random_recv_done() stores the device-reported used.len directly into
vi->data_avail. copy_data() then indexes vi->data[] using
vi->data_idx (advanced by previous copy_data() calls) and issues a
memcpy() without re-validating either value against the posted
buffer size sizeof(vi->data) (SMP_CACHE_BYTES bytes, typically 32
or 64).
A malicious or buggy virtio-rng backend can set used.len beyond
sizeof(vi->data), steering the memcpy() past the end of the inline
array into adjacent kmalloc-1k slab bytes. hwrng_fillfn() mixes
those bytes into the guest RNG, and guest root can also observe
them directly via /dev/hwrng.
Concrete impact is inside the guest:
- Memory-safety / hardening: any virtio-rng backend that
over-reports used.len causes the driver to read past vi->data
into unrelated slab contents. hwrng_fillfn() is a kernel thread
that runs as soon as the device is probed; no guest userspace
interaction is required to first-trigger the OOB.
- Cross-boundary leak (confidential-compute threat model): a
malicious hypervisor cooperating with a malicious or compromised
guest root userspace can use /dev/hwrng as a leak channel for
guest-kernel heap data. The host sets a large used.len, guest
root reads /dev/hwrng, and the returned bytes contain guest
kernel slab contents that were adjacent to vi->data. In
practice, confidential-compute guests (SEV-SNP, TDX) usually
disable virtio-rng entirely, so this path is narrow, but the
fix is still worth carrying because the underlying
memory-safety bug contaminates the guest RNG on any host.
KASAN confirms the OOB on a guest booted under a QEMU 9.0 whose
virtio-rng backend has been patched to report used.len = 0x10000:
BUG: KASAN: slab-out-of-bounds in virtio_read+0x394/0x5d0
Read of size 64 at addr ffff8880089c2220 by task hwrng/52
Call Trace:
__asan_memcpy
virtio_read+0x394/0x5d0
hwrng_fillfn+0xb2/0x470
kthread
Allocated by task 1:
probe_common+0xa5/0x660
virtio_dev_probe+0x549/0xbc0
The buggy address belongs to the object at ffff8880089c2000
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 0 bytes to the right of
allocated 544-byte region [ffff8880089c2000, ffff8880089c2220)
Same class of bug as commit c04db81cd028 ("net/9p: Fix buffer
overflow in USB transport layer"), which hardened
usb9pfs_rx_complete() against unchecked device-reported length in
the USB 9p transport.
With the clamp at point of use and array_index_nospec() in place,
the same harness boots cleanly: copy_data() returns zero for the
bogus report, the device-supplied bytes after data_idx are
discarded, and the driver issues a fresh request.
Changes in v2 (per Michael S. Tsirkin review):
- move the bound check from random_recv_done() into copy_data(),
so the clamp sits immediately next to the memcpy it protects
- clamp to sizeof(vi->data) rather than substituting len = 0, so a
previously-working but buggy device that occasionally over-reports
used.len does not start returning zero-length reads
- add array_index_nospec() on vi->data_idx to defeat a speculative
out-of-bounds read given the malicious-backend threat model
- expand the commit message to describe the /dev/hwrng observation
path and the hypervisor + guest-root cooperation scenario
Fixes: f7f510ec1957 ("virtio: An entropy device, as suggested by hpa.")
Cc: [email protected]
Suggested-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael Bommarito <[email protected]>
Assisted-by: Claude:claude-opus-4-7
---
drivers/char/hw_random/virtio-rng.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
index 0ce02d7e5048..5e83ffa105e4 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -7,6 +7,7 @@
#include <asm/barrier.h>
#include <linux/err.h>
#include <linux/hw_random.h>
+#include <linux/nospec.h>
#include <linux/scatterlist.h>
#include <linux/spinlock.h>
#include <linux/virtio.h>
@@ -69,8 +70,26 @@ static void request_entropy(struct virtrng_info *vi)
static unsigned int copy_data(struct virtrng_info *vi, void *buf,
unsigned int size)
{
- size = min_t(unsigned int, size, vi->data_avail);
- memcpy(buf, vi->data + vi->data_idx, size);
+ unsigned int idx, avail;
+
+ /*
+ * vi->data_avail was set from the device-reported used.len and
+ * vi->data_idx was advanced by previous copy_data() calls. A
+ * malicious or buggy virtio-rng backend can drive either past
+ * sizeof(vi->data). Clamp at point of use and harden the index
+ * with array_index_nospec() so the memcpy() below cannot be
+ * steered into adjacent slab memory, including under
+ * speculation.
+ */
+ avail = min_t(unsigned int, vi->data_avail, sizeof(vi->data));
+ if (vi->data_idx >= avail) {
+ vi->data_avail = 0;
+ request_entropy(vi);
+ return 0;
+ }
+ size = min_t(unsigned int, size, avail - vi->data_idx);
+ idx = array_index_nospec(vi->data_idx, sizeof(vi->data));
+ memcpy(buf, vi->data + idx, size);
vi->data_idx += size;
vi->data_avail -= size;
if (vi->data_avail == 0)
--
2.53.0
