[PATCH] Add missing bound checks for software 842 decompressor

[Joan Bruguera <joanbrugueram@gmail.com>]

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)
-- 
2.27.0