jail: fs: don't overwrite existing mount target

Using the creat() function overwrites existing files which is
unintended when it comes to making sure the target of a single-file
mount exists. Instead, use open() with the O_EXCL flag to make sure
mount targets are only created if actually needed.

While at it also clean up various error paths of the do_mounts
function, making sure the additionally allocated string being created
for the path inside the jail's root filesystem is always freed
and also making it a bit more readable and less bloated.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

diff --git a/jail/fs.c b/jail/fs.c
index c14027b5ae948454aa247bb8372e72314aa81ddf..423878b3398f1424ec823a75882de99e874243d6 100644 (file)
--- a/jail/fs.c
+++ b/jail/fs.c
@@ -61,20 +61,22 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
        struct stat s;
        char new[PATH_MAX];
        char *source = (char *)orig_source;
-       int fd;
+       int fd, ret = 0;
        bool is_bind = (orig_mountflags & MS_BIND);
        bool is_mask = (source == (void *)(-1));
        unsigned long mountflags = orig_mountflags;
 
+       assert(!(inner && is_mask));
+       assert(!(inner && !orig_source));
+
        if (source && is_bind && stat(source, &s)) {
                ERROR("stat(%s) failed: %m\n", source);
                return error;
        }
 
-       if (!is_mask && orig_source && inner) {
+       if (inner)
                if (asprintf(&source, "%s%s", root, orig_source) < 0)
                        return ENOMEM;
-       }
 
        snprintf(new, sizeof(new), "%s%s", root, target?target:source);
 
@@ -83,7 +85,7 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
                        return 0; /* doesn't exists, nothing to mask */
 
                if (S_ISDIR(s.st_mode)) {/* use empty 0-sized tmpfs for directories */
-                       if (mount(NULL, new, "tmpfs", MS_RDONLY | MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_NOATIME, "size=0,mode=000"))
+                       if (mount("none", new, "tmpfs", MS_RDONLY | MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_NOATIME, "size=0,mode=000"))
                                return error;
                } else {
                        /* mount-bind 0-sized file having mode 000 */
@@ -104,12 +106,16 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
        } else if (is_bind && source) {
                mkdir_p(dirname(new), 0755);
                snprintf(new, sizeof(new), "%s%s", root, target?target:source);
-               fd = creat(new, 0644);
-               if (fd < 0) {
-                       ERROR("creat(%s) failed: %m\n", new);
-                       return error;
+               fd = open(new, O_CREAT|O_WRONLY|O_TRUNC|O_EXCL, 0644);
+               if (fd >= 0)
+                       close(fd);
+
+               if (error && fd < 0 && errno != EEXIST) {
+                       ERROR("failed to create mount target %s: %m\n", new);
+
+                       ret = errno;
+                       goto free_source_out;
                }
-               close(fd);
        }
 
        if (is_bind) {
@@ -117,10 +123,8 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
                        if (error)
                                ERROR("failed to mount -B %s %s: %m\n", source, new);
 
-                       if (inner)
-                               free(source);
-
-                       return error;
+                       ret = error;
+                       goto free_source_out;
                }
                mountflags |= MS_REMOUNT;
        }
@@ -130,10 +134,8 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
                if (error)
                        ERROR("failed to mount %s %s: %m\n", source, new);
 
-               if (inner)
-                       free(source);
-
-               return error;
+               ret = error;
+               goto free_source_out;
        }
 
        DEBUG("mount %s%s %s (%s)\n", (mountflags & MS_BIND)?"-B ":"", source, new,
@@ -143,16 +145,14 @@ static int do_mount(const char *root, const char *orig_source, const char *targe
                if (error)
                        ERROR("failed to mount --make-... %s \n", new);
 
-               if (inner)
-                       free(source);
-
-               return error;
+               ret = error;
        }
 
+free_source_out:
        if (inner)
                free(source);
 
-       return 0;
+       return ret;
 }
 
 static int _add_mount(const char *source, const char *target, const char *filesystemtype,