On Tue, 2 Aug 2022 at 17:29, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Fixes: 1601fbad2b14 ("xattr: define vfs_getxattr_alloc and vfs_xattr_cmp")
> > > Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> > > ---
> > > 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...
The minimal semantic change that would fix all buggy callers is to
change the non-loop case (i.e. when called only once with a
xattr_value pointing to NULL.
In the looping case callers correctly handle the error, since an
explicit kfree in the helper is unexpected.
Posted v2.
Thanks,
Miklos
