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

[Joan Bruguera]

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@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>