On 19.08.20 22:19, Kees Cook wrote:
> On Sun, Aug 16, 2020 at 02:33:41AM +0200, Joan Bruguera wrote:
>> Any feedback?
> 
> Hi!
> 
> I just happened to see this email. I think this should likely be
> directed to the crypto (which also handled compress/decompress APIs)
> list and the original author...
> 
> Additional notes below...
> 

Thanks for the review, much appreciated.

>> - Joan
>>
>> On 05.06.20 17:44, Joan Bruguera wrote:
>>> The software 842 decompressor receives, through the initial value of the
>>> 'olen' parameter, the capacity of the buffer pointed to by 'out'. If this
>>> capacity is insufficient to decode the compressed bitstream, -ENOSPC
>>> should be returned.
>>>
>>> However, the bounds checks are missing for index references (for those
>>> ops. where decomp_ops includes a I2, I4 or I8 subop.), and also for
>>> OP_SHORT_DATA. Due to this, sw842_decompress can write past the capacity
>>> of the 'out' buffer.
> 
> Eek. :(

It isn't much exploitable though, since AFAICT, compressed data can only 
come from zram or pstore, not from userspace.

> 
>>>
>>> The case for index references can be triggered by compressing data that
>>> follows a 16-byte periodic pattern (excluding special cases which are
>>> better encoded by OP_ZEROS) and passing a 'olen' somewhat smaller than the
>>> original length.
>>> The case for OP_SHORT_DATA can be triggered by compressing an amount of
>>> data that is not a multiple of 8, and then a slightly smaller 'olen' that
>>> can't fit those last <8 bytes.
>>>
>>> Following is a small test module to demonstrate the issue.
>>>
>>> -
>>>
>>> #include <linux/module.h>
>>> #include <linux/kernel.h>
>>> #include <linux/sw842.h>
>>>
>>> static unsigned char workspace[1000000] = { 0 }; // Hacky
>>>
>>> static void test_bound(const char *name, unsigned ibound, unsigned dbound)
>>> {
>>> 	uint8_t in[ibound], out[ibound * 4], decomp[ibound /* Overallocated */];
>>> 	unsigned clen = ibound * 4, dlen = dbound, i;
>>> 	int ret;
>>>
>>> 	for (i = 0; i < ibound; i ++)
>>> 		in[i] = i % 16; // 0, 1, 2, ..., 14, 15, 0, 1, 2, ...
>>> 	for (i = dbound; i < ibound; i++)
>>> 		decomp[i] = 0xFF; // Place guard bytes
>>>
>>> 	ret = sw842_compress(in, ibound, out, &clen, workspace);
>>> 	BUG_ON(ret != 0);
>>>
>>> 	ret = sw842_decompress(out, clen, decomp, &dlen);
>>> 	if (ret != -ENOSPC) {
>>> 		printk(KERN_ERR "%s: Expected ENOSPC to be returned\n", name);
>>> 	}
>>> 	for (i = dbound; i < ibound; i++) {
>>> 		if (decomp[i] != 0xFF) {
>>> 			printk(KERN_ERR "%s: Guard overwritten\n", name);
>>> 			break;
>>> 		}
>>> 	}
>>> }
>>>
>>> int init_module(void)
>>> {
>>> 	test_bound("Index reference test", 256, 64);
>>> 	test_bound("Short data test", 12, 8);
>>> 	return -ECANCELED; // Do not leave this test module hanging around
>>> }
>>>
>>> void cleanup_module(void)
>>> {
>>> }
>>>
>>> MODULE_LICENSE("GPL");
>>> MODULE_SOFTDEP("pre: 842");
> 
> Can this test be added to the kernel source directly? It'd be nice to
> add such a regression test.
> 

I took a look and maybe it could go into the crypto self tests (in 
crypto/testmgr.c)... but I'm not really sure if it fits there, since 
this would be a regression test rather than the smoke tests that seem to 
make most of that file, and this bug requires adding a custom method 
testing whether the decompressor respects the output buffer size rather 
than a simple test with just a simple input / output data test vector.

Were thinking of that, something else (e.g. Linux Test Project), or just 
suggesting?

>>>
>>> Signed-off-by: Joan Bruguera <joanbrugueram@...il.com>
>>> ---
>>>    lib/842/842_decompress.c | 6 ++++++
>>>    1 file changed, 6 insertions(+)
>>>
>>> diff --git a/lib/842/842_decompress.c b/lib/842/842_decompress.c
>>> index 582085ef8b4..c29fbfc9d08 100644
>>> --- a/lib/842/842_decompress.c
>>> +++ b/lib/842/842_decompress.c
>>> @@ -202,6 +202,9 @@ static int __do_index(struct sw842_param *p, u8 size, u8 bits, u64 fsize)
>>>    			 (unsigned long)total,
>>>    			 (unsigned long)beN_to_cpu(&p->ostart[offset], size));
>>> +	if (size > p->olen)
>>> +		return -ENOSPC;
>>> +
>>>    	memcpy(p->out, &p->ostart[offset], size);
>>>    	p->out += size;
>>>    	p->olen -= size;
>>> @@ -345,6 +348,9 @@ int sw842_decompress(const u8 *in, unsigned int ilen,
>>>    			if (!bytes || bytes > SHORT_DATA_BITS_MAX)
>>>    				return -EINVAL;
>>> +			if (bytes > p.olen)
>>> +				return -ENOSPC;
>>> +
>>>    			while (bytes-- > 0) {
>>>    				ret = next_bits(&p, &tmp, 8);
>>>    				if (ret)
>>>
> 
> Reviewed-by: Kees Cook <keescook@...omium.org>
> 
> 

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