CVE-2022-48744

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: Avoid field-overflowing memcpy()

In preparation for FORTIFY_SOURCE performing compile-time and run-time field bounds checking for memcpy(), memmove(), and memset(), avoid intentionally writing across neighboring fields.

Use flexible arrays instead of zero-element arrays (which look like they are always overflowing) and split the cross-field memcpy() into two halves that can be appropriately bounds-checked by the compiler.

We were doing:

#define ETH_HLEN  14
#define VLAN_HLEN  4
...
#define MLX5E_XDP_MIN_INLINE (ETH_HLEN + VLAN_HLEN)
...
    struct mlx5e_tx_wqe      *wqe  = mlx5_wq_cyc_get_wqe(wq, pi);
...
    struct mlx5_wqe_eth_seg  *eseg = &wqe->eth;
    struct mlx5_wqe_data_seg *dseg = wqe->data;
...
memcpy(eseg->inline_hdr.start, xdptxd->data, MLX5E_XDP_MIN_INLINE);

target is wqe->eth.inline_hdr.start (which the compiler sees as being 2 bytes in size), but copying 18, intending to write across start (really vlan_tci, 2 bytes). The remaining 16 bytes get written into wqe->data[0], covering byte_count (4 bytes), lkey (4 bytes), and addr (8 bytes).

struct mlx5e_tx_wqe { struct mlx5_wqe_ctrl_seg ctrl; /* 0 16 / struct mlx5_wqe_eth_seg eth; / 16 16 / struct mlx5_wqe_data_seg data[]; / 32 0 */

    /* size: 32, cachelines: 1, members: 3 */
    /* last cacheline: 32 bytes */

};

struct mlx5_wqe_eth_seg { u8 swp_outer_l4_offset; /* 0 1 / u8 swp_outer_l3_offset; / 1 1 / u8 swp_inner_l4_offset; / 2 1 / u8 swp_inner_l3_offset; / 3 1 / u8 cs_flags; / 4 1 / u8 swp_flags; / 5 1 / __be16 mss; / 6 2 / __be32 flow_table_metadata; / 8 4 / union { struct { __be16 sz; / 12 2 / u8 start[2]; / 14 2 / } inline_hdr; / 12 4 / struct { __be16 type; / 12 2 / __be16 vlan_tci; / 14 2 / } insert; / 12 4 / __be32 trailer; / 12 4 / }; / 12 4 */

    /* size: 16, cachelines: 1, members: 9 */
    /* last cacheline: 16 bytes */

};

struct mlx5_wqe_data_seg { __be32 byte_count; /* 0 4 / __be32 lkey; / 4 4 / __be64 addr; / 8 8 */

    /* size: 16, cachelines: 1, members: 3 */
    /* last cacheline: 16 bytes */

};

So, split the memcpy() so the compiler can reason about the buffer sizes.

pahole shows no size nor member offset changes to struct mlx5e_tx_wqe nor struct mlx5e_umr_wqe. objdump -d shows no meaningful object code changes (i.e. only source line number induced differences and optimizations).

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• https://git.kernel.org/stable/c/8fbdf8c8b8ab82beab882175157650452c46493e
• https://git.kernel.org/stable/c/ad5185735f7dab342fdd0dd41044da4c9ccfef67
• https://git.kernel.org/stable/c/8fbdf8c8b8ab82beab882175157650452c46493e
• https://git.kernel.org/stable/c/ad5185735f7dab342fdd0dd41044da4c9ccfef67