Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

[Greg Kurz <groug@kaod.org>]

On Fri, 4 Feb 2022 19:19:39 +0300
"Dmitry V. Levin" <ldv@altlinux.org> wrote:

> On Fri, Feb 04, 2022 at 02:55:45PM +0100, Philippe Mathieu-Daudé wrote:
> > On 4/2/22 06:06, Vitaly Chikunov wrote:
> > > `struct dirent' returned from readdir(3) could be shorter (or longer)
> > > than `sizeof(struct dirent)', thus memcpy of sizeof length will overread
> > > into unallocated page causing SIGSEGV. Example stack trace:
> > > 
> > >   #0  0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + 0x497eed)
> > >   #1  0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 + 0x4982e9)
> > >   #2  0x0000555555eb7983 coroutine_trampoline (/usr/bin/qemu-system-x86_64 + 0x963983)
> > >   #3  0x00007ffff73e0be0 n/a (n/a + 0x0)
> > > 
> > > While fixing, provide a helper for any future `struct dirent' cloning.
> > > 
> > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841
> > > Cc: qemu-stable@nongnu.org
> > > Co-authored-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> > > ---
> > > Tested on x86-64 Linux again.
> > > 
> > > Changes from v2:
> > > - Make it work with a simulated dirent where d_reclen is 0, which was
> > >    caused abort in readdir qos-test, by using fallback at runtime.
> > > 
> > >   hw/9pfs/codir.c      |  3 +--
> > >   include/qemu/osdep.h | 13 +++++++++++++
> > >   util/osdep.c         | 18 ++++++++++++++++++
> > >   3 files changed, 32 insertions(+), 2 deletions(-)
> > 
> > > +struct dirent *
> > > +qemu_dirent_dup(struct dirent *dent)
> > > +{
> > > +    size_t sz = 0;
> > > +#if defined _DIRENT_HAVE_D_RECLEN
> > > +    /* Avoid use of strlen() if there's d_reclen. */
> > > +    sz = dent->d_reclen;
> > > +#endif
> > > +    if (sz == 0) {
> > 
> > If _DIRENT_HAVE_D_RECLEN is defined, this case is unlikely...
> 
> If it was so easy to be misled this way,
> I'd suggest adding an explicit comment, e.g.
> 
>     /*
>      * Test sz for non-zero even if d_reclen is available
>      * because some drivers may set d_reclen to zero.
>      */
> 

+1

>