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

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

[Vitaly Chikunov]

`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.

 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);
+}
-- 
2.33.0

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

[Greg Kurz]

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);
> +}

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

[Christian Schoenebeck]

On Montag, 7. Februar 2022 08:56:07 CET Greg Kurz wrote:
> 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>

Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

I wait a bit to see whether there are still objections by other people.

Philippe maybe?

> >  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);
> > +}

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

[Christian Schoenebeck]

On Sonntag, 6. Februar 2022 02:34:19 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>
> 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.
> 
>  hw/9pfs/codir.c      |  3 +--
>  include/qemu/osdep.h | 13 +++++++++++++
>  util/osdep.c         | 21 +++++++++++++++++++++
>  3 files changed, 35 insertions(+), 2 deletions(-)

Queued on 9p.next:
https://github.com/cschoenebeck/qemu/commits/9p.next

Thanks!

I plan to send a PR with my current queue to Peter tomorrow.

Best regards,
Christian Schoenebeck