[PATCH] storage: don't read storage volumes in nonblock mode

[Eric Blake <eblake@xxxxxxxxxx>]

Commit 348b4e2 introduced a potential problem (thankfully not
in any release): we are attempting to use virFileReadHeaderFD()
on a file that was opened with O_NONBLOCK.  While this
shouldn't be a problem in practice (because O_NONBLOCK
typically doesn't affect regular or block files, and fifos and
sockets cannot be storage volumes), it's better to play it safe
to avoid races from opening an unexpected file type while also
avoiding problems with having to handle EAGAIN while read()ing.

Based on a report by Dan Berrange.

* src/storage/storage_backend.c
(virStorageBackendVolOpenCheckMode): Fix up fd after avoiding race.

Signed-off-by: Eric Blake <eblake@xxxxxxxxxx>
---
 src/storage/storage_backend.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/storage/storage_backend.c b/src/storage/storage_backend.c
index 57c1728..bde39d6 100644
--- a/src/storage/storage_backend.c
+++ b/src/storage/storage_backend.c
@@ -1179,6 +1179,12 @@ virStorageBackendVolOpenCheckMode(const char *path, struct stat *sb,
         return -2;
     }

+    /* O_NONBLOCK should only matter during open() for fifos and
+     * sockets, which we already filtered; but using it prevents a
+     * TOCTTOU race.  However, later on we will want to read() the
+     * header from this fd, and virFileRead* routines require a
+     * blocking fd, so fix it up after verifying we avoided a
+     * race.  */
     if ((fd = open(path, O_RDONLY|O_NONBLOCK|O_NOCTTY)) < 0) {
         if ((errno == ENOENT || errno == ELOOP) &&
             S_ISLNK(sb->st_mode)) {
@@ -1200,13 +1206,13 @@ virStorageBackendVolOpenCheckMode(const char *path, struct stat *sb,
         return -1;
     }

-    if (S_ISREG(sb->st_mode))
+    if (S_ISREG(sb->st_mode)) {
         mode = VIR_STORAGE_VOL_OPEN_REG;
-    else if (S_ISCHR(sb->st_mode))
+    } else if (S_ISCHR(sb->st_mode)) {
         mode = VIR_STORAGE_VOL_OPEN_CHAR;
-    else if (S_ISBLK(sb->st_mode))
+    } else if (S_ISBLK(sb->st_mode)) {
         mode = VIR_STORAGE_VOL_OPEN_BLOCK;
-    else if (S_ISDIR(sb->st_mode)) {
+    } else if (S_ISDIR(sb->st_mode)) {
         mode = VIR_STORAGE_VOL_OPEN_DIR;

         if (STREQ(base, ".") ||
@@ -1215,6 +1221,17 @@ virStorageBackendVolOpenCheckMode(const char *path, struct stat *sb,
             VIR_INFO("Skipping special dir '%s'", base);
             return -2;
         }
+    } else {
+        VIR_WARN("ignoring unexpected type for file '%s'", path);
+        VIR_FORCE_CLOSE(fd);
+        return -2;
+    }
+
+    if (virSetBlocking(fd, true) < 0) {
+        virReportSystemError(errno, _("unable to set blocking mode for '%s'"),
+                             path);
+        VIR_FORCE_CLOSE(fd);
+        return -2;
     }

     if (!(mode & flags)) {
-- 
1.8.3.1

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