Re: [PATCH 2/3] x86/uprobes: Fix not using prefixes.nbytes for loop over prefixes.bytes

[Kees Cook]

On Wed, Dec 02, 2020 at 05:51:16PM +0900, Masami Hiramatsu wrote:
> Since the insn.prefixes.nbytes can be bigger than the size of
> insn.prefixes.bytes[] when a same prefix is repeated, we have to
> check whether the insn.prefixes.bytes[i] != 0 and i < 4 instead
> of insn.prefixes.nbytes.
> 
> Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Kees Cook <keescook@xxxxxxxxxxxx>

This should probably be:

Reported-by: syzbot+9b64b619f10f19d19a7c@xxxxxxxxxxxxxxxxxxxxxxxxx
Debugged-by: Kees Cook <keescook@xxxxxxxxxxxx>

> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  arch/x86/kernel/uprobes.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 3fdaa042823d..bb3ea3705b99 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -257,7 +257,7 @@ static bool is_prefix_bad(struct insn *insn)
>  {
>  	int i;
>  
> -	for (i = 0; i < insn->prefixes.nbytes; i++) {
> +	for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
>  		insn_attr_t attr;
>  
>  		attr = inat_get_opcode_attribute(insn->prefixes.bytes[i]);
> @@ -746,7 +746,7 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>  	 * Intel and AMD behavior differ in 64-bit mode: Intel ignores 66 prefix.
>  	 * No one uses these insns, reject any branch insns with such prefix.
>  	 */
> -	for (i = 0; i < insn->prefixes.nbytes; i++) {
> +	for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
>  		if (insn->prefixes.bytes[i] == 0x66)
>  			return -ENOTSUPP;
>  	}
> 


-- 
Kees Cook