CVE: CVE-2025-38327
Published: 2025-07-10
Description: 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=[PII_REDACTED], basem=[PII_REDACTED]00, base_local=0xffff88823c3ae040, base_global=0xffff88823c3af300, tevt=0xffff888100e47cb8);
           <idle>-0       [007] d..4.   358.966012:  7)               |          tmigr_cpu_deactivate(nextexp=[PII_REDACTED]00) {
           <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: [PII_REDACTED] RBX: 1ffff1102679ff31 RCX: [PII_REDACTED]
 RDX: 1ffffffff0b27a60 RSI: ffffffff8593d2f0 RDI: ffffffff85941140
 RBP:[PII_REDACTED]0c2041 R08: ffffffffffffffff R09: ffffed[PII_REDACTED]
 R10: ffff[PII_REDACTED]0f R11: ffffed[PII_REDACTED] R12: ffffffff8593d2f0
 R13: ffffffff8593d300 R14: ffffffff85941140 R15: ffffffff85631100
 FS:  00007f7ec6f28740(0000) GS:ffff8882b5251000(0000) knlGS:[PII_REDACTED]
 CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
 CR2: 00007f7ec6f181c0 CR3: 000000012f1d0005 CR4:[PII_REDACTED]172ef0
 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---
CVSS Score: 5.5 (MEDIUM)
References: https://git.kernel.org/stable/c/300dedd9fe182d4c7424550d81cee595994486d1; https://git.kernel.org/stable/c/327e28664307d49ce3fa71ba30dcc0007c270974