[libvirt] [PATCH] virt-aa-helper: Improve valid_path

[Michal Privoznik]

So, after some movement in virt-aa-helper, I've noticed the
virt-aa-helper-test failing. I've ran gdb (it took me a while to
realize how to do that) and this showed up immediately:

  Program received signal SIGSEGV, Segmentation fault.
  strlen () at ../sysdeps/x86_64/strlen.S:106
  106     ../sysdeps/x86_64/strlen.S: No such file or directory.
  (gdb) bt
  #0  strlen () at ../sysdeps/x86_64/strlen.S:106
  #1  0x0000555555561a13 in array_starts_with (str=0x5555557ce910 "/tmp/tmp.6nI2Fkv0KL/1.img", arr=0x7fffffffd160, size=-1540438016) at security/virt-aa-helper.c:525
  #2  0x0000555555561d49 in valid_path (path=0x5555557ce910 "/tmp/tmp.6nI2Fkv0KL/1.img", readonly=false) at security/virt-aa-helper.c:617
  #3  0x0000555555562506 in vah_add_path (buf=0x7fffffffd3e0, path=0x5555557cb910 "/tmp/tmp.6nI2Fkv0KL/1.img", perms=0x555555581585 "rw", recursive=false) at security/virt-aa-helper.c:823
  #4  0x0000555555562693 in vah_add_file (buf=0x7fffffffd3e0, path=0x5555557cb910 "/tmp/tmp.6nI2Fkv0KL/1.img", perms=0x555555581585 "rw") at security/virt-aa-helper.c:854
  #5  0x0000555555562918 in add_file_path (disk=0x5555557d4440, path=0x5555557cb910 "/tmp/tmp.6nI2Fkv0KL/1.img", depth=0, opaque=0x7fffffffd3e0) at security/virt-aa-helper.c:931
  #6  0x00007ffff78f18b1 in virDomainDiskDefForeachPath (disk=0x5555557d4440, ignoreOpenFailure=true, iter=0x5555555628a6 <add_file_path>, opaque=0x7fffffffd3e0) at conf/domain_conf.c:23286
  #7  0x0000555555562b5f in get_files (ctl=0x7fffffffd670) at security/virt-aa-helper.c:982
  #8  0x0000555555564100 in vahParseArgv (ctl=0x7fffffffd670, argc=5, argv=0x7fffffffd7e8) at security/virt-aa-helper.c:1277
  #9  0x00005555555643d6 in main (argc=5, argv=0x7fffffffd7e8) at security/virt-aa-helper.c:1332

So I've taken look at valid_path() because it is obviously
calling array_starts_with() with malformed @size. And here's the
result: there are two variables to hold the size of three arrays
and their value is recalculated before each call of
array_starts_with(). What if we just use three variables,
initialize them and do not touch them afterwards?

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 src/security/virt-aa-helper.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/src/security/virt-aa-helper.c b/src/security/virt-aa-helper.c
index a78c4c8..b4a8f27 100644
--- a/src/security/virt-aa-helper.c
+++ b/src/security/virt-aa-helper.c
@@ -546,9 +546,6 @@ array_starts_with(const char *str, const char * const *arr, const long size)
 static int
 valid_path(const char *path, const bool readonly)
 {
-    int npaths;
-    int nropaths;
-
     const char * const restricted[] = {
         "/bin/",
         "/etc/",
@@ -581,6 +578,10 @@ valid_path(const char *path, const bool readonly)
         "/etc/libvirt-sandbox/services/" /* for virt-sandbox service config */
     };
 
+    const int nropaths = ARRAY_CARDINALITY(restricted);
+    const int nrwpaths = ARRAY_CARDINALITY(restricted_rw);
+    const int nopaths = ARRAY_CARDINALITY(override);
+
     if (path == NULL) {
         vah_error(NULL, 0, _("bad pathname"));
         return -1;
@@ -600,21 +601,18 @@ valid_path(const char *path, const bool readonly)
         vah_warning(_("path does not exist, skipping file type checks"));
 
     /* overrides are always allowed */
-    npaths = sizeof(override)/sizeof(*(override));
-    if (array_starts_with(path, override, npaths) == 0)
+    if (array_starts_with(path, override, nopaths) == 0)
         return 0;
 
     /* allow read only paths upfront */
     if (readonly) {
-        nropaths = sizeof(restricted_rw)/sizeof(*(restricted_rw));
-        if (array_starts_with(path, restricted_rw, nropaths) == 0)
+        if (array_starts_with(path, restricted_rw, nrwpaths) == 0)
             return 0;
     }
 
     /* disallow RW acess to all paths in restricted and restriced_rw */
-    npaths = sizeof(restricted)/sizeof(*(restricted));
-    if ((array_starts_with(path, restricted, npaths) == 0
-        || array_starts_with(path, restricted_rw, nropaths) == 0))
+    if ((array_starts_with(path, restricted, nropaths) == 0 ||
+         array_starts_with(path, restricted_rw, nrwpaths) == 0))
         return 1;
 
     return 0;
-- 
2.4.6