Trap SIGBUS to handle truncated shared memory segments

If a client passes a section of memory via file descriptor and then
subsequently truncates that file, the underlying pages will be freed
and the addresses invalidated. Subsequent accesses to the page will
fail with a SIGBUS error.

Trap that SIGBUS, figure out which segment was causing the error and
then allocate new pages to fill in for that region. Mark the offending
shared segment as invalid and free the resource ID so that the client
will be able to tell when subsequently attempting to use the segment.

Signed-off-by: Keith Packard <keithp@keithp.com>

v2: Use MAP_FIXED to simplify the recovery logic (Mark Kettenis)
v3: Also catch errors in ShmCreateSegment

Conflicts:
	include/dix-config.h.in
	include/xorg-config.h.in

Xext/shm.c

@@ -438,9 +438,11 @@ ShmDetachSegment(pointer value, /* must conform to DeleteType */
     if (--shmdesc->refcnt)
         return TRUE;
 #if SHM_FD_PASSING
-    if (shmdesc->is_fd)
+    if (shmdesc->is_fd) {
+        if (shmdesc->busfault)
+            busfault_unregister(shmdesc->busfault);
         munmap(shmdesc->addr, shmdesc->size);
-    else
+    } else
 #endif
         shmdt(shmdesc->addr);
     for (prev = &Shmsegs; *prev != shmdesc; prev = &(*prev)->next);
@@ -1099,6 +1101,19 @@ ProcShmCreatePixmap(ClientPtr client)
 }
 
 #ifdef SHM_FD_PASSING
+
+static void
+ShmBusfaultNotify(void *context)
+{
+    ShmDescPtr shmdesc = context;
+
+    ErrorF("shared memory 0x%x truncated by client\n",
+           (unsigned int) shmdesc->resource);
+    busfault_unregister(shmdesc->busfault);
+    shmdesc->busfault = NULL;
+    FreeResource (shmdesc->resource, RT_NONE);
+}
+
 static int
 ProcShmAttachFd(ClientPtr client)
 {
@@ -1143,6 +1158,15 @@ ProcShmAttachFd(ClientPtr client)
     shmdesc->refcnt = 1;
     shmdesc->writable = !stuff->readOnly;
     shmdesc->size = statb.st_size;
+    shmdesc->resource = stuff->shmseg;
+
+    shmdesc->busfault = busfault_register_mmap(shmdesc->addr, shmdesc->size, ShmBusfaultNotify, shmdesc);
+    if (!shmdesc->busfault) {
+        munmap(shmdesc->addr, shmdesc->size);
+        free(shmdesc);
+        return BadAlloc;
+    }
+
     shmdesc->next = Shmsegs;
     Shmsegs = shmdesc;
 
@@ -1198,6 +1222,15 @@ ProcShmCreateSegment(ClientPtr client)
     shmdesc->refcnt = 1;
     shmdesc->writable = !stuff->readOnly;
     shmdesc->size = stuff->size;
+
+    shmdesc->busfault = busfault_register_mmap(shmdesc->addr, shmdesc->size, ShmBusfaultNotify, shmdesc);
+    if (!shmdesc->busfault) {
+        close(fd);
+        munmap(shmdesc->addr, shmdesc->size);
+        free(shmdesc);
+        return BadAlloc;
+    }
+
     shmdesc->next = Shmsegs;
     Shmsegs = shmdesc;
 

Xext/shmint.h

@@ -62,6 +62,10 @@ typedef struct _ShmFuncs {
 #define SHM_FD_PASSING  1
 #endif
 
+#ifdef SHM_FD_PASSING
+#include "busfault.h"
+#endif
+
 typedef struct _ShmDesc {
     struct _ShmDesc *next;
     int shmid;
@@ -71,6 +75,7 @@ typedef struct _ShmDesc {
     unsigned long size;
 #ifdef SHM_FD_PASSING
     Bool is_fd;
+    struct busfault *busfault;
     XID resource;
 #endif
 } ShmDescRec, *ShmDescPtr;

