9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

`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>
Acked-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20220206013419.849161-1-vt@altlinux.org>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

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) {

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

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);
+}