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References: <20170323175851.14342-1-bobby.prani@gmail.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <3dd9c1d8-bc6d-0b6e-14c2-b37bfd40d852@redhat.com>
Date: Fri, 24 Mar 2017 11:49:52 +0100
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In-Reply-To: <20170323175851.14342-1-bobby.prani@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] tcg/i386: Check the size of instruction
 being translated
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To: Pranith Kumar <bobby.prani@gmail.com>, Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, "open list:All patches CC here" <qemu-devel@nongnu.org>
Cc: Peter Maydell <peter.maydell@linaro.org>

Queued, thanks.

Paolo

On 23/03/2017 18:58, Pranith Kumar wrote:
> Sending again since I messed by pbonzini's email.
> 
> This fixes the bug: 'user-to-root privesc inside VM via bad translation
> caching' reported by Jann Horn here:
> https://bugs.chromium.org/p/project-zero/issues/detail?id=1122
> 
> CC: Richard Henderson <rth@twiddle.net>
> CC: Peter Maydell <peter.maydell@linaro.org>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> Reported-by: Jann Horn <jannh@google.com>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  target/i386/translate.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/target/i386/translate.c b/target/i386/translate.c
> index 72c1b03a2a..1d1372fb43 100644
> --- a/target/i386/translate.c
> +++ b/target/i386/translate.c
> @@ -4418,6 +4418,13 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
>      s->vex_l = 0;
>      s->vex_v = 0;
>   next_byte:
> +    /* x86 has an upper limit of 15 bytes for an instruction. Since we
> +     * do not want to decode and generate IR for an illegal
> +     * instruction, the following check limits the instruction size to
> +     * 25 bytes: 14 prefix + 1 opc + 6 (modrm+sib+ofs) + 4 imm */
> +    if (s->pc - pc_start > 14) {
> +        goto illegal_op;
> +    }
>      b = cpu_ldub_code(env, s->pc);
>      s->pc++;
>      /* Collect prefixes.  */
>