Re: [PATCH v5] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

[Vitaly Chikunov]

On Thu, Feb 17, 2022 at 10:26:37AM +0100, Christian Schoenebeck wrote:
> On Mittwoch, 16. Februar 2022 19:18:21 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 this, 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 qos-test -m slow.
> 
> Consider donating your Tested-by tag then.
> 
> > Changes since v4:
> > - Zero clear V9fsSynthOpenState on allocation. Uninitialised use of
> >   d_reclen bug found by fuzzing. Use g_new0 instead of g_malloc0 as
> >   Greg Kurz suggested.
> 
> I can confirm that v4 triggered the previously discussed ASan error when
> running the 9p 'synth' tests; and with this v5 ASan no longer triggers an
> error.
> 
> Tested-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

Tested-by: Vitaly Chikunov <vt@altlinux.org>

> 
> Maybe some of the new comments could be improved, for instance ...
> 
> > 
> >  hw/9pfs/9p-synth.c   | 18 +++++++++++++++---
> >  hw/9pfs/9p-synth.h   |  5 +++++
> >  hw/9pfs/codir.c      |  3 +--
> >  include/qemu/osdep.h | 13 +++++++++++++
> >  util/osdep.c         | 21 +++++++++++++++++++++
> >  5 files changed, 55 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/9pfs/9p-synth.c b/hw/9pfs/9p-synth.c
> > index b38088e066..396cda36f1 100644
> > --- a/hw/9pfs/9p-synth.c
> > +++ b/hw/9pfs/9p-synth.c
> > @@ -182,7 +182,12 @@ static int synth_opendir(FsContext *ctx,
> >      V9fsSynthOpenState *synth_open;
> >      V9fsSynthNode *node = *(V9fsSynthNode **)fs_path->data;
> > 
> > -    synth_open = g_malloc(sizeof(*synth_open));
> > +    /*
> > +     * V9fsSynthOpenState contains 'struct dirent' which have OS-specific
> > +     * properties, thus it's zero cleared on allocation here and below
> > +     * in synth_open.
> > +     */
> > +    synth_open = g_new0(V9fsSynthOpenState, 1);
> >      synth_open->node = node;
> >      node->open_count++;
> >      fs->private = synth_open;
> > @@ -220,7 +225,14 @@ static void synth_rewinddir(FsContext *ctx,
> > V9fsFidOpenState *fs) static void synth_direntry(V9fsSynthNode *node,
> >                                  struct dirent *entry, off_t off)
> >  {
> > -    strcpy(entry->d_name, node->name);
> > +    size_t sz = strlen(node->name) + 1;
> > +    /*
> > +     * 'entry' is always inside of V9fsSynthOpenState which have NAME_MAX
> > +     * back padding. Ensure we do not orerflow it.
> > +     */
> 
> ... here is a typo in the comment. But unless there are more issues, I could
> handle such minor comment issues on my end before pushing.

Ah, thanks!

Vitaly,

> 
> Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> 
> Best regards,
> Christian Schoenebeck
> 
> > +    g_assert(sizeof(struct dirent) + NAME_MAX >=
> > +             offsetof(struct dirent, d_name) + sz);
> > +    memcpy(entry->d_name, node->name, sz);
> >      entry->d_ino = node->attr->inode;
> >      entry->d_off = off + 1;
> >  }
> > @@ -266,7 +278,7 @@ static int synth_open(FsContext *ctx, V9fsPath *fs_path,
> > V9fsSynthOpenState *synth_open;
> >      V9fsSynthNode *node = *(V9fsSynthNode **)fs_path->data;
> > 
> > -    synth_open = g_malloc(sizeof(*synth_open));
> > +    synth_open = g_new0(V9fsSynthOpenState, 1);
> >      synth_open->node = node;
> >      node->open_count++;
> >      fs->private = synth_open;
> > diff --git a/hw/9pfs/9p-synth.h b/hw/9pfs/9p-synth.h
> > index 036d7e4a5b..eeb246f377 100644
> > --- a/hw/9pfs/9p-synth.h
> > +++ b/hw/9pfs/9p-synth.h
> > @@ -41,6 +41,11 @@ typedef struct V9fsSynthOpenState {
> >      off_t offset;
> >      V9fsSynthNode *node;
> >      struct dirent dent;
> > +    /*
> > +     * Ensure there is enough space for 'dent' above, some systems have a
> > +     * d_name size of just 1, which would cause a buffer overrun.
> > +     */
> > +    char dent_trailing_space[NAME_MAX];
> >  } V9fsSynthOpenState;
> > 
> >  int qemu_v9fs_synth_mkdir(V9fsSynthNode *parent, int mode,
> > 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);
> > +}
>