CVE-2025-38327 in Linux
Summary
by MITRE • 07/10/2025
In the Linux kernel, the following vulnerability has been resolved:
fgraph: Do not enable function_graph tracer when setting funcgraph-args
When setting the funcgraph-args option when function graph tracer is net enabled, it incorrectly enables it. Worse, it unregisters itself when it was never registered. Then when it gets enabled again, it will register itself a second time causing a WARNing.
~# echo 1 > /sys/kernel/tracing/options/funcgraph-args ~# head -20 /sys/kernel/tracing/trace # tracer: nop # # entries-in-buffer/entries-written: 813/26317372 #P:8 # # _-----=> irqs-off/BH-disabled # / _----=> need-resched # | / _---=> hardirq/softirq # || / _--=> preempt-depth # ||| / _-=> migrate-disable # |||| / delay # TASK-PID CPU# ||||| TIMESTAMP FUNCTION # | | | ||||| | | <idle>-0 [007] d..4. 358.966010: 7) 1.692 us | fetch_next_timer_interrupt(basej=4294981640, basem=357956000000, base_local=0xffff88823c3ae040, base_global=0xffff88823c3af300, tevt=0xffff888100e47cb8);
<idle>-0 [007] d..4. 358.966012: 7) | tmigr_cpu_deactivate(nextexp=357988000000) {
<idle>-0 [007] d..4. 358.966013: 7) | _raw_spin_lock(lock=0xffff88823c3b2320) {
<idle>-0 [007] d..4. 358.966014: 7) 0.981 us | preempt_count_add(val=1);
<idle>-0 [007] d..5. 358.966017: 7) 1.058 us | do_raw_spin_lock(lock=0xffff88823c3b2320);
<idle>-0 [007] d..4. 358.966019: 7) 5.824 us | }
<idle>-0 [007] d..5. 358.966021: 7) | tmigr_inactive_up(group=0xffff888100cb9000, child=0x0, data=0xffff888100e47bc0) {
<idle>-0 [007] d..5. 358.966022: 7) | tmigr_update_events(group=0xffff888100cb9000, child=0x0, data=0xffff888100e47bc0) {
Notice the "tracer: nop" at the top there. The current tracer is the "nop" tracer, but the content is obviously the function graph tracer.
Enabling function graph tracing will cause it to register again and trigger a warning in the accounting:
~# echo function_graph > /sys/kernel/tracing/current_tracer -bash: echo: write error: Device or resource busy
With the dmesg of:
------------[ cut here ]------------
WARNING: CPU: 7 PID: 1095 at kernel/trace/ftrace.c:3509 ftrace_startup_subops+0xc1e/0x1000 Modules linked in: kvm_intel kvm irqbypass CPU: 7 UID: 0 PID: 1095 Comm: bash Not tainted 6.16.0-rc2-test-00006-gea03de4105d3 #24 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:ftrace_startup_subops+0xc1e/0x1000 Code: 48 b8 22 01 00 00 00 00 ad de 49 89 84 24 88 01 00 00 8b 44 24 08 89 04 24 e9 c3 f7 ff ff c7 04 24 ed ff ff ff e9 b7 f7 ff ff <0f> 0b c7 04 24 f0 ff ff ff e9 a9 f7 ff ff c7 04 24 f4 ff ff ff e9 RSP: 0018:ffff888133cff948 EFLAGS: 00010202 RAX: 0000000000000001 RBX: 1ffff1102679ff31 RCX: 0000000000000000 RDX: 1ffffffff0b27a60 RSI: ffffffff8593d2f0 RDI: ffffffff85941140 RBP: 00000000000c2041 R08: ffffffffffffffff R09: ffffed1020240221 R10: ffff88810120110f R11: ffffed1020240214 R12: ffffffff8593d2f0 R13: ffffffff8593d300 R14: ffffffff85941140 R15: ffffffff85631100 FS: 00007f7ec6f28740(0000) GS:ffff8882b5251000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f7ec6f181c0 CR3: 000000012f1d0005 CR4: 0000000000172ef0 Call Trace: <TASK> ? __pfx_ftrace_startup_subops+0x10/0x10 ? find_held_lock+0x2b/0x80 ? ftrace_stub_direct_tramp+0x10/0x10 ? ftrace_stub_direct_tramp+0x10/0x10 ? trace_preempt_on+0xd0/0x110 ? __pfx_trace_graph_entry_args+0x10/ ---truncated---
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Analysis
by VulDB Data Team • 12/08/2025
The vulnerability described in CVE-2025-38327 resides within the Linux kernel's function graph tracer implementation, specifically in how it handles the interaction between the funcgraph-args option and the function graph tracer state management. This issue manifests when users attempt to enable the funcgraph-args option while the function graph tracer is not active, leading to improper state transitions and registration inconsistencies. The flaw results in a misreported tracer state where the system indicates the "nop" tracer is active while displaying function graph trace data, creating a discrepancy between the reported tracer and actual trace content. This behavior stems from the kernel's tracing subsystem not properly managing the registration lifecycle of the function graph tracer when the funcgraph-args option is set, causing the tracer to be both incorrectly enabled and subsequently unregistered without proper cleanup.
The technical root cause of this vulnerability lies in the improper handling of tracer registration and state transitions within the kernel's trace framework. When the funcgraph-args option is enabled, the system incorrectly activates the function graph tracer even though it should remain inactive. This misbehavior occurs due to flawed logic in the tracer initialization code that does not properly account for the current tracer state when processing funcgraph-args settings. The vulnerability is further exacerbated by the fact that the tracer unregisters itself when it was never properly registered, creating a state inconsistency that leads to double-registration attempts. This double registration triggers kernel warnings and can cause system instability, particularly in environments where precise tracing behavior is required for debugging or performance analysis. The issue is categorized under CWE-691 as an Insufficient Control Flow Management, where the control flow of the tracing subsystem is not properly managed during option setting operations.
The operational impact of this vulnerability extends beyond simple tracing inconsistencies, potentially affecting system reliability and debugging capabilities in kernel environments. When the function graph tracer is improperly enabled, it can cause unexpected behavior in kernel debugging sessions where developers rely on accurate tracer state reporting. The warning message generated during double registration attempts indicates a deeper issue in the kernel's tracing subsystem that could propagate to other components relying on proper tracer state management. This vulnerability affects systems using the Linux kernel's ftrace infrastructure and could be exploited in scenarios where precise tracing control is required, such as in production kernel environments or during security analysis. The improper handling of tracer registration can also interfere with automated tracing tools and monitoring systems that depend on consistent tracer behavior and state reporting.
Mitigation strategies for CVE-2025-38327 should focus on ensuring proper state management within the kernel's tracing subsystem and implementing defensive programming practices to prevent improper tracer registration sequences. The primary fix involves modifying the tracer initialization logic to properly validate the current tracer state before attempting to enable or disable tracing features, particularly when dealing with funcgraph-args options. System administrators should ensure they are running patched kernel versions that contain the corrected implementation of the function graph tracer state management. Organizations should also implement monitoring for kernel warnings related to ftrace operations and establish procedures for verifying tracer state consistency in kernel debugging environments. Additionally, developers working with kernel tracing should be aware of this vulnerability when creating tracing-based tools or applications that rely on the proper functioning of the function graph tracer. The fix should be validated through comprehensive testing of tracer state transitions and registration sequences to ensure that the corrected implementation properly handles all combinations of tracer options and states without generating warnings or inconsistent behavior. This vulnerability aligns with ATT&CK technique T1547.006 related to kernel rootkits and system modification, as improper kernel tracing behavior can be leveraged to hide malicious activities or interfere with security monitoring systems.