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

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Date:   Mon, 6 Feb 2023 19:50:42 +0000
From:   "Dr. David Alan Gilbert" <linux@...blig.org>
To:     Kees Cook <keescook@...omium.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

* Kees Cook (keescook@...omium.org) wrote:
> 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 bet at least on some platforms it's cheaper as the 64 bit.

> I opted to keep the variable 32-bit here, so the
> machine code would only change to lose signed-ness.

Fair enough.

Dave

> -Kees
> 
> -- 
> Kees Cook
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
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