Code: int saa7164_bus_get(struct saa7164_dev *dev, struct tmComResInfo* msg,
	void *buf, int peekonly)
{
	struct tmComResBusInfo *bus = &dev->bus;
	u32 bytes_to_read, write_distance, curr_grp, curr_gwp,
		new_grp, buf_size, space_rem;
	struct tmComResInfo msg_tmp;
	int ret = SAA_ERR_BAD_PARAMETER;

	saa7164_bus_verify(dev);

	if (msg == NULL)
		return ret;

	if (msg->size > dev->bus.m_wMaxReqSize) {
		printk(KERN_ERR "%s() Exceeded dev->bus.m_wMaxReqSize\n",
			__func__);
		return ret;
	}

	if ((peekonly == 0) && (msg->size > 0) && (buf == NULL)) {
		printk(KERN_ERR
			"%s() Missing msg buf, size should be %d bytes\n",
			__func__, msg->size);
		return ret;
	}

	mutex_lock(&bus->lock);

	/* Peek the bus to see if a msg exists, if it's not what we're expecting
	 * then return cleanly else read the message from the bus.
	 */
	curr_gwp = saa7164_readl(bus->m_dwGetWritePos);
	curr_grp = saa7164_readl(bus->m_dwGetReadPos);

	if (curr_gwp == curr_grp) {
		ret = SAA_ERR_EMPTY;
		goto out;
	}

	bytes_to_read = sizeof(*msg);

	/* Calculate write distance to current read position */
	write_distance = 0;
	if (curr_gwp >= curr_grp)
		/* Write doesn't wrap around the ring */
		write_distance = curr_gwp - curr_grp;
	else
		/* Write wraps around the ring */
		write_distance = curr_gwp + bus->m_dwSizeGetRing - curr_grp;

	if (bytes_to_read > write_distance) {
		printk(KERN_ERR "%s() No message/response found\n", __func__);
		ret = SAA_ERR_INVALID_COMMAND;
		goto out;
	}

	/* Calculate the new read position */
	new_grp = curr_grp + bytes_to_read;
	if (new_grp > bus->m_dwSizeGetRing) {

		/* Ring wraps */
		new_grp -= bus->m_dwSizeGetRing;
		space_rem = bus->m_dwSizeGetRing - curr_grp;

		memcpy_fromio(&msg_tmp, bus->m_pdwGetRing + curr_grp, space_rem);
		memcpy_fromio((u8 *)&msg_tmp + space_rem, bus->m_pdwGetRing,
			bytes_to_read - space_rem);

	} else {
		/* No wrapping */
		memcpy_fromio(&msg_tmp, bus->m_pdwGetRing + curr_grp, bytes_to_read);
	}
	/* Convert from little endian to CPU */
 	msg_tmp.size = le16_to_cpu((__force __le16)msg_tmp.size);
 	msg_tmp.command = le32_to_cpu((__force __le32)msg_tmp.command);
 	msg_tmp.controlselector = le16_to_cpu((__force __le16)msg_tmp.controlselector);
 
 	/* No need to update the read positions, because this was a peek */
 	/* If the caller specifically want to peek, return */
 	if (peekonly) {
		memcpy(msg, &msg_tmp, sizeof(*msg));
 		goto peekout;
 	}
 
	/* Check if the command/response matches what is expected */
	if ((msg_tmp.id != msg->id) || (msg_tmp.command != msg->command) ||
		(msg_tmp.controlselector != msg->controlselector) ||
		(msg_tmp.seqno != msg->seqno) || (msg_tmp.size != msg->size)) {

		printk(KERN_ERR "%s() Unexpected msg miss-match\n", __func__);
		saa7164_bus_dumpmsg(dev, msg, buf);
		saa7164_bus_dumpmsg(dev, &msg_tmp, NULL);
		ret = SAA_ERR_INVALID_COMMAND;
		goto out;
	}

