medium CVSS 5.5

CVE-2021-47608·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix kernel address leakage in atomic fetch The change in commit 37086bfdc737 ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH") around check_mem_access() handling is buggy since this would allow for unprivileged users to leak kernel pointers. For example, an atomic fetch/and with -1 on a stack destination which holds a spilled pointer will migrate the spilled register type into a scalar, which can then be exported out of the program (since scalar != pointer) by dumping it into a map value. The original implementation of XADD was preventing this situation by using a double call to check_mem_access() one with BPF_READ and a subsequent one with BPF_WRITE, in both cases passing -1 as a placeholder value instead of register as per XADD semantics since it didn't contain a value fetch. The BPF_READ also included a check in check_stack_read_fixed_off() which rejects the program if the stack slot is of __is_pointer_value() if dst_regno < 0. The latter is to distinguish whether we're dealing with a regular stack spill/ fill or some arithmetical operation which is disallowed on non-scalars, see also 6e7e63cbb023 ("bpf: Forbid XADD on spilled pointers for unprivileged users") for more context on check_mem_access() and its handling of placeholder value -1. One minimally intrusive option to fix the leak is for the BPF_FETCH case to initially check the BPF_READ case via check_mem_access() with -1 as register, followed by the actual load case with non-negative load_reg to propagate stack bounds to registers.

Severity
medium
Software
Kernel
Fixed in
5.15.11
Published
2024-06-19

Affected versions

From: 5.12

Until: 5.15.11

Fixed in: 5.15.11

How to fix this CVE

Update your Linux kernel to version 5.15.11 or later to patch a memory disclosure vulnerability in BPF atomic operations. Unprivileged users could previously leak kernel pointers by performing atomic fetch operations on spilled pointers on the stack. Apply the patch immediately, especially on systems running kernels 5.12 through 5.15.10.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check your kernel version with `uname -r` and verify if it falls in the range 5.12.x to 5.15.10
  2. Step 2: Confirm BPF is enabled in your kernel with `grep CONFIG_BPF /boot/config-$(uname -r)` — if it returns 'y', the vulnerable code path is compiled
  3. Step 3: Search system logs for failed BPF program loads with `journalctl -xe | grep -i bpf` or check dmesg with `dmesg | grep -i 'bpf\|atomic'`
  4. Step 4: After patching, verify the new kernel version with `uname -r` to confirm it is 5.15.11 or later

FAQ

What is CVE-2021-47608?

CVE-2021-47608 is a kernel memory disclosure flaw in BPF atomic operations that allows unprivileged users to leak kernel pointers by performing atomic fetch operations on spilled pointers stored on the stack, converting them to scalars that can be extracted from BPF programs.

Is CVE-2021-47608 being actively exploited?

No, according to CISA's Known Exploited Vulnerabilities catalog, CVE-2021-47608 is not being actively exploited, and no public exploits are currently available.

What versions of Kernel are affected by CVE-2021-47608?

Linux kernel versions 5.12 through 5.15.10 are affected; version 5.15.11 and later include the fix.

How do I check if my server is vulnerable to CVE-2021-47608?

Run `uname -r` and confirm your kernel version is between 5.12 and 5.15.10, then verify BPF is enabled with `grep CONFIG_BPF /boot/config-$(uname -r)` returning 'y'.

Does Defensia detect CVE-2021-47608?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47608 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2026-100075CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: RDMA/srpt: Fix srpt_alloc_rw_ctxs() unwind counters When srpt_alloc_rw_ctxs() fails partway through a multi-buffer indirect descriptor, the unwind path destroys RDMA contexts but leaves stale n_rw_ctx and n_rdma values (and a dangling rw_ctxs pointer). Later sq_wr_avail accounting in srpt_queue_response() or srpt_write_pending() can then subtract the wrong number of send queue credits. Reset the counters and clear rw_ctxs after freeing the heap allocation before returning an error.
CVE-2026-64439CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: crypto: krb5 - filter out async aead implementations at alloc krb5_aead_encrypt(), krb5_aead_decrypt() in rfc3961_simplified.c and rfc8009_encrypt(), rfc8009_decrypt() in rfc8009_aes2.c set a NULL completion callback and treat any negative return from crypto_aead_{encrypt,decrypt}() as terminal, falling through to kfree_sensitive(buffer). When the encrypt_name resolves to an async AEAD instance the request returns -EINPROGRESS, the buffer is freed while the backend's worker still holds a pointer, and the worker dereferences the freed slab on completion. KASAN report under UML+SLUB with a synthetic async aead backend bound to krb5->encrypt_name: BUG: KASAN: slab-use-after-free in t5_stub_complete+0x7d/0xc7 The helpers were written synchronously, so filter the async instances out at allocation time instead of plumbing crypto_wait_req() through every call site. Reachable via net/rxrpc/rxgk.c, fs/afs/cm_security.c and net/ceph/crypto.c on systems with an async AEAD provider bound to the krb5 enctype name.
CVE-2026-64047CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: net: tls: fix off-by-one in sg_chain entry count for wrapped sk_msg ring When an sk_msg scatterlist ring wraps (sg.end < sg.start), tls_push_record() chains the tail portion of the ring to the head using sg_chain(). An extra entry in the sg array is reserved for this: struct sk_msg_sg { [...] /* The extra two elements: * 1) used for chaining the front and sections when the list becomes * partitioned (e.g. end < start). The crypto APIs require the * chaining; * 2) to chain tailer SG entries after the message. */ struct scatterlist data[MAX_MSG_FRAGS + 2]; The current code uses MAX_SKB_FRAGS + 1 as the ring size: sg_chain(&msg_pl->sg.data[msg_pl->sg.start], MAX_SKB_FRAGS - msg_pl->sg.start + 1, msg_pl->sg.data); This places the chain pointer at sg_chain(data[start], (MAX_SKB_FRAGS - msg_start + 1) .. = &data[start] + (MAX_SKB_FRAGS - msg_start + 1) - 1 = data[start + (MAX_SKB_FRAGS - start + 1) - 1] = data[MAX_SKB_FRAGS] instead of the true last entry. This is likely due to a "race" of the commit under Fixes landing close to commit 031097d9e079 ("bpf: sk_msg, zap ingress queue on psock down") Convert to ARRAY_SIZE and drop the data[start] / - start (as suggested by Sabrina).
CVE-2025-38660CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: [ceph] parse_longname(): strrchr() expects NUL-terminated string ... and parse_longname() is not guaranteed that. That's the reason why it uses kmemdup_nul() to build the argument for kstrtou64(); the problem is, kstrtou64() is not the only thing that need it. Just get a NUL-terminated copy of the entire thing and be done with that...
CVE-2025-39682CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: tls: fix handling of zero-length records on the rx_list Each recvmsg() call must process either - only contiguous DATA records (any number of them) - one non-DATA record If the next record has different type than what has already been processed we break out of the main processing loop. If the record has already been decrypted (which may be the case for TLS 1.3 where we don't know type until decryption) we queue the pending record to the rx_list. Next recvmsg() will pick it up from there. Queuing the skb to rx_list after zero-copy decrypt is not possible, since in that case we decrypted directly to the user space buffer, and we don't have an skb to queue (darg.skb points to the ciphertext skb for access to metadata like length). Only data records are allowed zero-copy, and we break the processing loop after each non-data record. So we should never zero-copy and then find out that the record type has changed. The corner case we missed is when the initial record comes from rx_list, and it's zero length.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2021-47608. Free for 1 server.

Get started free