linux-kernel - Re: [PATCH] mctp: fix use after free

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Date:   Tue, 15 Feb 2022 08:44:57 +0800
From:   Jeremy Kerr <jk@...econstruct.com.au>
To:     trix@...hat.com, matt@...econstruct.com.au, davem@...emloft.net,
        kuba@...nel.org, nathan@...nel.org, ndesaulniers@...gle.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] mctp: fix use after free

Hi Tom,

> Clang static analysis reports this problem
> route.c:425:4: warning: Use of memory after it is freed
>   trace_mctp_key_acquire(key);
>   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> When mctp_key_add() fails, key is freed but then is later
> used in trace_mctp_key_acquire().  Add an else statement
> to use the key only when mctp_key_add() is successful.

Looks good to me, thanks for the fix.

However, the Fixes tag will need an update; at the point of 
4a992bbd3650 ("mctp: Implement message fragmentation"), there was no
use of 'key' after the kfree() there.

Instead, this is the hunk that introduced the trace event:

  @@ -365,12 +368,16 @@
                          if (rc)
                                  kfree(key);
   
  +                       trace_mctp_key_acquire(key);
  +
                          /* we don't need to release key->lock on exit */
                          key = NULL;
 
- which is from 4f9e1ba6de45. The unref() comes in later, but the
initial uaf is caused by this change.

So, I'd suggest this instead:

Fixes: 4f9e1ba6de45 ("mctp: Add tracepoints for tag/key handling")

(this just means we need the fix for 5.16+, rather than 5.15+).

Also, can you share how you're doing the clang static analysis there?
I'll get that included in my checks too.

Cheers,


Jeremy