Before going on content...

I originally mentioned that this bug will be disclosed 3 months after the patch. However, I decided to release it today since 3 months seems to be too long... As long as there is an another researcher who disclosed his bugs at the patch date, it would be fine releasing this bug today.


High-level overview of the vulnerability and the possible effect of using it

A divide by zero vulnerability exists in vmsvga3dSurfaceDMA, in SVGA3D of Oracle VirtualBox 6.1.16 Linux distributions. To use this vulnerability, 3D acceleration option should be turned for a guest OS, and an attacker must get a root privilege in the guest OS.

This vulnerability allows local or remote attackers to crash VMs with 3D acceleration, leading to a denial-of-service condition. Flaws exist in three points: (1) Invalid surfaces can be defined in vmsvga3dSurfaceDefine, (2) Textures can be bound to invalid surfaces, (3) No check for divisor in vmsvga3dSurfaceDMA.

Exact product that was found to be vulnerable including complete version information

Version Information

Oracle VirtualBox 6.1.16 releases (which is the latest release in 2020/11/17) except for Windows are affected by this vulnerability. PoC code is tested with the release and Ubuntu 20.04.1 desktop for host and guest OSes, but it is also expected to work with other operating systems, except with Windows.

Precondition

An attacker must get a root privilege in a guest OS in order to use this vulnerability. Also, 3D acceleration option must be turned on for the guest OS to use this vulnerability. Finally, graphics controller option must be set to VMSVGA, which is the default option for Linux guest OSes.

Root Cause Analysis

Overview

A divide by zero vulnerability exists in vmsvga3dSurfaceDMA, which handles the SVGA_3D_CMD_SURFACE_DMA command in the SVGA FIFO loop. A VM crashes when the vulnerability is triggered, leading to the denial-of-service condition on the software. The vulnerability lies in the simple overflow check of the guest offset, before the offset gets verified by vmsvgaR3GmrTransfer. (See Code 1)

// Code 1

// Devices/Graphics/DevVGA-SVGA3d.cpp:610
/* srcx, srcy and srcz values are used to calculate the guest offset.
 * The offset will be verified by vmsvgaR3GmrTransfer, so just check for overflows here.
 */
AssertReturn(srcz < UINT32_MAX / pMipLevel->mipmapSize.height / cbGuestPitch, VERR_INVALID_PARAMETER);

cbGuestPitch can be zero with forged surfaces, leading to a divide by zero vulnerability. No check exists for pMipLevel->cbSurfacePitch, so cbGuestPitch can be zero. AssertReturn function remains as an if-else statement in a release version, so the vulnerability exists in a release version.

Three steps are needed to reach the vulnerable condition. These steps each uses a bug contained in its process.

  1. Two invalid surfaces, which have invalid mipmap sizes, are defined using a SVGA_3D_CMD_SURFACE_DEFINE command. One of two surfaces may be a valid surface. It doesn't affect the next step.
  2. Textures are bound to two surfaces using SVGA_3D_CMD_SURFACE_STRETCHBLT command. Two surfaces are each sent as source and destination surfaces.
  3. Divide by zero is triggered by using SVGA_3D_CMD_SURFACE_DMA command with guest.pitch = 0

Code flow from input to vmsvga* functions

Commands are asynchronously handled with the FIFO loop. (vmsvgaR3FifoLoop in Devices/Graphics/DevVGA-SVGA.cpp) Commands sent via VMware SVGA II PCI device are first enqueued to a SVGA command FIFO buffer, and they are dequeued and handled asynchronously in FIFO order by the loop.

Parsing and executing commands is done in vmsvgaR3FifoLoop function, which contains a simple switch-case statement to handle commands. With a given command id, the loop selects the method of parsing and handling the command. The loop eventually calls vmsvga~~ function to handle a command.

Step 1. Definition of invalid surfaces

Definition of an invalid surface is needed for to reach the vulnerable condition. Our goal is to make a surface that has the following conditions:

  1. Mipmap size is not zero so the clip box is not considered as empty in DMA command.
  2. cbSurfacePitch (derived from cBlocksX, cBlocksY, ...) is zero to trigger divide by zero.
  3. Texture is bound to the surface so that oglId is not zero.

In this step, a surface is defined with a non-zero mipmap size and a zero cbSurfacePitch value. A surface should not be defined in this condition, so this can be considered as a bug.

