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

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Date:   Wed, 2 Dec 2020 11:04:41 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/uprobes: Fix not using prefixes.nbytes for loop
 over prefixes.bytes

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@...r.kernel.org
> Reported-by: Kees Cook <keescook@...omium.org>

This should probably be:

Reported-by: syzbot+9b64b619f10f19d19a7c@...kaller.appspotmail.com
Debugged-by: Kees Cook <keescook@...omium.org>

> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  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