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

[Philippe Mathieu-Daudé]

On 5/2/22 12:36, Christian Schoenebeck wrote:
> On Freitag, 4. Februar 2022 06:06:09 CET 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(-)
> >
> > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c
> > index 032cce04c4..c0873bde16 100644
> > --- a/hw/9pfs/codir.c
> > +++ b/hw/9pfs/codir.c
> > @@ -143,8 +143,7 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState
> > *fidp, } else {
> >               e = e->next = g_malloc0(sizeof(V9fsDirEnt));
> >           }
> > -        e->dent = g_malloc0(sizeof(struct dirent));
> > -        memcpy(e->dent, dent, sizeof(struct dirent));
> > +        e->dent = qemu_dirent_dup(dent);
> >
> >           /* perform a full stat() for directory entry if requested by caller
> > */ if (dostat) {
> > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > index d1660d67fa..ce12f64853 100644
> > --- a/include/qemu/osdep.h
> > +++ b/include/qemu/osdep.h
> > @@ -805,6 +805,19 @@ static inline int
> > platform_does_not_support_system(const char *command) }
> >   #endif /* !HAVE_SYSTEM_FUNCTION */
> >
> > +/**
> > + * Duplicate directory entry @dent.
> > + *
> > + * It is highly recommended to use this function instead of open coding
> > + * duplication of @c dirent objects, because the actual @c struct @c dirent
> > + * size may be bigger or shorter than @c sizeof(struct dirent) and correct
> > + * handling is platform specific (see gitlab issue #841).
> > + *
> > + * @dent - original directory entry to be duplicated
> > + * @returns duplicated directory entry which should be freed with g_free()
> > + */
> > +struct dirent *qemu_dirent_dup(struct dirent *dent);
> > +
> >   #ifdef __cplusplus
> >   }
> >   #endif
> > diff --git a/util/osdep.c b/util/osdep.c
> > index 42a0a4986a..2c80528a61 100644
> > --- a/util/osdep.c
> > +++ b/util/osdep.c
> > @@ -33,6 +33,7 @@
> >   extern int madvise(char *, size_t, int);
> >   #endif
> >
> > +#include <dirent.h>
> >   #include "qemu-common.h"
> >   #include "qemu/cutils.h"
> >   #include "qemu/sockets.h"
> > @@ -615,3 +616,20 @@ writev(int fd, const struct iovec *iov, int iov_cnt)
> >       return readv_writev(fd, iov, iov_cnt, true);
> >   }
> >   #endif
> > +
> > +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) {
>
> Philippe, Greg, apart from the additional comment, do you want to see this
> check to be changed in v4 to this?
>
>         if (unlikely(sz == 0)) {

This is not an hot path so probably not very useful.

> Best regards,
> Christian Schoenebeck
>
> > +        /* Fallback to the most portable way. */
> > +        sz = offsetof(struct dirent, d_name) +
> > +                      strlen(dent->d_name) + 1;
> > +    }
> > +    struct dirent *dst = g_malloc(sz);
> > +    return memcpy(dst, dent, sz);

However 'return g_memdup(dent, sz)' is simpler to review.

> > +}