On Tue, Jan 5, 2021 at 5:12 PM Dave Wysochanski <dwysocha@xxxxxxxxxx> wrote:
>
> When handling an auth_gss downcall, it's possible to get 0-length
> opaque object for the acceptor. In this case we should make sure
> simple_get_netobj() returns -EFAULT and does not continue to
> kmemdup() which will return a special pointer value of
> ZERO_SIZE_PTR. If it just so happens that the rpcgss_context
> trace event is enabled, the kernel will crash as follows:
>
> [ 162.887992] BUG: kernel NULL pointer dereference, address: 0000000000000010
> [ 162.898693] #PF: supervisor read access in kernel mode
> [ 162.900830] #PF: error_code(0x0000) - not-present page
> [ 162.902940] PGD 0 P4D 0
> [ 162.904027] Oops: 0000 [#1] SMP PTI
> [ 162.905493] CPU: 4 PID: 4321 Comm: rpc.gssd Kdump: loaded Not tainted 5.10.0 #133
> [ 162.908548] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> [ 162.910978] RIP: 0010:strlen+0x0/0x20
> [ 162.912505] Code: 48 89 f9 74 09 48 83 c1 01 80 39 00 75 f7 31 d2 44 0f b6 04 16 44 88 04 11 48 83 c2 01 45 84 c0 75 ee c3 0f 1f 80 00 00 00 00 <80> 3f 00 74 10 48 89 f8 48 83 c0 01 80 38 00 75 f7 48 29 f8 c3 31
> [ 162.920101] RSP: 0018:ffffaec900c77d90 EFLAGS: 00010202
> [ 162.922263] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000000fffde697
> [ 162.925158] RDX: 000000000000002f RSI: 0000000000000080 RDI: 0000000000000010
> [ 162.928073] RBP: 0000000000000010 R08: 0000000000000e10 R09: 0000000000000000
> [ 162.930976] R10: ffff8e698a590cb8 R11: 0000000000000001 R12: 0000000000000e10
> [ 162.933883] R13: 00000000fffde697 R14: 000000010034d517 R15: 0000000000070028
> [ 162.936777] FS: 00007f1e1eb93700(0000) GS:ffff8e6ab7d00000(0000) knlGS:0000000000000000
> [ 162.940067] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 162.942417] CR2: 0000000000000010 CR3: 0000000104eba000 CR4: 00000000000406e0
> [ 162.945300] Call Trace:
> [ 162.946428] trace_event_raw_event_rpcgss_context+0x84/0x140 [auth_rpcgss]
> [ 162.949308] ? __kmalloc_track_caller+0x35/0x5a0
> [ 162.951224] ? gss_pipe_downcall+0x3a3/0x6a0 [auth_rpcgss]
> [ 162.953484] gss_pipe_downcall+0x585/0x6a0 [auth_rpcgss]
> [ 162.955953] rpc_pipe_write+0x58/0x70 [sunrpc]
> [ 162.957849] vfs_write+0xcb/0x2c0
> [ 162.959264] ksys_write+0x68/0xe0
> [ 162.960706] do_syscall_64+0x33/0x40
> [ 162.962238] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 162.964346] RIP: 0033:0x7f1e1f1e57df
>
> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> ---
> include/linux/sunrpc/xdr.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
> index 8ef788ff80b9..aca38bdb0059 100644
> --- a/include/linux/sunrpc/xdr.h
> +++ b/include/linux/sunrpc/xdr.h
> @@ -53,7 +53,7 @@ simple_get_netobj(const void *p, const void *end, struct xdr_netobj *dest)
> if (IS_ERR(p))
> return p;
> q = (const void *)((const char *)p + len);
> - if (unlikely(q > end || q < p))
> + if (unlikely(q > end || q <= p))
> return ERR_PTR(-EFAULT);
> dest->data = kmemdup(p, len, GFP_NOFS);
> if (unlikely(dest->data == NULL))
> --
> 2.25.2
>
Just following up on this as I got no response, but didn't see
it in anyone's tree yet.
After talking offline with Bruce about this, I think the above
approach is wrong and i'm going to do a v2 of this patch.
The short reason is, I don't think simple_get_netobj() should be
returning the -EFAULT in the case of a 0-length acceptor,
which I explain in the cover letter why that is valid as an
object. Rather I think this patch should do a explicit check
for 0 length, and then fill in dest->data = NULL:
diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
index 8ef788ff80b9..b4f5bf104405 100644
--- a/include/linux/sunrpc/xdr.h
+++ b/include/linux/sunrpc/xdr.h
@@ -55,9 +55,12 @@ struct xdr_netobj {
q = (const void *)((const char *)p + len);
if (unlikely(q > end || q < p))
return ERR_PTR(-EFAULT);
- dest->data = kmemdup(p, len, GFP_NOFS);
- if (unlikely(dest->data == NULL))
- return ERR_PTR(-ENOMEM);
+ if (len) {
+ dest->data = kmemdup(p, len, GFP_NOFS);
+ if (unlikely(dest->data == NULL))
+ return ERR_PTR(-ENOMEM);
+ } else
+ dest->data = NULL;
dest->len = len;
return q;
