Re: [PATCH v2] virtio-gpu: use g_try_malloc to avoid guest-triggered abort

[Akihiko Odaki]

On 2026/07/28 23:33, Marc-André Lureau wrote:
Hi
On Tue, Jul 28, 2026 at 6:28 PM Akihiko Odaki
<od...@rsg.ci.i.u-tokyo.ac.jp> wrote:

On 2026/07/28 19:15, marcandre.lur...@redhat.com wrote:
From: Marc-André Lureau <marcandre.lur...@redhat.com>

Use g_try_malloc/g_try_new0 for guest-controlled allocation, so failure
returns an error to the guest rather than crashing the host (glib
behaviour).

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3898

Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com>
---
Based-on: <20260710134720.2317856-1-marcandre.lur...@redhat.com> ("[PATCH] 
vhost-user-gpu: fix integer overflow in buffer allocation")
v2:
   - accept NULL 0-size allocations
---
   contrib/vhost-user-gpu/vhost-user-gpu.c | 25 +++++++++++++++++--------
   contrib/vhost-user-gpu/virgl.c          |  6 +++++-
   contrib/vhost-user-gpu/vugbm.c          |  5 ++++-
   hw/display/virtio-gpu-rutabaga.c        | 14 ++++++++++++--
   hw/display/virtio-gpu-virgl.c           |  6 +++++-
   hw/display/virtio-gpu.c                 |  5 ++++-
   6 files changed, 47 insertions(+), 14 deletions(-)

diff --git a/contrib/vhost-user-gpu/vhost-user-gpu.c 
b/contrib/vhost-user-gpu/vhost-user-gpu.c
index ee9858c397ce..0fd115300425 100644
--- a/contrib/vhost-user-gpu/vhost-user-gpu.c
+++ b/contrib/vhost-user-gpu/vhost-user-gpu.c
@@ -487,7 +487,7 @@ vg_create_mapping_iov(VuGpu *g,
                         struct virtio_gpu_ctrl_command *cmd,
                         struct iovec **iov)
   {
-    struct virtio_gpu_mem_entry *ents;
+    g_autofree struct virtio_gpu_mem_entry *ents = NULL;
       size_t esize, s;
       int i;

@@ -498,17 +498,22 @@ vg_create_mapping_iov(VuGpu *g,
       }

       esize = sizeof(*ents) * ab->nr_entries;
-    ents = g_malloc(esize);
+    ents = g_try_malloc(esize);
+    if (!ents && esize) {
+        return -1;
+    }
       s = iov_to_buf(cmd->elem.out_sg, cmd->elem.out_num,
                      sizeof(*ab), ents, esize);
       if (s != esize) {
           g_critical("%s: command data size incorrect %zu vs %zu",
                      __func__, s, esize);
-        g_free(ents);
           return -1;
       }

-    *iov = g_new0(struct iovec, ab->nr_entries);
+    *iov = g_try_new0(struct iovec, ab->nr_entries);
+    if (!*iov && ab->nr_entries) {
+        return -1;
+    }
       for (i = 0; i < ab->nr_entries; i++) {
           uint64_t len = ents[i].length;
           (*iov)[i].iov_len = ents[i].length;
@@ -517,12 +522,10 @@ vg_create_mapping_iov(VuGpu *g,
               g_critical("%s: resource %d element %d",
                          __func__, ab->resource_id, i);
               g_free(*iov);
-            g_free(ents);
               *iov = NULL;
               return -1;
           }
       }
-    g_free(ents);
       return 0;
   }

@@ -828,8 +831,14 @@ vg_resource_flush(VuGpu *g,
                   PIXMAN_FORMAT_BPP(pixman_image_get_format(res->image)) / 8;
               size_t size = width * height * bpp;

-            void *p = g_malloc(VHOST_USER_GPU_HDR_SIZE +
-                               sizeof(VhostUserGpuUpdate) + size);
+            void *p = g_try_malloc(VHOST_USER_GPU_HDR_SIZE +
+                                   sizeof(VhostUserGpuUpdate) + size);
+            if (!p) {
+                pixman_region_fini(&region);
+                pixman_region_fini(&finalregion);
+                cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
+                break;
+            }
               VhostUserGpuMsg *msg = p;
               msg->request = VHOST_USER_GPU_UPDATE;
               msg->size = sizeof(VhostUserGpuUpdate) + size;
diff --git a/contrib/vhost-user-gpu/virgl.c b/contrib/vhost-user-gpu/virgl.c
index 550fd03bf5c4..20bae57d0fe4 100644
--- a/contrib/vhost-user-gpu/virgl.c
+++ b/contrib/vhost-user-gpu/virgl.c
@@ -209,7 +209,11 @@ virgl_cmd_submit_3d(VuGpu *g,
           return;
       }

