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-sta...@nongnu.org
> > Co-authored-by: Christian Schoenebeck <qemu_...@crudebyte.com>
> > Signed-off-by: Vitaly Chikunov <v...@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.
*/
--
ldv