Making an invalid surface is simple: a surface can be defined with a negative size. (i.e. (-1, -1, -1)) Then, the definition fails in the condition described below. (See Code 2)

// Code 2

// Devices/Graphics/DevVGA-SVGA3d.cpp:278
if (RT_LIKELY(pSurface->cxBlock == 1 && pSurface->cyBlock == 1))
{
    cBlocksX = pMipmapLevel->mipmapSize.width;
    cBlocksY = pMipmapLevel->mipmapSize.height;
}
else
{
    (...)
}

(...)

const uint32_t cMaxBlocksX = cbMemRemaining / pSurface->cbBlock;
if (cBlocksX > cMaxBlocksX)
    return VERR_INVALID_PARAMETER;

cBlocksX becomes a negative value (i.e. -1), which is given as the argument of the command. cMaxBlocksX is 0x80000000 / n since cbMemRemaining remains 0x80000000 and pSurface->cbBlock becomes a small integer which is calculated from a given surface format. (i.e. 1, 2, 4, ..., see DevVGA-SVGA3d-shard.cpp:263 for details) Since the check is done with an unsigned comparison, cBlocksX is always bigger than cMaxBlocksX, so the check fails and the function returns.

A mipmap is already allocated before checking the validity of cBlocks, and the default value of a mipmap is zero for all other remaining values that are not initialized. Therefore, *cbSurfacePitch also becomes zero since it is not initialized. Mipmap size is already assigned before validation, so it is not zero. (See Code 3)

// Code 3

// Devices/Graphics/DevVGA-SVGA3d.cpp:223
for (uint32_t i=0; i < cMipLevels; i++)
    pSurface->paMipmapLevels[i].mipmapSize = paMipLevelSizes[i];

The outcome of step 1 is an object that looks like below. (Size is given as (-1, -1, -1)) Note that mipmapSize is not zero, and cbSurfacePitch is zero.

[*] surface
$2441 = 0x7fff6c7c49a0
$2442 = {
  id = 0x0,
  idWeakContextAssociation = 0xffffffff,
  surfaceFlags = 0x2,
  format = SVGA3D_BUFFER,
  internalFormatGL = 0xffffffff,
  formatGL = 0xffffffff,
  typeGL = 0xffffffff,
  enmOGLResType = VMSVGA3D_OGLRESTYPE_NONE,
  oglId = {
    texture = 0x0,
    buffer = 0x0,
    renderbuffer = 0x0
  },
  targetGL = 0x0,
  bindingGL = 0x0,
  fEmulated = 0x0,
  idEmulated = 0x0,
  faces = {{
      numMipLevels = 0x1
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }},
  cFaces = 0x1,
  cMipmapLevels = 0x1,
  paMipmapLevels = 0x7fff6fca48a0,
  multiSampleCount = 0x0,
  autogenFilter = SVGA3D_TEX_FILTER_NONE,
  cbBlock = 0x1,
  cxBlock = 0x1,
  cyBlock = 0x1,
  cbBlockGL = 0x1,
  fDirty = 0x0
}
[*] paMipmapLevel[0]
$2443 = {
  mipmapSize = {
    width = 0xffffffff,
    height = 0xffffffff,
    depth = 0xffffffff
  },
  cBlocksX = 0x0,
  cBlocksY = 0x0,
  cBlocks = 0x0,
  cbSurfacePitch = 0x0,
  cbSurfacePlane = 0x0,
  cbSurface = 0x0,
  pSurfaceData = 0x0,
  fDirty = 0x0
}

Step 2. Texture binding

Texture binding to the surface made in step 1 is needed to reach the vulnerable condition. In step 2, SVGA_3D_CMD_SURFACE_STRETCHBLT command is used to bind a texture. Draw primitive functions may be used to bind textures, but they are more complicated than vmsvga3dSurfaceStretchBlt function, so the function is used in step 2. Also, a buffer may be bound to a surface instead of binding a texture to it, but binding a texture is chosen because of the same reason.

vmsvga3dSurfaceStretchBlt needs two surfaces: one as a source surface and the other as a destination surface. One of two surfaces may be valid, but one of them must be the surface which is made in step 1 to exploit the vulnerability.

Codes described in Code 4 is used to create and bind the texture to the surfaces. Even if a surface is invalid, a texture will be created and bound to the surface. vmsvga3dBackCreateTexture function will create a texture and bind it to the surface.

