Re: [PATCH v4 05/13] hvf: Fix OOB write in RDTSCP instruction decode

["Philippe Mathieu-Daudé via" <qemu-devel@nongnu.org>]

Hi Cameron,

On 11/2/22 17:34, Philippe Mathieu-Daudé wrote:
> From: Cameron Esfahani <dirty@apple.com>
> 
> A guest could craft a specific stream of instructions that will have QEMU
> write 0xF9 to inappropriate locations in memory.  Add additional asserts
> to check for this.  Generate a #UD if there are more than 14 prefix bytes.
> 
> Found by Julian Stecklina <julian.stecklina@cyberus-technology.de>
> 
> Signed-off-by: Cameron Esfahani <dirty@apple.com>
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
>   target/i386/hvf/x86_decode.c | 11 +++++++++--
>   target/i386/hvf/x86hvf.c     |  8 ++++++++
>   target/i386/hvf/x86hvf.h     |  1 +
>   3 files changed, 18 insertions(+), 2 deletions(-)

> @@ -1847,7 +1849,8 @@ void calc_modrm_operand(CPUX86State *env, struct x86_decode *decode,
>   
>   static void decode_prefix(CPUX86State *env, struct x86_decode *decode)
>   {
> -    while (1) {
> +    /* At most 14 prefix bytes. */
> +    for (int i = 0; i < 14; i++) {

Could we have a definition instead of this magic '14' number?

>           /*
>            * REX prefix must come after legacy prefixes.
>            * REX before legacy is ignored.
> @@ -1892,6 +1895,8 @@ static void decode_prefix(CPUX86State *env, struct x86_decode *decode)
>               return;
>           }
>       }
> +    /* Too many prefixes!  Generate #UD. */
> +    hvf_inject_ud(env);
>   }