	/* Get the actual command and response from the bus */
	buf_size = msg->size;

	bytes_to_read = sizeof(*msg) + msg->size;
	/* Calculate write distance to current read position */
	write_distance = 0;
	if (curr_gwp >= curr_grp)
		/* Write doesn't wrap around the ring */
		write_distance = curr_gwp - curr_grp;
	else
		/* Write wraps around the ring */
		write_distance = curr_gwp + bus->m_dwSizeGetRing - curr_grp;

	if (bytes_to_read > write_distance) {
		printk(KERN_ERR "%s() Invalid bus state, missing msg or mangled ring, faulty H/W / bad code?\n",
		       __func__);
		ret = SAA_ERR_INVALID_COMMAND;
		goto out;
	}

	/* Calculate the new read position */
	new_grp = curr_grp + bytes_to_read;
	if (new_grp > bus->m_dwSizeGetRing) {

		/* Ring wraps */
		new_grp -= bus->m_dwSizeGetRing;
 		space_rem = bus->m_dwSizeGetRing - curr_grp;
 
 		if (space_rem < sizeof(*msg)) {
			/* msg wraps around the ring */
			memcpy_fromio(msg, bus->m_pdwGetRing + curr_grp, space_rem);
			memcpy_fromio((u8 *)msg + space_rem, bus->m_pdwGetRing,
				sizeof(*msg) - space_rem);
 			if (buf)
 				memcpy_fromio(buf, bus->m_pdwGetRing + sizeof(*msg) -
 					space_rem, buf_size);
 
 		} else if (space_rem == sizeof(*msg)) {
			memcpy_fromio(msg, bus->m_pdwGetRing + curr_grp, sizeof(*msg));
 			if (buf)
 				memcpy_fromio(buf, bus->m_pdwGetRing, buf_size);
 		} else {
 			/* Additional data wraps around the ring */
			memcpy_fromio(msg, bus->m_pdwGetRing + curr_grp, sizeof(*msg));
 			if (buf) {
 				memcpy_fromio(buf, bus->m_pdwGetRing + curr_grp +
 					sizeof(*msg), space_rem - sizeof(*msg));
				memcpy_fromio(buf + space_rem - sizeof(*msg),
					bus->m_pdwGetRing, bytes_to_read -
					space_rem);
			}

		}
 
 	} else {
 		/* No wrapping */
		memcpy_fromio(msg, bus->m_pdwGetRing + curr_grp, sizeof(*msg));
 		if (buf)
 			memcpy_fromio(buf, bus->m_pdwGetRing + curr_grp + sizeof(*msg),
 				buf_size);
 	}
	/* Convert from little endian to CPU */
	msg->size = le16_to_cpu((__force __le16)msg->size);
	msg->command = le32_to_cpu((__force __le32)msg->command);
	msg->controlselector = le16_to_cpu((__force __le16)msg->controlselector);
 
 	/* Update the read positions, adjusting the ring */
 	saa7164_writel(bus->m_dwGetReadPos, new_grp);

peekout:
	ret = SAA_OK;
out:
	mutex_unlock(&bus->lock);
	saa7164_bus_verify(dev);
	return ret;
}

Vulnerability Type: DoS 
CWE ID: CWE-125
Summary: The saa7164_bus_get function in drivers/media/pci/saa7164/saa7164-bus.c in the Linux kernel through 4.11.5 allows local users to cause a denial of service (out-of-bounds array access) or possibly have unspecified other impact by changing a certain sequence-number value, aka a *double fetch* vulnerability.
Commit Message: [PATCH] saa7164: Bug - Double fetch PCIe access condition

Avoid a double fetch by reusing the values from the prior transfer.

Originally reported via https://bugzilla.kernel.org/show_bug.cgi?id=195559

Thanks to Pengfei Wang <wpengfeinudt@gmail.com> for reporting.

Signed-off-by: Steven Toth <stoth@kernellabs.com>