// Code 4

// Devices/Graphics/DevVGA-SVGA3d.cpp:437
if (!VMSVGA3DSURFACE_HAS_HW_SURFACE(pSrcSurface))
{
    /* Unknown surface type; turn it into a texture, which can be used for other purposes too. */
    LogFunc(("unknown src sid=%u type=%d format=%d -> create texture\n", sidSrc, pSrcSurface->surfaceFlags, pSrcSurface->format));
    rc = vmsvga3dBackCreateTexture(pState, pContext, pContext->id, pSrcSurface);
    AssertRCReturn(rc, rc);
}

if (!VMSVGA3DSURFACE_HAS_HW_SURFACE(pDstSurface))
{
    /* Unknown surface type; turn it into a texture, which can be used for other purposes too. */
    LogFunc(("unknown dest sid=%u type=%d format=%d -> create texture\n", sidDst, pDstSurface->surfaceFlags, pDstSurface->format));
    rc = vmsvga3dBackCreateTexture(pState, pContext, pContext->id, pDstSurface);
    AssertRCReturn(rc, rc);
}

A texture should not be bound if a surface is invalid. Therefore, this can be also considered as a bug.

The outcome of step 2 is an object that looks like below. Note that oglId is not zero anymore.

[*] pSurface
$2453 = {
  id = 0x1,
  idWeakContextAssociation = 0xffffffff,
  surfaceFlags = 0x22,
  format = SVGA3D_BUFFER,
  internalFormatGL = 0xffffffff,
  formatGL = 0xffffffff,
  typeGL = 0xffffffff,
  enmOGLResType = VMSVGA3D_OGLRESTYPE_TEXTURE,
  oglId = {
    texture = 0x38,
    buffer = 0x38,
    renderbuffer = 0x38
  },
  targetGL = 0x806f,
  bindingGL = 0x806a,
  fEmulated = 0x0,
  idEmulated = 0x0,
  faces = {{
      numMipLevels = 0x1
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }, {
      numMipLevels = 0x0
    }},
  cFaces = 0x1,
  cMipmapLevels = 0x1,
  paMipmapLevels = 0x7fff6c7c17a0,
  multiSampleCount = 0x0,
  autogenFilter = SVGA3D_TEX_FILTER_NONE,
  cbBlock = 0x1,
  cxBlock = 0x1,
  cyBlock = 0x1,
  cbBlockGL = 0x1,
  fDirty = 0x0
}
[*] pMipLevel
$2454 = {
  mipmapSize = {
    width = 0xffffffff,
    height = 0xffffffff,
    depth = 0xffffffff
  },
  cBlocksX = 0x0,
  cBlocksY = 0x0,
  cBlocks = 0x0,
  cbSurfacePitch = 0x0,
  cbSurfacePlane = 0x0,
  cbSurface = 0x0,
  pSurfaceData = 0x0,
  fDirty = 0x0
}

Step 3. Triggering the bug

Divide by zero bug in vmsvga3dSurfaceDMA can now be triggered with the surface made in step 1 and step 2. The problematic code is described in Code 5.

// Code 5

// Devices/Graphics/DevVGA-SVGA3d.cpp:610
/* srcx, srcy and srcz values are used to calculate the guest offset.
 * The offset will be verified by vmsvgaR3GmrTransfer, so just check for overflows here.
 */
AssertReturn(srcz < UINT32_MAX / pMipLevel->mipmapSize.height / cbGuestPitch, VERR_INVALID_PARAMETER);

Here, divide by zero occurs when cbGuestPitch is zero. cbGuestPitch becomes zero if we give guest.pitch = 0. (guest.pitch is an argument of a DMA command given by the user) AssertReturn function remains as an if-else statement in a release version, so the vulnerability exists in a release version. See Code 6.

// Code 6

// Devices/Graphics/DevVGA-SVGA3d.cpp:597
uint32_t cbGuestPitch = guest.pitch;
if (cbGuestPitch == 0)
{
    /* Host must "assume image is tightly packed". Our surfaces are. */
    cbGuestPitch = pMipLevel->cbSurfacePitch;
}
else { (...) }

There are two checks that need to be bypassed to reach the vulnerable condition. One (Code 7) is bypassed by defining an invalid surface in step 1, and the other (Code 8) is bypassed by binding a texture to the invalid surface defined in step 2. The check in Code 7 requires a non-zero mipmap size, and the other check in Code 8 requires a non-zero oglId.