configure.ac

@@ -1171,6 +1171,25 @@ case "$DRI3,$HAVE_DRI3PROTO" in
 		;;
 esac
 
+AC_CHECK_FUNCS([sigaction])
+
+BUSFAULT=no
+
+case x"$ac_cv_func_sigaction" in
+	xyes)
+		AC_DEFINE(HAVE_SIGACTION, 1, [Have sigaction function])
+		BUSFAULT=yes
+		;;
+esac
+
+case x"$BUSFAULT" in
+	xyes)
+		AC_DEFINE(BUSFAULT, 1, [Include busfault OS API])
+		;;
+esac
+
+AM_CONDITIONAL(BUSFAULT, test x"$BUSFAULT" = xyes)
+
 PKG_CHECK_MODULES([XSHMFENCE], $XSHMFENCE,
 		  [HAVE_XSHMFENCE=yes], [HAVE_XSHMFENCE=no])
 

include/Makefile.am

@@ -2,6 +2,7 @@ if XORG
 sdk_HEADERS =		\
 	XIstubs.h	\
 	Xprintf.h	\
+	busfault.h	\
 	callback.h	\
 	client.h	\
 	closestr.h	\

include/busfault.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright © 2013 Keith Packard
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting documentation, and
+ * that the name of the copyright holders not be used in advertising or
+ * publicity pertaining to distribution of the software without specific,
+ * written prior permission.  The copyright holders make no representations
+ * about the suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
+ * OF THIS SOFTWARE.
+ */
+
+#ifndef _BUSFAULT_H_
+#define _BUSFAULT_H_
+
+#include <dix-config.h>
+
+#ifdef BUSFAULT
+
+#include <sys/types.h>
+
+typedef void (*busfault_notify_ptr) (void *context);
+
+struct busfault *
+busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void *context);
+
+void
+busfault_unregister(struct busfault *busfault);
+
+void
+busfault_check(void);
+
+Bool
+busfault_init(void);
+
+#endif
+
+#endif /* _BUSFAULT_H_ */

include/dix-config.h.in

@@ -455,4 +455,7 @@
 /* Use XTrans FD passing support */
 #undef XTRANS_SEND_FDS
 
+/* Wrap SIGBUS to catch MIT-SHM faults */
+#undef BUSFAULT
+
 #endif /* _DIX_CONFIG_H_ */

os/Makefile.am

@@ -5,6 +5,7 @@ AM_CFLAGS = $(DIX_CFLAGS) $(SHA1_CFLAGS)
 SECURERPC_SRCS = rpcauth.c
 XDMCP_SRCS = xdmcp.c
 XORG_SRCS = log.c
+BUSFAULT_SRCS = busfault.c
 
 libos_la_SOURCES = 	\
 	WaitFor.c	\
@@ -39,6 +40,10 @@ AM_CFLAGS += $(LIBUNWIND_CFLAGS)
 libos_la_LIBADD += $(LIBUNWIND_LIBS)
 endif
 
+if BUSFAULT
+libos_la_SOURCES += $(BUSFAULT_SRCS)
+endif
+
 EXTRA_DIST = $(SECURERPC_SRCS) $(XDMCP_SRCS)
 
 if SPECIAL_DTRACE_OBJECTS

os/WaitFor.c

@@ -72,6 +72,7 @@ SOFTWARE.
 #ifdef DPMSExtension
 #include "dpmsproc.h"
 #endif
+#include "busfault.h"
 
 #ifdef WIN32
 /* Error codes from windows sockets differ from fileio error codes  */
@@ -162,6 +163,10 @@ WaitForSomething(int *pClientsReady)
         SmartScheduleStopTimer();
     nready = 0;
 
