Re: [PATCH v2 1/3] x86/uprobes: Fix not using prefixes.nbytes for loop over prefixes.bytes

[Borislav Petkov <bp@alien8.de>]

On Thu, Dec 03, 2020 at 01:50:37PM +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.
> This introduces for_each_insn_prefix() macro for this purpose.
> 
> Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints")
> Reported-by: syzbot+9b64b619f10f19d19a7c@syzkaller.appspotmail.com
> Debugged-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>  Changes in v2:
>   - Add for_each_insn_prefix() macro and fix to check index first.
> ---
>  arch/x86/include/asm/insn.h |   15 +++++++++++++++
>  arch/x86/kernel/uprobes.c   |   10 ++++++----
>  2 files changed, 21 insertions(+), 4 deletions(-)

Warning: Kernel ABI header at 'tools/arch/x86/include/asm/insn.h' differs from latest version at 'arch/x86/include/asm/insn.h'

> diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
> index 5c1ae3eff9d4..e6b38ebd3a1c 100644
> --- a/arch/x86/include/asm/insn.h
> +++ b/arch/x86/include/asm/insn.h
> @@ -201,6 +201,21 @@ static inline int insn_offset_immediate(struct insn *insn)
>  	return insn_offset_displacement(insn) + insn->displacement.nbytes;
>  }
>  
> +/**
> + * for_each_insn_prefix() -- Iterate prefixes in the instruction
> + * @insn: Pointer to struct insn.
> + * @idx:  Index storage.
> + * @prefix: Prefix byte.
> + *
> + * Iterate prefix bytes of given @insn. Each prefix byte is stored in @prefix
> + * and the index is stored in @idx (note that this @idx is just for a cursor,
> + * do not change it.)
> + * Since prefixes.nbytes can be bigger than 4 if some prefixes are repeated,
> + * we can not use it for looping over the prefixes.

Please use passive voice: no "we" or "I", etc,

> + */
> +#define for_each_insn_prefix(insn, idx, prefix)	\
> +	for (idx = 0; idx < 4 && (prefix = insn->prefixes.bytes[idx]) != 0; idx++)

Btw, looking at the struct insn definition, that prefixes member should
have a comment above it that those are the legacy prefixes which can be
<= 4. But that's minor.

In any case, I'll fix up the minor issues now but pls remember to do
them in the future.

Thx.

-- 
Regards/Gruss,
    Boris.

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