// Code 7

// Devices/Graphics/DevVGA-SVGA3d.cpp:536
/* The copybox's "dest" is coords in the host surface. Verify them against the surface's mipmap size. */
SVGA3dBox hostBox;
hostBox.x = paBoxes[i].x;
hostBox.y = paBoxes[i].y;
hostBox.z = paBoxes[i].z;
hostBox.w = paBoxes[i].w;
hostBox.h = paBoxes[i].h;
hostBox.d = paBoxes[i].d;
vmsvgaR3ClipBox(&pMipLevel->mipmapSize, &hostBox);

if (   !hostBox.w
    || !hostBox.h
    || !hostBox.d)
{
    Log(("Skip empty box\n"));
    continue;
}

// Devices/Graphics/DevVGA-SVGA.cpp:5274
/**
 * Unsigned coordinates in pBox. Clip to [0; pSize).
 *
 * @param   pSize   Source surface dimensions.
 * @param   pBox    Coordinates to be clipped.
 */
void vmsvgaR3ClipBox(const SVGA3dSize *pSize, SVGA3dBox *pBox)
{
    /* x, w */
    if (pBox->x > pSize->width)
        pBox->x = pSize->width;
    if (pBox->w > pSize->width - pBox->x)
        pBox->w = pSize->width - pBox->x;

    /* y, h */
    if (pBox->y > pSize->height)
        pBox->y = pSize->height;
    if (pBox->h > pSize->height - pBox->y)
        pBox->h = pSize->height - pBox->y;

    /* z, d */
    if (pBox->z > pSize->depth)
        pBox->z = pSize->depth;
    if (pBox->d > pSize->depth - pBox->z)
        pBox->d = pSize->depth - pBox->z;
}
// Code 8

// Devices/Graphics/DevVGA-SVGA3d.cpp:504
if (!VMSVGA3DSURFACE_HAS_HW_SURFACE(pSurface))
{
    /*
     * Not realized in host hardware/library yet, we have to work with
     * the copy of the data we've got in VMSVGA3DMIMAPLEVEL::pSurfaceData.
     */
    AssertReturn(pMipLevel->pSurfaceData, VERR_INTERNAL_ERROR);
}

// Devices/Graphics/DevVGA-SVGA3d-internal.h:683 (VMSVGA3D_DIRECT3D not defined in linux release)
/** @def VMSVGA3DSURFACE_HAS_HW_SURFACE
 * Checks whether the surface has a host hardware/library surface.
 * @returns true/false
 * @param   a_pSurface      The VMSVGA3d surface.
 */
#ifdef VMSVGA3D_DIRECT3D
# define VMSVGA3DSURFACE_HAS_HW_SURFACE(a_pSurface) ((a_pSurface)->u.pSurface != NULL)
#else
# define VMSVGA3DSURFACE_HAS_HW_SURFACE(a_pSurface) ((a_pSurface)->oglId.texture != OPENGL_INVALID_ID)
#endif

Vulnerable code can be reached by bypassing these two conditions. VM crashes with divide by zero, trying to divide with cbGuestPitch, which has zero value.

gdb-peda$ c
Continuing.

