linux-hardening - Re: [PATCH] jfs: Use unsigned variable for length calculations

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Date:   Mon, 6 Feb 2023 10:35:21 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Dr. David Alan Gilbert" <linux@...blig.org>
Cc:     Dave Kleikamp <shaggy@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] jfs: Use unsigned variable for length calculations

On Sat, Feb 04, 2023 at 08:25:45PM +0000, Dr. David Alan Gilbert wrote:
> * Kees Cook (keescook@...omium.org) wrote:
> > To avoid confusing the compiler about possible negative sizes, switch
> > "ssize" which can never be negative from int to u32.  Seen with GCC 13:
> > 
> > ../fs/jfs/namei.c: In function 'jfs_symlink': ../include/linux/fortify-string.h:57:33: warning: '__builtin_memcpy' pointer overflow between offset 0 and size [-2147483648, -1]
> > [-Warray-bounds=]
> >    57 | #define __underlying_memcpy     __builtin_memcpy
> >       |                                 ^
> > ...
> > ../fs/jfs/namei.c:950:17: note: in expansion of macro 'memcpy'
> >   950 |                 memcpy(ip->i_link, name, ssize);
> >       |                 ^~~~~~
> > 
> > Cc: Dave Kleikamp <shaggy@...nel.org>
> > Cc: Christian Brauner <brauner@...nel.org>
> > Cc: Dave Chinner <dchinner@...hat.com>
> > Cc: jfs-discussion@...ts.sourceforge.net
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  fs/jfs/namei.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
> > index b29d68b5eec5..494b9f4043cf 100644
> > --- a/fs/jfs/namei.c
> > +++ b/fs/jfs/namei.c
> > @@ -876,7 +876,7 @@ static int jfs_symlink(struct mnt_idmap *idmap, struct inode *dip,
> >  	tid_t tid;
> >  	ino_t ino = 0;
> >  	struct component_name dname;
> > -	int ssize;		/* source pathname size */
> > +	u32 ssize;		/* source pathname size */
> 
> Had you considered using size_t - this is set from a strlen and used by a memcpy
> that both talk size_t.

I considered that, but I've had other maintainers upset about doubling
the variable size. I opted to keep the variable 32-bit here, so the
machine code would only change to lose signed-ness.

-Kees

-- 
Kees Cook