Re: [PATCH v4] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

[Greg Kurz <groug@kaod.org>]

On Sun, 6 Feb 2022 04:34:19 +0300
Vitaly Chikunov <vt@altlinux.org> 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>
> Reviewed-by: Dmitry V. Levin <ldv@altlinux.org>
> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> ---
> Tested on x68-64 Linux with btrfs-progs tests and slow qos-test.
> Changes since v3:
> - Update commentary on qemu_dirent_dup logic.
> - Use g_memdup as suggested by Greg Kurz.
> 

The synth driver should really be made compliant with the
_DIRENT_HAVE_D_* macros at some point, but this patch fixes
a crash and the comment is clear enough to understand the
sz == 0 weirdness so I won't object any further.

Acked-by: Greg Kurz <groug@kaod.org>

>  hw/9pfs/codir.c      |  3 +--
>  include/qemu/osdep.h | 13 +++++++++++++
>  util/osdep.c         | 21 +++++++++++++++++++++
>  3 files changed, 35 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..67fbf22778 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 +616,23 @@ 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 platform supports d_reclen. */
> +    sz = dent->d_reclen;
> +#endif
> +    /*
> +     * Test sz for zero even if d_reclen is available
> +     * because some drivers may set d_reclen to zero.
> +     */
> +    if (sz == 0) {
> +        /* Fallback to the most portable way. */
> +        sz = offsetof(struct dirent, d_name) +
> +                      strlen(dent->d_name) + 1;
> +    }
> +    return g_memdup(dent, sz);
> +}