[PATCH 1/2] erofs-utils: tar: fix out-of-bounds access when trimming pax path

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

When a PAX extended header contains a path consisting entirely of '/'
characters (e.g., "path=/"), the trailing-slash trimming loop in
tarerofs_parse_pax_header() decrements j to 0, then accesses
eh->path[-1] which is an out-of-bounds heap read.

The tar header path trimming had a similar issue fixed by commit
dcd06f421003 ("erofs-utils: mkfs: tar: fix SIGSEGV on `/` entry"),
but the PAX header path trimming was not addressed.

Add a j > 0 guard to the while condition.

Fixes: 95d315fd7958 ("erofs-utils: introduce tarerofs")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 lib/tar.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/tar.c b/lib/tar.c
index eca29f5..3d92f48 100644
--- a/lib/tar.c
+++ b/lib/tar.c
@@ -509,7 +509,7 @@ int tarerofs_parse_pax_header(struct erofs_iostream *ios,
 				int j = p - 1 - value;
 				free(eh->path);
 				eh->path = strdup(value);
-				while (eh->path[j - 1] == '/')
+				while (j > 0 && eh->path[j - 1] == '/')
 					eh->path[--j] = '\0';
 			} else if (!strncmp(kv, "linkpath=",
 					sizeof("linkpath=") - 1)) {
-- 
2.43.0