Re: [PATCH] dcache: Limit the minimal number of bucket to two

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2026/1/30 11:48, Zhihao Cheng 写道:
friendly ping...
> 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>
> ---
>   fs/dcache.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 66dd1bb830d1..957a44d2c44a 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -3260,7 +3260,7 @@ static void __init dcache_init_early(void)
>   					HASH_EARLY | HASH_ZERO,
>   					&d_hash_shift,
>   					NULL,
> -					0,
> +					2,
>   					0);
>   	d_hash_shift = 32 - d_hash_shift;
>   
> @@ -3292,7 +3292,7 @@ static void __init dcache_init(void)
>   					HASH_ZERO,
>   					&d_hash_shift,
>   					NULL,
> -					0,
> +					2,
>   					0);
>   	d_hash_shift = 32 - d_hash_shift;
>   
>