Commit 35aae182 authored by Nathan Lynch's avatar Nathan Lynch Committed by Michael Ellerman
Browse files

powerpc/pseries/papr-sysparm: Validate buffer object lengths 



The ability to get and set system parameters will be exposed to user
space, so let's get a little more strict about malformed
papr_sysparm_buf objects.

* Create accessors for the length field of struct papr_sysparm_buf.
  The length is always stored in MSB order and this is better than
  spreading the necessary conversions all over.

* Reject attempts to submit invalid buffers to RTAS.

* Warn if RTAS returns a buffer with an invalid length, clamping the
  returned length to a safe value that won't overrun the buffer.

These are meant as precautionary measures to mitigate both firmware
and kernel bugs in this area, should they arise, but I am not aware of
any.

Signed-off-by: default avatarNathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-10-e9eafd0c8c6c@linux.ibm.com
parent 514f6ff4

arch/powerpc/platforms/pseries/papr-sysparm.c

+47 −0
Original line number Diff line number Diff line
@@ -23,6 +23,46 @@ void papr_sysparm_buf_free(struct papr_sysparm_buf *buf) 
	kfree(buf);

}



static size_t papr_sysparm_buf_get_length(const struct papr_sysparm_buf *buf)

{

	return be16_to_cpu(buf->len);

}



static void papr_sysparm_buf_set_length(struct papr_sysparm_buf *buf, size_t length)

{

	WARN_ONCE(length > sizeof(buf->val),

		  "bogus length %zu, clamping to safe value", length);

	length = min(sizeof(buf->val), length);

	buf->len = cpu_to_be16(length);

}



/*

 * For use on buffers returned from ibm,get-system-parameter before

 * returning them to callers. Ensures the encoded length of valid data

 * cannot overrun buf->val[].

 */

static void papr_sysparm_buf_clamp_length(struct papr_sysparm_buf *buf)

{

	papr_sysparm_buf_set_length(buf, papr_sysparm_buf_get_length(buf));

}



/*

 * Perform some basic diligence on the system parameter buffer before

 * submitting it to RTAS.

 */

static bool papr_sysparm_buf_can_submit(const struct papr_sysparm_buf *buf)

{

	/*

	 * Firmware ought to reject buffer lengths that exceed the

	 * maximum specified in PAPR, but there's no reason for the

	 * kernel to allow them either.

	 */

	if (papr_sysparm_buf_get_length(buf) > sizeof(buf->val))

		return false;



	return true;

}



/**

 * papr_sysparm_get() - Retrieve the value of a PAPR system parameter.

 * @param: PAPR system parameter token as described in
@@ -63,6 +103,9 @@ int papr_sysparm_get(papr_sysparm_t param, struct papr_sysparm_buf *buf) 
	if (token == RTAS_UNKNOWN_SERVICE)

		return -ENOENT;



	if (!papr_sysparm_buf_can_submit(buf))

		return -EINVAL;



	work_area = rtas_work_area_alloc(sizeof(*buf));



	memcpy(rtas_work_area_raw_buf(work_area), buf, sizeof(*buf));
@@ -77,6 +120,7 @@ int papr_sysparm_get(papr_sysparm_t param, struct papr_sysparm_buf *buf) 
	case 0:

		ret = 0;

		memcpy(buf, rtas_work_area_raw_buf(work_area), sizeof(*buf));

		papr_sysparm_buf_clamp_length(buf);

		break;

	case -3: /* parameter not implemented */

		ret = -EOPNOTSUPP;
@@ -115,6 +159,9 @@ int papr_sysparm_set(papr_sysparm_t param, const struct papr_sysparm_buf *buf) 
	if (token == RTAS_UNKNOWN_SERVICE)

		return -ENOENT;



	if (!papr_sysparm_buf_can_submit(buf))

		return -EINVAL;



	work_area = rtas_work_area_alloc(sizeof(*buf));



	memcpy(rtas_work_area_raw_buf(work_area), buf, sizeof(*buf));