+#ifdef BUSFAULT
+    busfault_check();
+#endif
+
     /* We need a while loop here to handle 
        crashed connections and the screen saver timeout */
     while (1) {

os/busfault.c

@@ -0,0 +1,149 @@
+/*
+ * Copyright © 2013 Keith Packard
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting documentation, and
+ * that the name of the copyright holders not be used in advertising or
+ * publicity pertaining to distribution of the software without specific,
+ * written prior permission.  The copyright holders make no representations
+ * about the suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
+ * OF THIS SOFTWARE.
+ */
+
+#ifdef HAVE_DIX_CONFIG_H
+#include <dix-config.h>
+#endif
+
+#include <X11/Xos.h>
+#include <X11/Xdefs.h>
+#include "misc.h"
+#include <busfault.h>
+#include <list.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <sys/mman.h>
+#include <signal.h>
+
+struct busfault {
+    struct xorg_list    list;
+
+    void                *addr;
+    size_t              size;
+
+    Bool                valid;
+
+    busfault_notify_ptr notify;
+    void                *context;
+};
+
+static Bool             busfaulted;        
+static struct xorg_list busfaults;
+
+struct busfault *
+busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void *context)
+{
+    struct busfault     *busfault;
+
+    busfault = calloc(1, sizeof (struct busfault));
+    if (!busfault)
+        return NULL;
+
+    busfault->addr = addr;
+    busfault->size = size;
+    busfault->notify = notify;
+    busfault->context = context;
+    busfault->valid = TRUE;
+
+    xorg_list_add(&busfault->list, &busfaults);
+    return busfault;
+}
+
+void
+busfault_unregister(struct busfault *busfault)
+{
+    xorg_list_del(&busfault->list);
+    free(busfault);
+}
+
+void
+busfault_check(void)
+{
+    struct busfault     *busfault, *tmp;
+
+    if (!busfaulted)
+        return;
+
+    busfaulted = FALSE;
+
+    xorg_list_for_each_entry_safe(busfault, tmp, &busfaults, list) {
+        if (!busfault->valid)
+            (*busfault->notify)(busfault->context);
+    }
+}
+
+static void (*previous_busfault_sigaction)(int sig, siginfo_t *info, void *param);
+
+static void
+busfault_sigaction(int sig, siginfo_t *info, void *param)
+{
+    void                *fault = info->si_addr;
+    struct busfault     *busfault = NULL;
+    void                *new_addr;
+
+    /* Locate the faulting address in our list of shared segments
+     */
+    xorg_list_for_each_entry(busfault, &busfaults, list) {
+        if ((char *) busfault->addr <= (char *) fault && (char *) fault < (char *) busfault->addr + busfault->size) {
+            break;
+        }
+    }
+    if (!busfault)
+        goto panic;
+
+    if (!busfault->valid)
+        goto panic;
+
+    busfault->valid = FALSE;
+    busfaulted = TRUE;
+
+    /* The client truncated the file; unmap the shared file, map
+     * /dev/zero over that area and keep going
+     */
+
+    new_addr = mmap(busfault->addr, busfault->size, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED, -1, 0);
+
+    if (new_addr == MAP_FAILED)
+        goto panic;
+
+    return;
+panic:
+    if (previous_busfault_sigaction)
+        (*previous_busfault_sigaction)(sig, info, param);
+    else
+        FatalError("bus error");
+}
+
+Bool
+busfault_init(void)
+{
+    struct sigaction    act, old_act;
+
+    act.sa_sigaction = busfault_sigaction;
+    act.sa_flags = SA_SIGINFO;
+    if (sigaction(SIGBUS, &act, &old_act) < 0)
+        return FALSE;
+    previous_busfault_sigaction = old_act.sa_sigaction;
+    xorg_list_init(&busfaults);
+    return TRUE;
+}

os/osinit.c

@@ -149,6 +149,8 @@ OsSigHandler(int signo)
 }
 #endif /* !WIN32 || __CYGWIN__ */
 
+#include "busfault.h"
+
 void
 OsInit(void)
 {
@@ -185,6 +187,9 @@ OsInit(void)
             }
         }
 #endif /* !WIN32 || __CYGWIN__ */
+#ifdef BUSFAULT
+        busfault_init();
+#endif
 
 #ifdef HAVE_BACKTRACE
         /*