CVE-2024-41009 in Linux
Summary
by MITRE • 07/17/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix overrunning reservations in ringbuf
The BPF ring buffer internally is implemented as a power-of-2 sized circular buffer, with two logical and ever-increasing counters: consumer_pos is the consumer counter to show which logical position the consumer consumed the data, and producer_pos which is the producer counter denoting the amount of data reserved by all producers.
Each time a record is reserved, the producer that "owns" the record will successfully advance producer counter. In user space each time a record is read, the consumer of the data advanced the consumer counter once it finished processing. Both counters are stored in separate pages so that from user space, the producer counter is read-only and the consumer counter is read-write.
One aspect that simplifies and thus speeds up the implementation of both producers and consumers is how the data area is mapped twice contiguously back-to-back in the virtual memory, allowing to not take any special measures for samples that have to wrap around at the end of the circular buffer data area, because the next page after the last data page would be first data page again, and thus the sample will still appear completely contiguous in virtual memory.
Each record has a struct bpf_ringbuf_hdr { u32 len; u32 pg_off; } header for
book-keeping the length and offset, and is inaccessible to the BPF program. Helpers like bpf_ringbuf_reserve() return `(void *)hdr + BPF_RINGBUF_HDR_SZ` for the BPF program to use. Bing-Jhong and Muhammad reported that it is however possible to make a second allocated memory chunk overlapping with the first chunk and as a result, the BPF program is now able to edit first chunk's header.
For example, consider the creation of a BPF_MAP_TYPE_RINGBUF map with size of 0x4000. Next, the consumer_pos is modified to 0x3000 /before/ a call to bpf_ringbuf_reserve() is made. This will allocate a chunk A, which is in [0x0,0x3008], and the BPF program is able to edit [0x8,0x3008]. Now, lets
allocate a chunk B with size 0x3000. This will succeed because consumer_pos was edited ahead of time to pass the `new_prod_pos - cons_pos > rb->mask` check. Chunk B will be in range [0x3008,0x6010], and the BPF program is able
to edit [0x3010,0x6010]. Due to the ring buffer memory layout mentioned
earlier, the ranges [0x0,0x4000] and [0x4000,0x8000] point to the same data
pages. This means that chunk B at [0x4000,0x4008] is chunk A's header.
bpf_ringbuf_submit() / bpf_ringbuf_discard() use the header's pg_off to then locate the bpf_ringbuf itself via bpf_ringbuf_restore_from_rec(). Once chunk B modified chunk A's header, then bpf_ringbuf_commit() refers to the wrong page and could cause a crash.
Fix it by calculating the oldest pending_pos and check whether the range from the oldest outstanding record to the newest would span beyond the ring buffer size. If that is the case, then reject the request. We've tested with the ring buffer benchmark in BPF selftests (./benchs/run_bench_ringbufs.sh) before/after the fix and while it seems a bit slower on some benchmarks, it is still not significantly enough to matter.
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Analysis
by VulDB Data Team • 09/26/2024
The vulnerability identified as CVE-2024-41009 resides within the Linux kernel's BPF (Berkeley Packet Filter) ring buffer implementation, specifically in how the system manages memory reservations and counter tracking for circular buffer operations. This flaw represents a critical issue that could enable privilege escalation or system instability through improper memory management. The BPF ring buffer serves as a high-performance communication mechanism between kernel space and user space, facilitating efficient data transfer for various networking and tracing operations. The vulnerability stems from insufficient validation of memory reservation boundaries, particularly when dealing with overlapping memory chunks that can corrupt internal header structures used for tracking record positions.
The technical implementation of the ring buffer relies on a power-of-2 sized circular buffer with two logically distinct counters: consumer_pos and producer_pos, which track consumption and production positions respectively. These counters are stored on separate memory pages to maintain atomicity and prevent race conditions during concurrent access. Each record in the buffer contains a header structure with length and offset information that is managed internally and inaccessible to BPF programs directly. The optimization of mapping the data area twice contiguously in virtual memory allows for seamless handling of samples that wrap around the buffer boundary, eliminating the need for complex memory management operations. However, this optimization creates a potential attack surface when BPF programs can manipulate memory reservation logic through carefully crafted counter modifications.
The vulnerability manifests when an attacker can manipulate the consumer_pos counter to an arbitrary value before invoking bpf_ringbuf_reserve(), thereby bypassing normal boundary checks that should prevent overlapping memory reservations. This manipulation allows for the allocation of memory chunks that overlap with previously allocated chunks, specifically enabling modification of the first chunk's header data. In the described scenario with a 0x4000 sized ring buffer, when consumer_pos is set to 0x3000 before reservation, the system allocates chunk A covering [0x0,0x3008] while the BPF program can modify [0x8,0x3008]. Subsequently, a second chunk B with size 0x3000 gets allocated at [0x3008,0x6010], but due to the ring buffer's memory mapping, the portion [0x4000,0x4008] of chunk B overlaps with chunk A's header at [0x0,0x8]. When bpf_ringbuf_submit() or bpf_ringbuf_discard() attempt to process these records, they reference corrupted header information through bpf_ringbuf_restore_from_rec(), potentially causing memory corruption and system crashes.
The fix implemented addresses this vulnerability by introducing a more rigorous validation mechanism that calculates the oldest pending position and checks whether the range from the oldest outstanding record to the newest would exceed the ring buffer boundaries. This approach prevents overlapping memory reservations that could lead to header corruption by rejecting allocation requests that would create such overlaps. The solution aligns with common security principles for preventing buffer overflows and memory corruption vulnerabilities, specifically addressing the CWE-129 weakness category related to inadequate bounds checking. From an operational security perspective, this vulnerability could be exploited by malicious actors to gain elevated privileges or cause denial-of-service conditions, making it particularly concerning in environments where BPF programs execute with elevated privileges or where the system's stability is critical.
The mitigation strategy implemented in the kernel patch specifically targets the root cause by enforcing stricter boundary checking during memory reservation operations. This approach prevents the scenario where a second memory chunk can overlap with a previously allocated chunk's header, thereby eliminating the potential for header corruption. The fix maintains the performance characteristics of the ring buffer while adding necessary validation to prevent the memory corruption exploit. Testing conducted with BPF selftests shows that while there may be minor performance impacts in some benchmarks, the overall system stability and security are significantly improved. This vulnerability demonstrates the importance of careful memory management in kernel space, particularly in high-performance systems where optimizations can inadvertently create security weaknesses. The implementation follows established security patterns for preventing memory corruption issues, ensuring that the ring buffer's atomic operations remain reliable even under adversarial conditions.