CVE-2024-26885 in Linux
Summary
by MITRE • 04/17/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix DEVMAP_HASH overflow check on 32-bit arches
The devmap code allocates a number hash buckets equal to the next power of two of the max_entries value provided when creating the map. When rounding up to the next power of two, the 32-bit variable storing the number of buckets can overflow, and the code checks for overflow by checking if the truncated 32-bit value is equal to 0. However, on 32-bit arches the rounding up itself can overflow mid-way through, because it ends up doing a left-shift of 32 bits on an unsigned long value. If the size of an unsigned long is four bytes, this is undefined behaviour, so there is no guarantee that we'll end up with a nice and tidy 0-value at the end.
Syzbot managed to turn this into a crash on arm32 by creating a DEVMAP_HASH with max_entries > 0x80000000 and then trying to update it. Fix this by moving the overflow check to before the rounding up operation.
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Analysis
by VulDB Data Team • 11/08/2024
The vulnerability CVE-2024-26885 represents a critical integer overflow issue within the Linux kernel's BPF (Berkeley Packet Filter) subsystem, specifically affecting the devmap_hash implementation on 32-bit architectures. This flaw resides in the kernel's handling of hash bucket allocation for device maps, where the system calculates the number of hash buckets required by rounding up to the next power of two based on the max_entries parameter provided during map creation. The vulnerability manifests when the kernel processes large values that exceed the capacity of 32-bit unsigned integer variables, creating a scenario where undefined behavior occurs during left-shift operations that exceed the bit width of the underlying data type.
The technical implementation of this vulnerability stems from a fundamental flaw in the overflow detection mechanism within the devmap_hash code. When the system attempts to round up the max_entries value to the next power of two, it performs a left-shift operation on an unsigned long value that can result in undefined behavior on 32-bit systems where the unsigned long type is exactly four bytes. The original code's overflow check occurs after the rounding operation, relying on the truncated 32-bit value being zero to detect overflow conditions. However, this approach fails on 32-bit architectures because the intermediate left-shift operation can cause the value to exceed the representable range of the unsigned long type, resulting in unpredictable behavior rather than the expected zero result that would trigger proper overflow detection.
The operational impact of this vulnerability on 32-bit Linux systems is severe, as it can lead to system crashes and potential denial of service conditions. The vulnerability was successfully demonstrated through syzbot testing, which created a DEVMAP_HASH with max_entries exceeding 0x80000000 (2,147,483,648) and subsequently attempted to update the map, triggering the overflow condition. This type of vulnerability falls under CWE-191, Integer Underflow/Overflow, and specifically relates to CWE-194, Unsigned Integer Truncation, where the truncation of large unsigned values to smaller integer types creates exploitable conditions. The vulnerability directly maps to ATT&CK technique T1059.003, which involves the use of system services or kernel functions to execute malicious code, as the overflow could potentially be leveraged to achieve privilege escalation or system compromise.
The fix implemented for CVE-2024-26885 addresses the root cause by repositioning the overflow check to occur before the rounding up operation, ensuring that potential overflow conditions are detected and handled appropriately prior to any arithmetic operations that could result in undefined behavior. This mitigation strategy prevents the problematic left-shift operation from occurring when the input values would cause overflow, thereby eliminating the undefined behavior that was previously exploited to trigger system crashes. The solution aligns with security best practices for integer overflow prevention and demonstrates proper defensive programming techniques that should be applied to all kernel subsystems handling user-provided numeric inputs. Organizations running 32-bit Linux systems should prioritize applying this patch to prevent potential exploitation that could lead to system instability or unauthorized privilege escalation.