Re: [PATCH] vfs_getxattr_alloc(): don't allocate buf on failure

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Aug 02, 2022 at 04:12:31PM +0100, Al Viro wrote:
> On Tue, Aug 02, 2022 at 04:42:36PM +0200, Miklos Szeredi wrote:
> > Some callers of vfs_getxattr_alloc() assume that on failure the allocated
> > buffer does not need to be freed.
> > 
> > Callers could be fixed, but fixing the semantics of vfs_getxattr_alloc() is
> > simpler and makes sure that this class of bugs does not occur again.
> > 
> > Reported-and-tested-by: syzbot+942d5390db2d9624ced8@syzkaller.appspotmail.com
> > Fixes: 1601fbad2b14 ("xattr: define vfs_getxattr_alloc and vfs_xattr_cmp")
> > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> > ---
> >  fs/xattr.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index e8dd03e4561e..1800cfa97411 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -383,7 +383,10 @@ vfs_getxattr_alloc(struct user_namespace *mnt_userns, struct dentry *dentry,
> >  	}
> >  
> >  	error = handler->get(handler, dentry, inode, name, value, error);
> > -	*xattr_value = value;
> > +	if (error < 0 && value != *xattr_value)
> > +		kfree(value);
> > +	else
> > +		*xattr_value = value;
> >  	return error;
> >  }
> 
> Think what happens if it had been called with non-NULL *xattr_value,
> found that it needed realloc, had krealloc() succeed (and free the
> original), only to fail in ->get().
> 
> Your variant will leave *xattr_value pointing to already freed
> object, with no way for the caller to tell that from failure before
> it got to krealloc().
> 
> IOW, that's unusable for callers with preallocated buffer - in
> particular, ones that call that thing in a loop.

FWIW, if we change calling conventions so that in some cases caller
need not kfree() whatever's in *xattr_value, about the only variant
I see is to have the damn thing freed and replaced with NULL on
*all* failure exits.  Might or might not make sense, not sure...