On Thu, Nov 17, 2022 at 12:37:33PM -0800, Darrick J. Wong wrote:
> On Wed, Nov 09, 2022 at 02:23:35PM -0800, Catherine Hoang wrote:
> > Add support for the fsuuid command to retrieve the UUID of a mounted
> > filesystem.
> >
> > Signed-off-by: Catherine Hoang <catherine.hoang@xxxxxxxxxx>
> > ---
> > spaceman/Makefile | 4 +--
> > spaceman/fsuuid.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++
> > spaceman/init.c | 1 +
> > spaceman/space.h | 1 +
> > 4 files changed, 67 insertions(+), 2 deletions(-)
> > create mode 100644 spaceman/fsuuid.c
> >
> > diff --git a/spaceman/Makefile b/spaceman/Makefile
> > index 1f048d54..901e4e6d 100644
> > --- a/spaceman/Makefile
> > +++ b/spaceman/Makefile
> > @@ -7,10 +7,10 @@ include $(TOPDIR)/include/builddefs
> >
> > LTCOMMAND = xfs_spaceman
> > HFILES = init.h space.h
> > -CFILES = info.c init.c file.c health.c prealloc.c trim.c
> > +CFILES = info.c init.c file.c health.c prealloc.c trim.c fsuuid.c
> > LSRCFILES = xfs_info.sh
> >
> > -LLDLIBS = $(LIBXCMD) $(LIBFROG)
> > +LLDLIBS = $(LIBXCMD) $(LIBFROG) $(LIBUUID)
> > LTDEPENDENCIES = $(LIBXCMD) $(LIBFROG)
> > LLDFLAGS = -static
> >
> > diff --git a/spaceman/fsuuid.c b/spaceman/fsuuid.c
> > new file mode 100644
> > index 00000000..be12c1ad
> > --- /dev/null
> > +++ b/spaceman/fsuuid.c
> > @@ -0,0 +1,63 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (c) 2022 Oracle.
> > + * All Rights Reserved.
> > + */
> > +
> > +#include "libxfs.h"
> > +#include "libfrog/fsgeom.h"
> > +#include "libfrog/paths.h"
> > +#include "command.h"
> > +#include "init.h"
> > +#include "space.h"
> > +#include <sys/ioctl.h>
> > +
> > +#ifndef FS_IOC_GETFSUUID
> > +#define FS_IOC_GETFSUUID _IOR('f', 44, struct fsuuid)
> > +#define UUID_SIZE 16
> > +struct fsuuid {
> > + __u32 fsu_len;
> > + __u32 fsu_flags;
> > + __u8 fsu_uuid[];
>
> This is a flex array ^^ which has no size. struct fsuuid therefore
> has a size of 8 bytes (i.e. enough to cover the two u32 fields) and no
> more. It's assumed that the caller will allocate the memory for
> fsu_uuid...
>
> > +};
> > +#endif
> > +
> > +static cmdinfo_t fsuuid_cmd;
> > +
> > +static int
> > +fsuuid_f(
> > + int argc,
> > + char **argv)
> > +{
> > + struct fsuuid fsuuid;
> > + int error;
>
> ...which makes this usage a problem, because we've not reserved any
> space on the stack to hold the UUID. The kernel will blindly assume
> that there are fsuuid.fsu_len bytes after fsuuid and write to them,
> which will clobber something on the stack.
>
> If you're really unlucky, the C compiler will put the fsuuid right
> before the call frame, which is how stack smashing attacks work. It
> might also lay out bp[] immediately afterwards, which will give you
> weird results as the unparse function overwrites its source buffer. The
> C compiler controls the stack layout, which means this can go bad in
> subtle ways.
>
> Either way, gcc complains about this (albeit in an opaque manner)...
>
> In file included from ../include/xfs.h:9,
> from ../include/libxfs.h:15,
> from fsuuid.c:7:
> In function ‘platform_uuid_unparse’,
> inlined from ‘fsuuid_f’ at fsuuid.c:45:3:
> ../include/xfs/linux.h:100:9: error: ‘uuid_unparse’ reading 16 bytes from a region of size 0 [-Werror=stringop-overread]
> 100 | uuid_unparse(*uu, buffer);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~
> ../include/xfs/linux.h: In function ‘fsuuid_f’:
> ../include/xfs/linux.h:100:9: note: referencing argument 1 of type ‘const unsigned char *’
> In file included from ../include/xfs/linux.h:13,
> from ../include/xfs.h:9,
> from ../include/libxfs.h:15,
> from fsuuid.c:7:
> /usr/include/uuid/uuid.h:107:13: note: in a call to function ‘uuid_unparse’
> 107 | extern void uuid_unparse(const uuid_t uu, char *out);
> | ^~~~~~~~~~~~
> cc1: all warnings being treated as errors
>
> ...so please allocate the struct fsuuid object dynamically.
So, follow common convention and you'll get it wrong, eh? That a
score of -4 on Rusty's API Design scale.
http://sweng.the-davies.net/Home/rustys-api-design-manifesto
Flex arrays in user APIs like this just look plain dangerous to me.
Really, this says that the FSUUID API should have a fixed length
buffer size defined in the API and the length used can be anything
up to the maximum.
We already have this being added for the ioctl API:
#define UUID_SIZE 16
So why isn't the API definition this:
struct fsuuid {
__u32 fsu_len;
__u32 fsu_flags;
__u8 fsu_uuid[UUID_SIZE];
};
Or if we want to support larger ID structures:
#define MAX_FSUUID_SIZE 256
struct fsuuid {
__u32 fsu_len;
__u32 fsu_flags;
__u8 fsu_uuid[MAX_FSUUID_SIZE];
};
Then the structure can be safely placed on the stack, which means
"the obvious use is (probably) the correct one" (a score of 7 on
Rusty's API Design scale). It also gives the kernel a fixed upper
bound that it can use to validate the incoming fsu_len variable
against...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
