[PATCH 3/3] perf tools: Fix use-after-free in help_unknown_cmd()

[Namhyung Kim <namhyung@kernel.org>]

Currently perf aborts when it finds an invalid command.  I guess it
depends on the environment as I have some custom commands in the path.

  $ perf bad-command
  perf: 'bad-command' is not a perf-command. See 'perf --help'.
  Aborted (core dumped)

It's because the exclude_cmds() in libsubcmd has a use-after-free when
it removes some entries.  After copying one to another entry, it keeps
the pointer in the both position.  And the next copy operation will free
the later one but it's the same entry in the previous one.

For example, let's say cmds = { A, B, C, D, E } and excludes = { B, E }.

  ci  cj  ei   cmds-name  excludes
  -----------+--------------------
   0   0   0 |     A         B       :    cmp < 0, ci == cj
   1   1   0 |     B         B       :    cmp == 0
   2   1   1 |     C         E       :    cmp < 0, ci != cj

At this point, it frees cmds->names[1] and cmds->names[1] is assigned to
cmds->names[2].

   3   2   1 |     D         E       :    cmp < 0, ci != cj

Now it frees cmds->names[2] but it's the same as cmds->names[1].  So
accessing cmds->names[1] will be invalid.

This makes the subcmd tests succeed.

  $ perf test subcmd
   69: libsubcmd help tests                                            :
   69.1: Load subcmd names                                             : Ok
   69.2: Uniquify subcmd names                                         : Ok
   69.3: Exclude duplicate subcmd names                                : Ok

Fixes: 657a3efee43a ("libsubcmd: Avoid SEGV/use-after-free when commands aren't excluded")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/lib/subcmd/help.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/tools/lib/subcmd/help.c b/tools/lib/subcmd/help.c
index 8561b0f01a247690..9ef569492560efd7 100644
--- a/tools/lib/subcmd/help.c
+++ b/tools/lib/subcmd/help.c
@@ -9,6 +9,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 #include <dirent.h>
+#include <assert.h>
 #include "subcmd-util.h"
 #include "help.h"
 #include "exec-cmd.h"
@@ -82,10 +83,11 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
 				ci++;
 				cj++;
 			} else {
-				zfree(&cmds->names[cj]);
-				cmds->names[cj++] = cmds->names[ci++];
+				cmds->names[cj++] = cmds->names[ci];
+				cmds->names[ci++] = NULL;
 			}
 		} else if (cmp == 0) {
+			zfree(&cmds->names[ci]);
 			ci++;
 			ei++;
 		} else if (cmp > 0) {
@@ -94,12 +96,12 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
 	}
 	if (ci != cj) {
 		while (ci < cmds->cnt) {
-			zfree(&cmds->names[cj]);
-			cmds->names[cj++] = cmds->names[ci++];
+			cmds->names[cj++] = cmds->names[ci];
+			cmds->names[ci++] = NULL;
 		}
 	}
 	for (ci = cj; ci < cmds->cnt; ci++)
-		zfree(&cmds->names[ci]);
+		assert(cmds->names[ci] == NULL);
 	cmds->cnt = cj;
 }
 
-- 
2.50.0.727.gbf7dc18ff4-goog