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

On 2026/07/28 23:33, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Jul 28, 2026 at 6:28 PM Akihiko Odaki
> <[email protected]> wrote:
>>
>> On 2026/07/28 19:15, [email protected] wrote:
>>> From: Marc-André Lureau <[email protected]>
>>>
>>> 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 <[email protected]>
>>> ---
>>> Based-on: <[email protected]> ("[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