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SubjectRe: [PATCH] jfs: Fix fortify moan in symlink
* Kees Cook (keescook@chromium.org) wrote:
> On Sat, Oct 22, 2022 at 09:39:14PM +0100, linux@treblig.org wrote:
> > From: "Dr. David Alan Gilbert" <linux@treblig.org>
> >
> > JFS has in jfs_incore.h:
> >
> > /* _inline may overflow into _inline_ea when needed */
> > /* _inline_ea may overlay the last part of
> > * file._xtroot if maxentry = XTROOTINITSLOT
> > */
> > union {
> > struct {
> > /* 128: inline symlink */
> > unchar _inline[128];
> > /* 128: inline extended attr */
> > unchar _inline_ea[128];
> > };
> > unchar _inline_all[256];
> >
> > and currently the symlink code copies into _inline;
> > if this is larger than 128 bytes it triggers a fortify warning of the
> > form:
> >
> > memcpy: detected field-spanning write (size 132) of single field
> > "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615)
>
> Which compiler are you using for this build?

I think that report was the same on gcc on Fedora 37 and whatever
syzkaller was running.

> This size report (SIZE_MAX)
> should be impossible to reach. But also, the size is just wrong --
> i_inline is 128 bytes, not SIZE_MAX. So, the detection is working
> (132 > 128), but the report is broken, and I can't see how...

Yeh, and led me down a blind alley for a while thinking something had
really managed to screwup the strlen somehow.

>
> >
> > when it's actually OK.
> >
> > Copy it into _inline_all instead.
> >
> > Reported-by: syzbot+5fc38b2ddbbca7f5c680@syzkaller.appspotmail.com
> > Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
> > ---
> > fs/jfs/namei.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
> > index 9db4f5789c0ec..4fbbf88435e69 100644
> > --- a/fs/jfs/namei.c
> > +++ b/fs/jfs/namei.c
> > @@ -946,7 +946,7 @@ static int jfs_symlink(struct user_namespace *mnt_userns, struct inode *dip,
> > if (ssize <= IDATASIZE) {
> > ip->i_op = &jfs_fast_symlink_inode_operations;
> >
> > - ip->i_link = JFS_IP(ip)->i_inline;
> > + ip->i_link = JFS_IP(ip)->i_inline_all;
> > memcpy(ip->i_link, name, ssize);
> > ip->i_size = ssize - 1;
> >
>
> Regardless, the fix looks correct to me!
>
> Reviewed-by: Kees Cook <keescook@chromium.org>

Thanks!

Dave

> --
> 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 /
\ _________________________|_____ http://www.treblig.org |_______/

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