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

[Kees Cook]

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...

> - 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. :(

> > 
> > 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.

> > 
> > Signed-off-by: Joan Bruguera <joanbrugueram@xxxxxxxxx>
> > ---
> >   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@xxxxxxxxxxxx>


-- 
Kees Cook