Thread 24 "VMSVGA FIFO" received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0x7f3acc41f700 (LWP 73906)]
[----------------------------------registers-----------------------------------]
RAX: 0x1 
RBX: 0x0 
RCX: 0xffffffff 
RDX: 0x0 
RSI: 0x0 
RDI: 0x0 
RBP: 0x7f3acc41ed10 --> 0x7f3acc41ee50 --> 0x7f3acc41ee90 --> 0x7f3acc41eeb0 --> 0x7f3acc41eef0 --> 0x0 
RSP: 0x7f3acc41ec10 --> 0x400000001 
RIP: 0x7f3acd87b333 (div    ebx)
R8 : 0x0 
R9 : 0x0 
R10: 0x1 
R11: 0x2a ('*')
R12: 0x0 
R13: 0x7f3a94603ed0 --> 0xffffffff00000001 
R14: 0x7f3a9775f260 --> 0xffffffffffffffff 
R15: 0x7f3a94000b80 --> 0x0
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x7f3acd87b32b:    mov    eax,ecx
   0x7f3acd87b32d:    div    DWORD PTR [r14+0x4]
   0x7f3acd87b331:    xor    edx,edx
=> 0x7f3acd87b333:    div    ebx
   0x7f3acd87b335:    cmp    eax,DWORD PTR [rbp-0xbc]
   0x7f3acd87b33b:    jbe    0x7f3acd87b568
   0x7f3acd87b341:    mov    eax,ecx
   0x7f3acd87b343:    xor    edx,edx
[------------------------------------stack-------------------------------------]
0000| 0x7f3acc41ec10 --> 0x400000001 
0008| 0x7f3acc41ec18 --> 0x7f3a94000b64 --> 0xfffffffe 
0016| 0x7f3acc41ec20 --> 0x7f3ad8134568 --> 0xffffeeee 
0024| 0x7f3acc41ec28 --> 0x22057251 
0032| 0x7f3acc41ec30 --> 0x0 
0040| 0x7f3acc41ec38 --> 0x7f3ad8134250 --> 0x5000000010 
0048| 0x7f3acc41ec40 --> 0x7f3acc6e4140 --> 0x7f3ab8000000 --> 0xff1d1d1dff1d1d1d 
0056| 0x7f3acc41ec48 --> 0x7f3acc6e4800 --> 0x8000000 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGFPE
0x00007f3acd87b333 in ?? () from /usr/lib/virtualbox/VBoxDD.so
gdb-peda$ bt
#0  0x00007f3acd87b333 in  () at /usr/lib/virtualbox/VBoxDD.so
#1  0x00007f3acd876bcb in  () at /usr/lib/virtualbox/VBoxDD.so
#2  0x00007f3b10724c8a in  () at /usr/lib/virtualbox/components/VBoxVMM.so
#3  0x00007f3b21f879e8 in  () at /usr/lib/virtualbox/VBoxRT.so
#4  0x00007f3b2204b452 in  () at /usr/lib/virtualbox/VBoxRT.so
#5  0x00007f3b2235f609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#6  0x00007f3b22280293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
gdb-peda$ vmmap
(...)
0x00007f3acd7c6000 0x00007f3acd7e8000 r--p    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acd7e8000 0x00007f3acd94d000 r-xp    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acd94d000 0x00007f3acdb7f000 r--p    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acdb7f000 0x00007f3acdb80000 ---p    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acdb80000 0x00007f3acdb93000 r--p    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acdb93000 0x00007f3acdb9b000 rw-p    /usr/lib/virtualbox/VBoxDD.so
0x00007f3acdb9b000 0x00007f3ace201000 rw-p    mapped
(...)

Suggested Fixes

Three main causes exist in this vulnerability.

  1. An invalid surface, which has a non-zero mipmap size and zero cBlocks* and cbSurfacePitch, can be defined.
  2. A texture can be bound to an invalid surface.
  3. No check for the divisor, which is cbGuestPitch, in DMA command.

Fixing this vulnerability depends on the design choice that developers make. There are two options to fix this vulnerability.

  1. Deallocate the surface when defining a surface fails.
  2. Validate mipmaps of surfaces whether they have proper fields whenever surfaces are used in commands. For this vulnerability, there are problems in vmsvga3dSurfaceStretchBlt, vmsvga3dBackCreateTexture, and vmsvga3dSurfaceDMA. However, other commands may also contain problems.

Proof-of-Concept

Environment

Proof-of-Concept code is tested on VirtualBox 6.1.16 version (recent version in 2020-11-20), Linux distribution. Version for host and guest OSes are Ubuntu 20.04.1 desktop, but other OSes except Windows are supposed to work as well. 3D acceleration under display option should be turned on, and graphics controller option should remain VMSVGA, which is the default setting for display in Linux distribution.

Running PoC code

https://github.com/candymate/pwn/tree/master/CVE/CVE-2021-2127

PoC code should be run inside a guest. Thus, download the attached file inside a guest and extract the file.

libpciaccess-dev should be installed first to run the attached code. Run sudo apt install libpciaccess-dev first before running the code.

After installing the library, run make to compile the code. Next, run the compiled code with root privilege. (Do sudo ./poc)

Additional Information

Most of the codes are based on one of released PoC codes, which are in https://github.com/renorobert/virtualbox-vmsvga-bugs.

Software Download Link

VirtualBox release can be downloaded in https://www.virtualbox.org/wiki/Downloads. Linux distribution needs to be downloaded for this vulnerability. (PoC code is tested on Ubuntu 20.04.1 desktop for guest and host OSes)

Ubuntu 20.04.1 desktop image can be downloaded in https://releases.ubuntu.com/20.04/. This may be optional.


Patch

Patched at 6.1.18. Allocation (or Definition) of invalid surfaces is now prohibited. (See VBox/Devices/Graphics/DevVGA-SVGA3d.cpp)

 

  1. candymate 2021.02.18 20:19 신고

    Added PoC Code : https://github.com/candymate/pwn/tree/master/CVE/CVE-2021-2127

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