Re: [PATCH] Add missing bound checks for software 842 decompressor

[Joan Bruguera <joanbrugueram@gmail.com>]

Any feedback?

- 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.
> 
> 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");
> 
> Signed-off-by: Joan Bruguera <joanbrugueram@gmail.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)
>