[PATCH 6.6 096/166] dcache: Limit the minimal number of bucket to two

Greg Kroah-Hartman
2026-04-24T13:37:34Z

From: Zhihao Cheng <chengzhihao1@huawei.com>

commit f08fe8891c3eeb63b73f9f1f6d97aa629c821579 upstream.

There is an OOB read problem on dentry_hashtable when user sets
'dhash_entries=1':
BUG: unable to handle page fault for address: ffff888b30b774b0
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
Oops: Oops: 0000 [#1] SMP PTI
RIP: 0010:__d_lookup+0x56/0x120
Call Trace:
d_lookup.cold+0x16/0x5d
lookup_dcache+0x27/0xf0
lookup_one_qstr_excl+0x2a/0x180
start_dirop+0x55/0xa0
simple_start_creating+0x8d/0xa0
debugfs_start_creating+0x8c/0x180
debugfs_create_dir+0x1d/0x1c0
pinctrl_init+0x6d/0x140
do_one_initcall+0x6d/0x3d0
kernel_init_freeable+0x39f/0x460
kernel_init+0x2a/0x260

There will be only one bucket in dentry_hashtable when dhash_entries is
set as one, and d_hash_shift is calculated as 32 by dcache_init(). Then,
following process will access more than one buckets(which memory region
is not allocated) in dentry_hashtable:
d_lookup
b = d_hash(hash)
dentry_hashtable + ((u32)hashlen >> d_hash_shift)
// The C standard defines the behavior of right shift amounts
// exceeding the bit width of the operand as undefined. The
// result of '(u32)hashlen >> d_hash_shift' becomes 'hashlen',
// so 'b' will point to an unallocated memory region.
hlist_bl_for_each_entry_rcu(b)
hlist_bl_first_rcu(head)
h->first // read OOB!

Fix it by limiting the minimal number of dentry_hashtable bucket to two,
so that 'd_hash_shift' won't exceeds the bit width of type u32.

Cc: stable@vger.kernel.org
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://patch.msgid.link/20260130034853.215819-1-chengzhihao1@huawei.com
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/dcache.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3296,7 +3296,7 @@ static void __init dcache_init_early(voi
HASH_EARLY | HASH_ZERO,
&d_hash_shift,
NULL,
- 0,
+ 2,
0);
d_hash_shift = 32 - d_hash_shift;
}
@@ -3324,7 +3324,7 @@ static void __init dcache_init(void)
HASH_ZERO,
&d_hash_shift,
NULL,
- 0,
+ 2,
0);
d_hash_shift = 32 - d_hash_shift;
}