linux-kernel - Re: [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse

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Message-ID: <d31f6069-bda7-2cdb-b770-0c9cddac7537@suse.cz>
Date:   Wed, 15 Jan 2020 13:54:34 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lee Schermerhorn <lee.schermerhorn@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        syzbot <syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com>,
        aarcange@...hat.com, hughd@...gle.com, mhocko@...e.com,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        yang.shi@...ux.alibaba.com
Subject: Re: [PATCH] mm/mempolicy.c: Fix out of bounds write in
 mpol_parse_str()

On 1/15/20 6:54 AM, Dan Carpenter wrote:
> What we are trying to do is change the '=' character to a NUL terminator
> and then at the end of the function we restore it back to an '='.  The
> problem is there are two error paths where we jump to the end of the
> function before we have replaced the '=' with NUL.  We end up putting
> the '=' in the wrong place (possibly one element before the start of
> the buffer).

Bleh.

> Reported-by: syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com
> Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

CC stable perhaps? Can this (tmpfs mount options parsing AFAICS?) become
part of unprivileged operation in some scenarios?

> ---
>  mm/mempolicy.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 067cf7d3daf5..1340c5c496b5 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2817,6 +2817,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
>  	char *flags = strchr(str, '=');
>  	int err = 1, mode;
>  
> +	if (flags)
> +		*flags++ = '\0';	/* terminate mode string */
> +
>  	if (nodelist) {
>  		/* NUL-terminate mode or flags string */
>  		*nodelist++ = '\0';
> @@ -2827,9 +2830,6 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
>  	} else
>  		nodes_clear(nodes);
>  
> -	if (flags)
> -		*flags++ = '\0';	/* terminate mode string */
> -
>  	mode = match_string(policy_modes, MPOL_MAX, str);
>  	if (mode < 0)
>  		goto out;
>