ash: fix use-after-free in bash pattern substitution

function                                             old     new   delta
subevalvar                                          1566    1564      -2

Signed-off-by: Sören Tempel <soeren@soeren-tempel.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 105edd4..55c1034 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -7357,6 +7357,13 @@ subevalvar(char *start, char *str, int strloc,
 				idx = loc;
 			}
 
+			/* The STPUTC invocations above may resize and move the
+			 * stack via realloc(3). Since repl is a pointer into the
+			 * stack, we need to reconstruct it relative to stackblock().
+			 */
+			if (slash_pos >= 0)
+				repl = (char *)stackblock() + strloc + slash_pos + 1;
+
 			//bb_error_msg("repl:'%s'", repl);
 			for (loc = (char*)repl; *loc; loc++) {
 				char *restart_detect = stackblock();