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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Donnerstag, 27. Januar 2022 22:27:34 CET Vitaly Chikunov wrote:
> `struct dirent' returned from readdir(3) could be shorter than
> `sizeof(struct dirent)', thus memcpy of sizeof length will overread
> into unallocated page causing SIGSEGV. Example stack trace:

It could be shorter or longer. Your patch is fixing both possibilities.

I just realized BTW that there are other places that should be fixed as well. 
:/

For instance hw/9pfs/9p-synth.c (used for the 9p test cases [1][2]) is copying 
dirents, too.

And in hw/9pfs/9p.c v9fs_do_readdir_with_stat() used by 9p2000.u doesn't even 
bother to copy at all, which is undefined behaviour as fs driver is running on 
a background thread and the dirent pointer might have mutated in the meantime 
with the next readdir() call.

[1] https://wiki.qemu.org/Documentation/9p#Test_Cases
[2] https://wiki.qemu.org/Documentation/9p#9p_Filesystem_Drivers

>  #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)
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841
> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>

Adding qemu-stable on CC for making sure this patch will be handled for the 
stable branches as well.

> ---
> Tested on x86-64 Linux.
> 
>  hw/9pfs/codir.c      |  7 +++++--
>  include/qemu/osdep.h |  6 ++++++
>  util/osdep.c         | 23 +++++++++++++++++++++++
>  3 files changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c
> index 032cce04c4..ea7f5e6578 100644
> --- a/hw/9pfs/codir.c
> +++ b/hw/9pfs/codir.c
> @@ -143,8 +143,11 @@ 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);

That's the actual fix.

> +        if (!e->dent) {
> +            err = -errno;
> +            break;
> +        }

e->dent is never NULL, so this check can be removed. See explanation about 
g_malloc() below.

> 
>          /* 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..b54d22db04 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -805,6 +805,12 @@ static inline int
> platform_does_not_support_system(const char *command) }
>  #endif /* !HAVE_SYSTEM_FUNCTION */
> 
> +/**
> + * Actual 'struct dirent' size may be bigger or shorter than
> + * sizeof(struct dirent) in many cases.
> + */
> +struct dirent *qemu_dirent_dup(struct dirent *dent);
> +

I think this issue deserves a more verbose API doc comment, something like:

/**
 * 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..e39d1071fd 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -33,6 +33,7 @@
>  extern int madvise(char *, size_t, int);
>  #endif
> 
> +#include <dirent.h>

Wouldn't that break Windows builds?

>  #include "qemu-common.h"
>  #include "qemu/cutils.h"
>  #include "qemu/sockets.h"
> @@ -615,3 +616,25 @@ 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)
> +{
> +    struct dirent *dst;
> +#if defined _DIRENT_HAVE_D_RECLEN
> +    /* Avoid use of strlen() if there's d_reclen. */
> +    dst = g_malloc(dent->d_reclen);
> +#else
> +    /* Fallback to a most portable way. */
> +    const size_t reclen = offsetof(struct dirent, d_name) +
> strlen(dent->d_name) + 1; +
> +    dst = g_malloc(reclen);
> +#endif
> +    if (!dst)
> +        return NULL;
> +#ifdef _DIRENT_HAVE_D_RECLEN
> +    return memcpy(dst, dent, dent->d_reclen);
> +#else
> +    return memcpy(dst, dent, reclen);
> +#endif
> +}

On the long run we probably should have a configure check whether d_reclen
exists, but I would not insist on that now as there is a valid fallback
solution at least.

Note that g_malloc() never returns NULL, if it runs out of memory it
terminates instead, so the NULL checks are unncessary:
https://developer.gimp.org/api/2.0/glib/glib-Memory-Allocation.html#g-try-malloc

Also I would prefer g_malloc0() over g_malloc().

Then by adding a variable for the d_reclen yes/no case, overall code can be
reduced. So I would suggest something like this instead:

struct dirent *
qemu_dirent_dup(struct dirent *dent)
{
#if defined _DIRENT_HAVE_D_RECLEN
    /* Avoid use of strlen() if there's d_reclen. */
    const size_t sz = dent->d_reclen;
#else
    /* Fallback to a most portable way. */
    const size_t sz = offsetof(struct dirent, d_name) +
                      strlen(dent->d_name) + 1;
#endif
    struct dirent *dst = g_malloc(sz);
    return memcpy(dst, dent, sz);
}

Best regards,
Christian Schoenebeck