[BUG] tracing/uprobe: oversized dynamic ustring triggers WARN_ON_ONCE panic

[BUG] tracing/uprobe: oversized dynamic ustring triggers WARN_ON_ONCE panic

[Yifei Chu]

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Hello,

Short version: I can make trace_uprobe hit WARN_ON_ONCE() by creating an
uprobe/uretprobe event with several dynamic ustring fetch args. With
panic_on_warn=1, this becomes a reproducible panic.

The setup is pretty direct. The reproducers mount tracefs, create a trace
event with several ustring arguments pointing at a 4095-byte userspace
string, and then trigger the event. At probe hit time, the dynamic string
sizes are accumulated and prepare_uprobe_buffer() sees a payload larger
than MAX_UCB_BUFFER_SIZE/PAGE_SIZE:

WARN_ON_ONCE(ucb->dsize > MAX_UCB_BUFFER_SIZE)

I reproduced the same class through both uprobe and uretprobe events.

Tested environment:

Linux version 7.0.9, x86_64 QEMU
gcc 12.3.0, GNU ld 2.38
Boot args included: panic_on_warn=1 nokaslr console=ttyS0

Uprobe result:

WARNING: kernel/trace/trace_uprobe.c:982 at
prepare_uprobe_buffer.part.0+0x458/0x5b0
Kernel panic - not syncing: kernel: panic_on_warn set …

Uretprobe result:

triggering uretprobe oversized ustring buffer at offset 0x1db0
WARNING: kernel/trace/trace_uprobe.c:982 at
prepare_uprobe_buffer.part.0+0x458/0x5b0
uretprobe_dispatcher+0x328/0x3e0
Kernel panic - not syncing: kernel: panic_on_warn set …

I checked current mainline source and still see the runtime WARN path in
kernel/trace/trace_uprobe.c. I have reproduced the panic on the 7.0.9 QEMU
build above; I have not yet runtime-tested current mainline.

My expectation is that oversized user-controlled dynamic trace data should
be rejected, capped, or dropped before it reaches a WARN invariant. A
tracefs user should not be able to turn a long string fetch into a kernel
warning/panic.

The attached tarball has README files, both C reproducers, and the full
QEMU logs.

Thanks,
Chuyifei

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Re: [BUG] tracing/uprobe: oversized dynamic ustring triggers WARN_ON_ONCE panic

[Masami Hiramatsu]

On Sun, 24 May 2026 10:44:09 -0400
Yifei Chu <yifeichu24@gmail.com> wrote:

> Hello,
> 
> Short version: I can make trace_uprobe hit WARN_ON_ONCE() by creating an
> uprobe/uretprobe event with several dynamic ustring fetch args. With
> panic_on_warn=1, this becomes a reproducible panic.
> 
> The setup is pretty direct. The reproducers mount tracefs, create a trace
> event with several ustring arguments pointing at a 4095-byte userspace
> string, and then trigger the event. At probe hit time, the dynamic string
> sizes are accumulated and prepare_uprobe_buffer() sees a payload larger
> than MAX_UCB_BUFFER_SIZE/PAGE_SIZE:
> 
> WARN_ON_ONCE(ucb->dsize > MAX_UCB_BUFFER_SIZE)
> 
> I reproduced the same class through both uprobe and uretprobe events.

This should be fixed by [1]

[1] https://lore.kernel.org/all/20260428122302.706610ba@gandalf.local.home/

Thanks,

> 
> Tested environment:
> 
> Linux version 7.0.9, x86_64 QEMU
> gcc 12.3.0, GNU ld 2.38
> Boot args included: panic_on_warn=1 nokaslr console=ttyS0
> 
> Uprobe result:
> 
> WARNING: kernel/trace/trace_uprobe.c:982 at
> prepare_uprobe_buffer.part.0+0x458/0x5b0
> Kernel panic - not syncing: kernel: panic_on_warn set …
> 
> Uretprobe result:
> 
> triggering uretprobe oversized ustring buffer at offset 0x1db0
> WARNING: kernel/trace/trace_uprobe.c:982 at
> prepare_uprobe_buffer.part.0+0x458/0x5b0
> uretprobe_dispatcher+0x328/0x3e0
> Kernel panic - not syncing: kernel: panic_on_warn set …
> 
> I checked current mainline source and still see the runtime WARN path in
> kernel/trace/trace_uprobe.c. I have reproduced the panic on the 7.0.9 QEMU
> build above; I have not yet runtime-tested current mainline.
> 
> My expectation is that oversized user-controlled dynamic trace data should
> be rejected, capped, or dropped before it reaches a WARN invariant. A
> tracefs user should not be able to turn a long string fetch into a kernel
> warning/panic.
> 
> The attached tarball has README files, both C reproducers, and the full
> QEMU logs.
> 
> Thanks,
> Chuyifei


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>