netdev - Re: [PATCH net] ebpf, x86: fix general protection fault when tail call is invoked

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Message-Id: <20150729.170453.375462793871538401.davem@davemloft.net>
Date:	Wed, 29 Jul 2015 17:04:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	daniel@...earbox.net
Cc:	ast@...mgrid.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] ebpf, x86: fix general protection fault when tail
 call is invoked

From: Daniel Borkmann <daniel@...earbox.net>
Date: Tue, 28 Jul 2015 15:26:36 +0200

> With eBPF JIT compiler enabled on x86_64, I was able to reliably trigger
> the following general protection fault out of an eBPF program with a simple
> tail call, f.e. tracex5 (or a stripped down version of it):
 ...
> Changing the emitter to always use the 4 byte displacement in the lea
> instruction fixes the panic on my side. It increases the tail call instruction
> emission by 3 more byte, but it should cover us from various combinations
> (and perhaps other future increases on related structures).
> 
> After patch, disassembly:
> 
>   [...]
>   9e:   lea    0x80(%rsi,%rdx,8),%rax   <--- CONFIG_LOCKDEP/CONFIG_LOCK_STAT
>         48 8d 84 d6 80 00 00 00
>   a6:   mov    (%rax),%rax
>         48 8b 00
>   [...]
> 
>   [...]
>   9e:   lea    0x50(%rsi,%rdx,8),%rax   <--- No CONFIG_LOCKDEP
>         48 8d 84 d6 50 00 00 00
>   a6:   mov    (%rax),%rax
>         48 8b 00
>   [...]
> 
> Fixes: b52f00e6a715 ("x86: bpf_jit: implement bpf_tail_call() helper")
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>

Applied, thanks.
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