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

[Philippe Mathieu-Daudé]

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...

> +        /* 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);
> +}

What about this?

struct dirent *
qemu_dirent_dup(struct dirent *dent)
{
    size_t sz;

#if defined _DIRENT_HAVE_D_RECLEN
    /* Avoid use of strlen() if there's d_reclen. */
    sz = dent->d_reclen;
#else
    /* Fallback to the most portable way. */
    sz = offsetof(struct dirent, d_name) +
                  strlen(dent->d_name) + 1;
#endif

    return g_memdup(dent, sz);
}