[PATCH] bcachefs: fix use-after-free in bch2_dirent_to_text()

[Nirbhay Sharma]

Add bounds checking before calculating pointer offsets in dirent name
accessor functions to prevent out-of-bounds memory access when
processing corrupted filesytem metadata.

When d_name_len contains a corrupted value, the pointer calculation
&d.v->d_cf_name_block.d_names[name_len] results in an offset far
outside the dirent structure, triggering KASAN use-after-free erors.

While bch2_dirent_validate() detects such corruption,
bch2_dirent_to_text() may still be called for debug output, so the
accessor functions must handle invalid data gracefully.

Fixes: c21f41f6905be4fc5059a10a5bba94105ba87269 ("bcachefs: 
bch2_dirent_to_text() shows casefolded dirents")
Reported-by: syzbot+7f176adb30b21606c...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7f176adb30b21606c5fc
Signed-off-by: Nirbhay Sharma <nirbhay....@gmail.com>
---
 fs/bcachefs/dirent.c | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/fs/bcachefs/dirent.c b/fs/bcachefs/dirent.c
index d198001838f3..8be31b41c32b 100644
--- a/fs/bcachefs/dirent.c
+++ b/fs/bcachefs/dirent.c
@@ -58,8 +58,16 @@ static unsigned bch2_dirent_name_bytes(struct 
bkey_s_c_dirent d)
 
 struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
 {
+       unsigned int name_len, max_len;
+
        if (d.v->d_casefold) {
-               unsigned name_len = 
le16_to_cpu(d.v->d_cf_name_block.d_name_len);
+               name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
+               max_len = bkey_val_bytes(d.k) -
+                       offsetof(struct bch_dirent, d_cf_name_block.d_names);
+
+               if (name_len > max_len)
+                       return (struct qstr) QSTR_INIT(NULL, 0);
+
                return (struct qstr) 
QSTR_INIT(&d.v->d_cf_name_block.d_names[0], name_len);
        } else {
                return (struct qstr) QSTR_INIT(d.v->d_name, 
bch2_dirent_name_bytes(d));
@@ -68,13 +76,19 @@ struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
 
 static struct qstr bch2_dirent_get_casefold_name(struct bkey_s_c_dirent d)
 {
-       if (d.v->d_casefold) {
-               unsigned name_len = 
le16_to_cpu(d.v->d_cf_name_block.d_name_len);
-               unsigned cf_name_len = 
le16_to_cpu(d.v->d_cf_name_block.d_cf_name_len);
-               return (struct qstr) 
QSTR_INIT(&d.v->d_cf_name_block.d_names[name_len], cf_name_len);
-       } else {
+       unsigned int name_len, cf_name_len, max_len;
+
+       if (!d.v->d_casefold)
                return (struct qstr) QSTR_INIT(NULL, 0);
-       }
+
+       name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
+       cf_name_len = le16_to_cpu(d.v->d_cf_name_block.d_cf_name_len);
+       max_len = bkey_val_bytes(d.k) - offsetof(struct bch_dirent, 
d_cf_name_block.d_names);
+
+       if (name_len > max_len || cf_name_len > max_len || name_len + 
cf_name_len > max_len)
+               return (struct qstr) QSTR_INIT(NULL, 0);
+
+       return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[name_len], 
cf_name_len);
 }
 
 static inline struct qstr bch2_dirent_get_lookup_name(struct bkey_s_c_dirent d)
@@ -212,11 +226,18 @@ void bch2_dirent_to_text(struct printbuf *out, struct 
bch_fs *c, struct bkey_s_c
        struct bkey_s_c_dirent d = bkey_s_c_to_dirent(k);
        struct qstr d_name = bch2_dirent_get_name(d);
 
+       if (!d_name.name || !d_name.len) {
+               prt_str(out, "(invalid)");
+               return;
+       }
+
        prt_printf(out, "%.*s", d_name.len, d_name.name);
 
        if (d.v->d_casefold) {
-               struct qstr d_name = bch2_dirent_get_lookup_name(d);
-               prt_printf(out, " (casefold %.*s)", d_name.len, d_name.name);
+               struct qstr d_cf_name = bch2_dirent_get_lookup_name(d);
+
+               if (d_cf_name.name && d_cf_name.len)
+                       prt_printf(out, " (casefold %.*s)", d_name.len, 
d_name.name);
        }
 
        prt_str(out, " ->");
-- 
2.51.0