linux-hardening - Re: [PATCH] seq_file: Unconditionally use vmalloc for buffer

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Date:   Sat, 13 Mar 2021 09:54:21 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Lee Duncan <lduncan@...e.com>, Chris Leech <cleech@...hat.com>,
        Adam Nichols <adam@...mm-co.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] seq_file: Unconditionally use vmalloc for buffer

On Fri, Mar 12, 2021 at 12:55:58PM -0800, Kees Cook wrote:
> The sysfs interface to seq_file continues to be rather fragile, as seen
> with some recent exploits[1]. Move the seq_file buffer to the vmap area
> (while retaining the accounting flag), since it has guard pages that
> will catch and stop linear overflows. This seems justified given that
> seq_file already uses kvmalloc(), that allocations are normally short
> lived, and that they are not normally performance critical.
> 
> [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  fs/seq_file.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index cb11a34fb871..ad78577d4c2c 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -32,7 +32,7 @@ static void seq_set_overflow(struct seq_file *m)
>  
>  static void *seq_buf_alloc(unsigned long size)
>  {
> -	return kvmalloc(size, GFP_KERNEL_ACCOUNT);
> +	return __vmalloc(size, GFP_KERNEL_ACCOUNT);

Maybe a small comment here like:
	/* use vmalloc as it has good bounds checking */
so we know why this is being used instead of kmalloc() or anything else?

Other than that, no objection from me:

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>