CVE-2025-38279 in Linux
Summary
by MITRE • 07/10/2025
In the Linux kernel, the following vulnerability has been resolved:
bpf: Do not include stack ptr register in precision backtracking bookkeeping
Yi Lai reported an issue ([1]) where the following warning appears
in kernel dmesg: [ 60.643604] verifier backtracking bug
[ 60.643635] WARNING: CPU: 10 PID: 2315 at kernel/bpf/verifier.c:4302 __mark_chain_precision+0x3a6c/0x3e10
[ 60.648428] Modules linked in: bpf_testmod(OE)
[ 60.650471] CPU: 10 UID: 0 PID: 2315 Comm: test_progs Tainted: G OE 6.15.0-rc4-gef11287f8289-dirty #327 PREEMPT(full)
[ 60.654385] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
[ 60.656682] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 60.660475] RIP: 0010:__mark_chain_precision+0x3a6c/0x3e10
[ 60.662814] Code: 5a 30 84 89 ea e8 c4 d9 01 00 80 3d 3e 7d d8 04 00 0f 85 60 fa ff ff c6 05 31 7d d8 04
01 48 c7 c7 00 58 30 84 e8 c4 06 a5 ff <0f> 0b e9 46 fa ff ff 48 ... [ 60.668720] RSP: 0018:ffff888116cc7298 EFLAGS: 00010246
[ 60.671075] RAX: 54d70e82dfd31900 RBX: ffff888115b65e20 RCX: 0000000000000000
[ 60.673659] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
[ 60.676241] RBP: 0000000000000400 R08: ffff8881f6f23bd3 R09: 1ffff1103ede477a
[ 60.678787] R10: dffffc0000000000 R11: ffffed103ede477b R12: ffff888115b60ae8
[ 60.681420] R13: 1ffff11022b6cbc4 R14: 00000000fffffff2 R15: 0000000000000001
[ 60.684030] FS: 00007fc2aedd80c0(0000) GS:ffff88826fa8a000(0000) knlGS:0000000000000000
[ 60.686837] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 60.689027] CR2: 000056325369e000 CR3: 000000011088b002 CR4: 0000000000370ef0
[ 60.691623] Call Trace:
[ 60.692821] <TASK>
[ 60.693960] ? __pfx_verbose+0x10/0x10
[ 60.695656] ? __pfx_disasm_kfunc_name+0x10/0x10
[ 60.697495] check_cond_jmp_op+0x16f7/0x39b0
[ 60.699237] do_check+0x58fa/0xab10
...
Further analysis shows the warning is at line 4302 as below:
4294 /* static subprog call instruction, which 4295 * means that we are exiting current subprog, 4296 * so only r1-r5 could be still requested as 4297 * precise, r0 and r6-r10 or any stack slot in 4298 * the current frame should be zero by now 4299 */ 4300 if (bt_reg_mask(bt) & ~BPF_REGMASK_ARGS) {
4301 verbose(env, "BUG regs %x\n", bt_reg_mask(bt)); 4302 WARN_ONCE(1, "verifier backtracking bug"); 4303 return -EFAULT; 4304 }
With the below test (also in the next patch): __used __naked static void __bpf_jmp_r10(void) {
asm volatile ( "r2 = 2314885393468386424 ll;" "goto +0;" "if r2 <= r10 goto +3;" "if r1 >= -1835016 goto +0;" "if r2 <= 8 goto +0;" "if r3 <= 0 goto +0;" "exit;" ::: __clobber_all); }
SEC("?raw_tp") __naked void bpf_jmp_r10(void) {
asm volatile ( "r3 = 0 ll;" "call __bpf_jmp_r10;" "r0 = 0;" "exit;" ::: __clobber_all); }
The following is the verifier failure log: 0: (18) r3 = 0x0 ; R3_w=0 2: (85) call pc+2 caller: R10=fp0 callee: frame1: R1=ctx() R3_w=0 R10=fp0 5: frame1: R1=ctx() R3_w=0 R10=fp0 ; asm volatile (" \ @ verifier_precision.c:184 5: (18) r2 = 0x20202000256c6c78 ; frame1: R2_w=0x20202000256c6c78 7: (05) goto pc+0 8: (bd) if r2 <= r10 goto pc+3 ; frame1: R2_w=0x20202000256c6c78 R10=fp0 9: (35) if r1 >= 0xffe3fff8 goto pc+0 ; frame1: R1=ctx() 10: (b5) if r2 <= 0x8 goto pc+0 mark_precise: frame1: last_idx 10 first_idx 0 subseq_idx -1 mark_precise: frame1: regs=r2 stack= before 9: (35) if r1 >= 0xffe3fff8 goto pc+0 mark_precise: frame1: regs=r2 stack= before 8: (bd) if r2 <= r10 goto pc+3 mark_preci ---truncated---
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Analysis
by VulDB Data Team • 01/27/2026
The vulnerability CVE-2025-38279 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) verifier subsystem, specifically in how it handles precision backtracking during bytecode analysis. This issue manifests as a verifier backtracking bug that occurs when processing certain BPF programs, leading to potential instability or denial of service in kernel environments that utilize eBPF. The flaw is triggered during conditional jump operations and involves incorrect handling of register state tracking, particularly with the stack pointer register r10, which is not supposed to be included in precision backtracking bookkeeping but is inadvertently being considered.
The technical root cause of this vulnerability lies in the __mark_chain_precision function located in kernel/bpf/verifier.c at line 4302. When the verifier encounters a conditional jump instruction, it attempts to maintain precision tracking for register values across different execution paths. However, the code incorrectly includes the stack pointer register r10 in this precision tracking, violating the expected behavior where only specific registers should be tracked for precision during backtracking. This leads to a warning message indicating a verifier backtracking bug, ultimately resulting in a return value of -EFAULT that terminates the verification process.
The vulnerability is demonstrated through a specific test case involving nested BPF function calls where register r2 and r10 are used in conditional jumps. The test program creates a scenario where the verifier's precision tracking mechanism fails to properly distinguish between registers that should be tracked for precision and those that should not, particularly when dealing with frame-based stack operations. The stack pointer r10, which should not be part of precision backtracking, is being included in the register mask check, causing the verifier to incorrectly flag a bug condition.
This vulnerability impacts systems running Linux kernels with eBPF support, particularly those utilizing BPF programs for network filtering, tracing, or security monitoring. The operational impact includes potential kernel crashes, denial of service conditions, and the inability to load or execute certain BPF programs. The flaw aligns with CWE-691, which addresses insufficient control flow management in security-critical code, and maps to ATT&CK technique T1059.007 for execution through kernel modules, highlighting the potential for privilege escalation or system compromise through malicious BPF programs.
Mitigation strategies include upgrading to a patched kernel version that resolves the precision backtracking issue, applying the specific patch that excludes the stack pointer register from precision backtracking bookkeeping, and implementing proper input validation for BPF programs to prevent exploitation. Organizations should also monitor for any related vulnerabilities in the BPF subsystem and consider restricting BPF program loading where possible. The fix ensures that only the appropriate registers are tracked for precision during backtracking, preventing the false positive bug warnings and maintaining the integrity of the kernel's verification process.