-    buf = g_malloc(cs.size);
+    buf = g_try_malloc(cs.size);
+    if (!buf && cs.size) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
+        return;
+    }
       s = iov_to_buf(cmd->elem.out_sg, cmd->elem.out_num,
                      sizeof(cs), buf, cs.size);
       if (s != cs.size) {
diff --git a/contrib/vhost-user-gpu/vugbm.c b/contrib/vhost-user-gpu/vugbm.c
index 710d54529779..e2d8385fd857 100644
--- a/contrib/vhost-user-gpu/vugbm.c
+++ b/contrib/vhost-user-gpu/vugbm.c
@@ -13,7 +13,10 @@
   static bool
   mem_alloc_bo(struct vugbm_buffer *buf)
   {
-    buf->mmap = g_malloc((uint64_t)buf->width * buf->height * 4);
+    buf->mmap = g_try_malloc((uint64_t)buf->width * buf->height * 4);
+    if (!buf->mmap && buf->width && buf->height) {
+        return false;
+    }
       buf->stride = buf->width * 4;
       return true;
   }
diff --git a/hw/display/virtio-gpu-rutabaga.c b/hw/display/virtio-gpu-rutabaga.c
index e28aad94eead..60f64d9a8fa7 100644
--- a/hw/display/virtio-gpu-rutabaga.c
+++ b/hw/display/virtio-gpu-rutabaga.c
@@ -360,10 +360,20 @@ rutabaga_cmd_submit_3d(VirtIOGPU *g,
           return;
       }

-    buf = g_new0(uint8_t, cs.size);
+    buf = g_try_new0(uint8_t, cs.size);
+    if (!buf && cs.size) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
+        return;
+    }
       s = iov_to_buf(cmd->elem.out_sg, cmd->elem.out_num,
                      sizeof(cs), buf, cs.size);
-    CHECK(s == cs.size, cmd);
+    if (s != cs.size) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: size mismatch (%zu/%u)\n",
+                      __func__, s, cs.size);
+        cmd->error = VIRTIO_GPU_RESP_ERR_INVALID_PARAMETER;
+        return;
+    }

       rutabaga_cmd.ctx_id = cs.hdr.ctx_id;
       rutabaga_cmd.cmd = buf;
diff --git a/hw/display/virtio-gpu-virgl.c b/hw/display/virtio-gpu-virgl.c
index d9e5b0110497..028ab776df6a 100644
--- a/hw/display/virtio-gpu-virgl.c
+++ b/hw/display/virtio-gpu-virgl.c
@@ -615,7 +615,11 @@ static void virgl_cmd_submit_3d(VirtIOGPU *g,
           return;
       }

-    buf = g_malloc(cs.size);
+    buf = g_try_malloc(cs.size);
+    if (!buf && cs.size) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
+        return;
+    }
       s = iov_to_buf(cmd->elem.out_sg, cmd->elem.out_num,
                      sizeof(cs), buf, cs.size);
       if (s != cs.size) {
diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index 8607d0c0b581..9bacd38b4521 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -892,7 +892,10 @@ int virtio_gpu_create_mapping_iov(VirtIOGPU *g,
       }

       esize = sizeof(*ents) * nr_entries;
-    ents = g_malloc(esize);
+    ents = g_try_malloc(esize);
+    if (!ents && esize) {
+        return -1;
+    }

This converts the entry allocation to g_try_malloc(), but the resulting
mapping arrays still grow with aborting g_renew() calls.
dma_memory_map() may return only part of an entry; with fragmented IOMMU
mappings, guest-controlled entries can expand into more IOVs than
nr_entries.


The goal is not to make QEMU OOM-safe. Even when malloc returns
non-zero we may reach that and get killed. This patch is more a safety
guard for easy guest triggered allocations that can crash QEMU.
I agree we are not going to make QEMU OOM-safe, but I'm not sure where 
we draw the line between "easy" and "hard" guest-triggered allocations. 
Even if the IOMMU mapping isn't fragmented, the data from ents is 
essentially duplicated into iov right after, doubling the allocation 
size anyway. I feel it's easier to avoid reasoning by just consistently 
using g_try_* in this function.

Regards,
Akihiko Odaki