[PATCH 3.17 038/141] hwrng: pseries - port to new read API and fix stack corruption

Greg Kroah-Hartman
2014-11-19T22:17:06Z

From: Greg Kurz <gkurz@linux.vnet.ibm.com>

commit 24c65bc7037e7d0f362c0df70d17dd72ee64b8b9 upstream.

The add_early_randomness() function in drivers/char/hw_random/core.c passes
a 16-byte buffer to pseries_rng_data_read(). Unfortunately, plpar_hcall()
returns four 64-bit values and trashes 16 bytes on the stack.

This bug has been lying around for a long time. It got unveiled by:

commit d3cc7996473a7bdd33256029988ea690754e4e2a
Author: Amit Shah <amit.shah@redhat.com>
Date: Thu Jul 10 15:42:34 2014 +0530

hwrng: fetch randomness only after device init

It may trig a oops while loading or unloading the pseries-rng module for both
PowerVM and PowerKVM guests.

This patch does two things:
- pass an intermediate well sized buffer to plpar_hcall(). This is acceptalbe
since we're not on a hot path.
- move to the new read API so that we know the return buffer size for sure.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/char/hw_random/pseries-rng.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

--- a/drivers/char/hw_random/pseries-rng.c
+++ b/drivers/char/hw_random/pseries-rng.c
@@ -25,18 +25,21 @@
#include <asm/vio.h>

-static int pseries_rng_data_read(struct hwrng *rng, u32 *data)
+static int pseries_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
{
+ u64 buffer[PLPAR_HCALL_BUFSIZE];
+ size_t size = max < 8 ? max : 8;
int rc;

- rc = plpar_hcall(H_RANDOM, (unsigned long *)data);
+ rc = plpar_hcall(H_RANDOM, (unsigned long *)buffer);
if (rc != H_SUCCESS) {
pr_err_ratelimited("H_RANDOM call failed %d\n", rc);
return -EIO;
}
+ memcpy(data, buffer, size);

/* The hypervisor interface returns 64 bits */
- return 8;
+ return size;
}

/**
@@ -55,7 +58,7 @@ static unsigned long pseries_rng_get_des

static struct hwrng pseries_rng = {
.name = KBUILD_MODNAME,
- .data_read = pseries_rng_data_read,
+ .read = pseries_rng_read,
};

static int __init pseries_rng_probe(struct vio_dev *dev,