Re: [PATCH v2] nfsd: clear acl_access/acl_default after releasing them

[Jeff Layton <jlayton@kernel.org>]

On Sun, 2025-01-26 at 17:47 +0800, Li Lingfeng wrote:
> If getting acl_default fails, acl_access and acl_default will be released
> simultaneously. However, acl_access will still retain a pointer pointing
> to the released posix_acl, which will trigger a WARNING in
> nfs3svc_release_getacl like this:
> 
> ------------[ cut here ]------------
> refcount_t: underflow; use-after-free.
> WARNING: CPU: 26 PID: 3199 at lib/refcount.c:28
> refcount_warn_saturate+0xb5/0x170
> Modules linked in:
> CPU: 26 UID: 0 PID: 3199 Comm: nfsd Not tainted
> 6.12.0-rc6-00079-g04ae226af01f-dirty #8
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.16.1-2.fc37 04/01/2014
> RIP: 0010:refcount_warn_saturate+0xb5/0x170
> Code: cc cc 0f b6 1d b3 20 a5 03 80 fb 01 0f 87 65 48 d8 00 83 e3 01 75
> e4 48 c7 c7 c0 3b 9b 85 c6 05 97 20 a5 03 01 e8 fb 3e 30 ff <0f> 0b eb
> cd 0f b6 1d 8a3
> RSP: 0018:ffffc90008637cd8 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff83904fde
> RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88871ed36380
> RBP: ffff888158beeb40 R08: 0000000000000001 R09: fffff520010c6f56
> R10: ffffc90008637ab7 R11: 0000000000000001 R12: 0000000000000001
> R13: ffff888140e77400 R14: ffff888140e77408 R15: ffffffff858b42c0
> FS:  0000000000000000(0000) GS:ffff88871ed00000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000562384d32158 CR3: 000000055cc6a000 CR4: 00000000000006f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  ? refcount_warn_saturate+0xb5/0x170
>  ? __warn+0xa5/0x140
>  ? refcount_warn_saturate+0xb5/0x170
>  ? report_bug+0x1b1/0x1e0
>  ? handle_bug+0x53/0xa0
>  ? exc_invalid_op+0x17/0x40
>  ? asm_exc_invalid_op+0x1a/0x20
>  ? tick_nohz_tick_stopped+0x1e/0x40
>  ? refcount_warn_saturate+0xb5/0x170
>  ? refcount_warn_saturate+0xb5/0x170
>  nfs3svc_release_getacl+0xc9/0xe0
>  svc_process_common+0x5db/0xb60
>  ? __pfx_svc_process_common+0x10/0x10
>  ? __rcu_read_unlock+0x69/0xa0
>  ? __pfx_nfsd_dispatch+0x10/0x10
>  ? svc_xprt_received+0xa1/0x120
>  ? xdr_init_decode+0x11d/0x190
>  svc_process+0x2a7/0x330
>  svc_handle_xprt+0x69d/0x940
>  svc_recv+0x180/0x2d0
>  nfsd+0x168/0x200
>  ? __pfx_nfsd+0x10/0x10
>  kthread+0x1a2/0x1e0
>  ? kthread+0xf4/0x1e0
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork+0x34/0x60
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork_asm+0x1a/0x30
>  </TASK>
> Kernel panic - not syncing: kernel: panic_on_warn set ...
> 
> Clear acl_access/acl_default after posix_acl_release is called to prevent
> UAF from being triggered.
> 
> Fixes: a257cdd0e217 ("[PATCH] NFSD: Add server support for NFSv3 ACLs.")
> Link: https://lore.kernel.org/all/20241107014705.2509463-1-lilingfeng@huaweicloud.com/
> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
> ---
> Changes in v2:
> - Clear acl_access/acl_default after releasing them, instead of
>   modifying the logic for setting them.
> - Clear acl_access/acl_default in nfs2acl.c.
> ---
>  fs/nfsd/nfs2acl.c | 2 ++
>  fs/nfsd/nfs3acl.c | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/fs/nfsd/nfs2acl.c b/fs/nfsd/nfs2acl.c
> index 4e3be7201b1c..5fb202acb0fd 100644
> --- a/fs/nfsd/nfs2acl.c
> +++ b/fs/nfsd/nfs2acl.c
> @@ -84,6 +84,8 @@ static __be32 nfsacld_proc_getacl(struct svc_rqst *rqstp)
>  fail:
>  	posix_acl_release(resp->acl_access);
>  	posix_acl_release(resp->acl_default);
> +	resp->acl_access = NULL;
> +	resp->acl_default = NULL;
>  	goto out;
>  }
>  
> diff --git a/fs/nfsd/nfs3acl.c b/fs/nfsd/nfs3acl.c
> index 5e34e98db969..7b5433bd3019 100644
> --- a/fs/nfsd/nfs3acl.c
> +++ b/fs/nfsd/nfs3acl.c
> @@ -76,6 +76,8 @@ static __be32 nfsd3_proc_getacl(struct svc_rqst *rqstp)
>  fail:
>  	posix_acl_release(resp->acl_access);
>  	posix_acl_release(resp->acl_default);
> +	resp->acl_access = NULL;
> +	resp->acl_default = NULL;
>  	goto out;
>  }
>  

Nice catch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>