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: ? __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—

Affected Software

References

• https://git.kernel.org/stable/c/300dedd9fe182d4c7424550d81cee595994486d1
• https://git.kernel.org/stable/c/327e28664307d49ce3fa71ba30dcc0007c270974