mtab: handle ENOSPC/EFBIG condition properly when altering mtab

It's possible that when mount.cifs goes to append the mtab that there
won't be enough space to do so, and the mntent won't be appended to the
file in its entirety.

Add a my_endmntent routine that will fflush and then fsync the FILE if
that succeeds. If either fails then it will truncate the file back to
its provided size. It will then call endmntent unconditionally.

Have add_mtab call fstat on the opened mtab file in order to get the
size of the file before it has been appended. Assuming that that
succeeds, use my_endmntent to ensure that the file is not corrupted
before closing it. It's possible that we'll have a small race window
where the mtab is incorrect, but it should be quickly corrected.

This was reported some time ago as CVE-2011-1678:

    http://openwall.com/lists/oss-security/2011/03/04/9

...and it seems to fix the reproducer that I was able to come up with.

Signed-off-by: Jeff Layton <jlayton@samba.org>
Reviewed-by: Suresh Jayaraman <sjayaraman@suse.de>

diff --git a/mount.cifs.c b/mount.cifs.c
index 9d7e10777747e2d2dd65ebd84879f71e5f8ea7a5..107a5a5c5280abe02614cdebdc052b91a13d19d9 100644 (file)
--- a/mount.cifs.c
+++ b/mount.cifs.c
@@ -1428,10 +1428,11 @@ static int check_mtab(const char *progname, const char *devname,
 static int
 add_mtab(char *devname, char *mountpoint, unsigned long flags, const char *fstype)
 {
-       int rc = 0;
+       int rc = 0, tmprc, fd;
        uid_t uid;
        char *mount_user = NULL;
        struct mntent mountent;
+       struct stat statbuf;
        FILE *pmntfile;
        sigset_t mask, oldmask;
 
@@ -1483,6 +1484,23 @@ add_mtab(char *devname, char *mountpoint, unsigned long flags, const char *fstyp
                goto add_mtab_exit;
        }
 
+       fd = fileno(pmntfile);
+       if (fd < 0) {
+               fprintf(stderr, "mntent does not appear to be valid\n");
+               unlock_mtab();
+               rc = EX_FILEIO;
+               goto add_mtab_exit;
+       }
+
+       rc = fstat(fd, &statbuf);
+       if (rc != 0) {
+               fprintf(stderr, "unable to fstat open mtab\n");
+               endmntent(pmntfile);
+               unlock_mtab();
+               rc = EX_FILEIO;
+               goto add_mtab_exit;
+       }
+
        mountent.mnt_fsname = devname;
        mountent.mnt_dir = mountpoint;
        mountent.mnt_type = (char *)(void *)fstype;
@@ -1514,9 +1532,14 @@ add_mtab(char *devname, char *mountpoint, unsigned long flags, const char *fstyp
        rc = addmntent(pmntfile, &mountent);
        if (rc) {
                fprintf(stderr, "unable to add mount entry to mtab\n");
+               ftruncate(fd, statbuf.st_size);
+               rc = EX_FILEIO;
+       }
+       tmprc = my_endmntent(pmntfile, statbuf.st_size);
+       if (tmprc) {
+               fprintf(stderr, "error %d detected on close of mtab\n", tmprc);
                rc = EX_FILEIO;
        }
-       endmntent(pmntfile);
        unlock_mtab();
        SAFE_FREE(mountent.mnt_opts);
 add_mtab_exit:

diff --git a/mount.h b/mount.h
index d49c2ea9f2661459df5cf279aef7da50afe3f304..80bdbe7e329135b38921d1c16a17c241349e6b4c 100644 (file)
--- a/mount.h
+++ b/mount.h
@@ -35,5 +35,6 @@
 extern int mtab_unusable(void);
 extern int lock_mtab(void);
 extern void unlock_mtab(void);
+extern int my_endmntent(FILE *stream, off_t size);
 
 #endif /* ! _MOUNT_H_ */

diff --git a/mtab.c b/mtab.c
index 9cd50d8cf2a5089b5d5dc576044590d3b96488c8..de545b7d2c82990c09396edc54262fddb81cc943 100644 (file)
--- a/mtab.c
+++ b/mtab.c
@@ -251,3 +251,30 @@ lock_mtab (void) {
        return 0;
 }
 
+/*
+ * Call fflush and fsync on the mtab, and then endmntent. If either fflush
+ * or fsync fails, then truncate the file back to "size". endmntent is called
+ * unconditionally, and the errno (if any) from fflush and fsync are returned.
+ */
+int
+my_endmntent(FILE *stream, off_t size)
+{
+       int rc, fd;
+
+       fd = fileno(stream);
+       if (fd < 0)
+               return -EBADF;
+
+       rc = fflush(stream);
+       if (!rc)
+               rc = fsync(fd);
+
+       /* truncate file back to "size" -- best effort here */
+       if (rc) {
+               rc = errno;
+               ftruncate(fd, size);
+       }
+
+       endmntent(stream);
+       return rc;
+}