Re: [PATCH] tracing/fprobe: Fix NULL pointer dereference in fprobe_fgraph_entry()

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

On Fri, 19 Jun 2026 18:44:24 +0000
Sechang Lim <rhkrqnwk98@gmail.com> wrote:

> fprobe_fgraph_entry() sizes a shadow-stack reservation in one walk of
> the per-ip fprobe list and fills it in a second walk, both under
> rcu_read_lock() only. A fprobe registered on an already-live ip can
> become visible between the two walks, so the fill walk processes an
> exit_handler the sizing walk did not count and used runs past
> reserved_words. If the sizing walk counted nothing, fgraph_data is NULL
> and the first write_fprobe_header() faults:
> 
>   Oops: general protection fault, probably for non-canonical address ...
>   KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>   RIP: 0010:fprobe_fgraph_entry+0xa38/0xf10 kernel/trace/fprobe.c:167
>   Call Trace:
>    <TASK>
>    function_graph_enter_regs+0x44c/0xa10 kernel/trace/fgraph.c:677
>    ftrace_graph_func+0xc5/0x140 arch/x86/kernel/ftrace.c:671
>    __kernel_text_address+0x9/0x40 kernel/extable.c:78
>    arch_stack_walk+0x117/0x170 arch/x86/kernel/stacktrace.c:26
>    kmem_cache_free+0x188/0x580 mm/slub.c:6378
>    tcp_data_queue+0x18d/0x6550 net/ipv4/tcp_input.c:5590
>    [...]
>    </TASK>
> 
> The list cannot be frozen across the two walks, so skip a node that does
> not fit the reservation and count it as missed.
> 

Ah, good catch! Yes, if the list is scanned repeatedly, there is a
possibility that it could be updated even when using RCU guards.
This is rare case, but hmm, I think we need something like
fgraph_increment_reserved_data() to avoid skipping.
Anyway, this looks good to me. Let me pick it.

Thanks!

> Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
> ---
>  kernel/trace/fprobe.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index f378613ad120..f215990b9061 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -613,6 +613,16 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
>  			continue;
>  
>  		data_size = fp->entry_data_size;
> +		/*
> +		 * The list may have grown since it was sized, so this node
> +		 * may not fit. Skip it as missed rather than overrun the
> +		 * reservation.
> +		 */
> +		if (fp->exit_handler &&
> +		    used + FPROBE_HEADER_SIZE_IN_LONG + SIZE_IN_LONG(data_size) > reserved_words) {
> +			fp->nmissed++;
> +			continue;
> +		}
>  		if (data_size && fp->exit_handler)
>  			data = fgraph_data + used + FPROBE_HEADER_SIZE_IN_LONG;
>  		else
> -- 
> 2.43.0
> 


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