linux-kernel - Re: [RFC PATCH v1] tracing: Fix syscall tracepoint use-after-free

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Message-ID: <b961bc6b-331c-4315-b424-60d514cc112e@efficios.com>
Date: Sat, 26 Oct 2024 10:25:31 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Jordan Rife <jrife@...gle.com>, acme@...nel.org,
 alexander.shishkin@...ux.intel.com, andrii.nakryiko@...il.com,
 ast@...nel.org, bpf@...r.kernel.org, joel@...lfernandes.org,
 linux-kernel@...r.kernel.org, mark.rutland@....com, mhiramat@...nel.org,
 mingo@...hat.com, mjeanson@...icios.com, namhyung@...nel.org,
 paulmck@...nel.org, peterz@...radead.org,
 syzbot+b390c8062d8387b6272a@...kaller.appspotmail.com, yhs@...com
Subject: Re: [RFC PATCH v1] tracing: Fix syscall tracepoint use-after-free

On 2024-10-26 03:13, Steven Rostedt wrote:
> On Fri, 25 Oct 2024 15:38:48 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>>> I'm curious if it might be better to add some field to struct
>>> tracepoint like "sleepable" rather than adding a special case here
>>> based on the name? Of course, if it's only ever going to be these
>>> two cases then maybe adding a new field doesn't make sense.
>>
>> I know Steven is reluctant to bloat the tracepoint struct because there
>> are lots of tracepoint instances (thousands). So for now I thought that
>> just comparing the name would be a good start.
> 
> You are correct. I really trying to keep the footprint of
> tracepoints/events down.
> 
>>
>> We can eventually go a different route as well: introduce a section just
>> to put the syscall tracepoints, and compare the struct tracepoint
>> pointers to the section begin/end range. But it's rather complex
>> for what should remain a simple fix.
> 
> A separate section could work.

I have another approach to suggest: it shrinks the
size of struct tracepoint from 80 bytes down to 72 bytes
on x86-64, we don't have to do any section/linker
script trickery, and it's extensible for future flags:

struct static_key {
         int enabled;
         void *p;
};

struct static_key_false {
         struct static_key key;
};

struct static_call_key {
         void *func;
         void *p;
};

struct tracepoint {
         const char *name;               /* Tracepoint name */
         struct static_key_false key;
         struct static_call_key *static_call_key;
         void *static_call_tramp;
         void *iterator;
         void *probestub;
         void *funcs;
         /* Flags. */
         unsigned int regfunc:1,
                      syscall:1;
};

struct tracepoint_regfunc {
         struct tracepoint tp;
         int (*regfunc)(void);
         void (*unregfunc)(void);
};

Basically, a tracepoint with regfunc would define a
struct tracepoint_regfunc rather than a struct tracepoint.
So we remove both regfunc and unregfunc NULL pointers in
the common case, which gives us plenty of room for flags.

When we want to access the regfunc/unregfunc from
a struct tracepoint, we check the regfunc flag, and
if set, we can use container_of() to get the struct
tracepoint_regfunc.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com