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

net: dsa: sja1105: avoid out of bounds access in sja1105_init_l2_policing()

The SJA1105 family has 45 L2 policing table entries (SJA1105_MAX_L2_POLICING_COUNT) and SJA1110 has 110 (SJA1110_MAX_L2_POLICING_COUNT). Keeping the table structure but accounting for the difference in port count (5 in SJA1105 vs 10 in SJA1110) does not fully explain the difference. Rather, the SJA1110 also has L2 ingress policers for multicast traffic. If a packet is classified as multicast, it will be processed by the policer index 99 + SRCPORT.

The sja1105_init_l2_policing() function initializes all L2 policers such that they dont interfere with normal packet reception by default. To have a common code between SJA1105 and SJA1110, the index of the multicast policer for the port is calculated because its an index that is out of bounds for SJA1105 but in bounds for SJA1110, and a bounds check is performed.

The code fails to do the proper thing when determining what to do with the multicast policer of port 0 on SJA1105 (ds->num_ports = 5). The mcast index will be equal to 45, which is also equal to table->ops->max_entry_count (SJA1105_MAX_L2_POLICING_COUNT). So it passes through the check. But at the same time, SJA1105 doesnt have multicast policers. So the code programs the SHARINDX field of an out-of-bounds element in the L2 Policing table of the static config.

The comparison between index 45 and 45 entries should have determined the code to not access this policer index on SJA1105, since its memory wasnt even allocated.

With enough bad luck, the out-of-bounds write could even overwrite other valid kernel data, but in this case, the issue was detected using KASAN.

Kernel log:

sja1105 spi5.0: Probed switch chip: SJA1105Q

BUG: KASAN: slab-out-of-bounds in sja1105_setup+0x1cbc/0x2340 Write of size 8 at addr ffffff880bd57708 by task kworker/u8:0/8 … Workqueue: events_unbound deferred_probe_work_func Call trace: … sja1105_setup+0x1cbc/0x2340 dsa_register_switch+0x1284/0x18d0 sja1105_probe+0x748/0x840 … Allocated by task 8: … sja1105_setup+0x1bcc/0x2340 dsa_register_switch+0x1284/0x18d0 sja1105_probe+0x748/0x840 …

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/147f3e3d84054117ae6b9bf317ec4fda9f991192
• https://git.kernel.org/stable/c/5e88c6f4aaa70c542e59e5a9d2244bcc99cd245d
• https://git.kernel.org/stable/c/f8bac7f9fdb0017b32157957ffffd490f95faa07