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Re: [Xen-devel] [PATCH 4/5] xen/arm: optee: handle share buffer translation error
Hi Volodymyr,
On 9/11/19 7:32 PM, Volodymyr Babchuk wrote:
Julien Grall writes:
Hi Volodymyr,
On 8/23/19 7:48 PM, Volodymyr Babchuk wrote:
There is a case possible, when OP-TEE asks guest to allocate shared
buffer, but Xen for some reason can't translate buffer's addresses. In
this situation we should do two things:
1. Tell guest to free allocated buffer, so there will be no memory
leak for guest.
2. Tell OP-TEE that buffer allocation failed.
To ask guest to free allocated buffer we should perform the same
thing, as OP-TEE does - issue RPC request. This is done by filling
request buffer (luckily we can reuse the same buffer, that OP-TEE used
to issue original request) and then return to guest with special
return code.
Then we need to handle next call from guest in a special way: as RPC
was issued by Xen, not by OP-TEE, it should be handled by Xen.
Basically, this is the mechanism to preempt OP-TEE mediator.
The same mechanism can be used in the future to preempt mediator
during translation large (>512 pages) shared buffers.
Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
---
xen/arch/arm/tee/optee.c | 167 +++++++++++++++++++++++++++++++--------
1 file changed, 136 insertions(+), 31 deletions(-)
diff --git a/xen/arch/arm/tee/optee.c b/xen/arch/arm/tee/optee.c
index 3ce6e7fa55..4eebc60b62 100644
--- a/xen/arch/arm/tee/optee.c
+++ b/xen/arch/arm/tee/optee.c
@@ -96,6 +96,11 @@
OPTEE_SMC_SEC_CAP_UNREGISTERED_SHM | \
OPTEE_SMC_SEC_CAP_DYNAMIC_SHM)
+enum optee_call_state {
+ OPTEEM_CALL_NORMAL = 0,
enum always start counting at 0. Also, looking at the code, it does
not seem you need to know the value. Right?
Yep. This is a bad habit. Will remove.
+ OPTEEM_CALL_XEN_RPC,
I am a bit confused, the enum is called optee_call_state but all the
enum are prefixed with OPTEEM_CALL_. Why the discrepancy?
Because I'm bad at naming things :)
OPTEEM_CALL_STATE_XEN_RPC looks too long. But you are right, so I'll
rename the enum values. Unless, you have a better idea for this.
My point was not about adding _STATE to the enum values but the fact you
call the enum optee but the value OPTEEM (note the extra M in the later).
So my only request here is to call the enum opteem_call_state or prefix
all the enum value with OPTEE.
+};
+
static unsigned int __read_mostly max_optee_threads;
/*
@@ -112,6 +117,9 @@ struct optee_std_call {
paddr_t guest_arg_ipa;
int optee_thread_id;
int rpc_op;
+ /* Saved buffer type for the last buffer allocate request */
Looking at the code, it feels to me you are saving the buffer type for
the current command and not the last. Did I miss anything?
Yes, right. Will rename.
+ unsigned int rpc_buffer_type;
+ enum optee_call_state state;
uint64_t rpc_data_cookie;
bool in_flight;
register_t rpc_params[2];
@@ -299,6 +307,7 @@ static struct optee_std_call *allocate_std_call(struct
optee_domain *ctx)
call->optee_thread_id = -1;
call->in_flight = true;
+ call->state = OPTEEM_CALL_NORMAL;
spin_lock(&ctx->lock);
list_add_tail(&call->list, &ctx->call_list);
@@ -1075,6 +1084,10 @@ static int handle_rpc_return(struct optee_domain *ctx,
ret = -ERESTART;
}
+ /* Save the buffer type in case we will want to free it
*/
+ if ( shm_rpc->xen_arg->cmd == OPTEE_RPC_CMD_SHM_ALLOC )
+ call->rpc_buffer_type = shm_rpc->xen_arg->params[0].u.value.a;
+
unmap_domain_page(shm_rpc->xen_arg);
}
@@ -1239,18 +1252,102 @@ err:
return;
}
+/*
+ * Prepare RPC request to free shared buffer in the same way, as
+ * OP-TEE does this.
+ *
+ * Return values:
+ * true - successfully prepared RPC request
+ * false - there was an error
+ */
+static bool issue_rpc_cmd_free(struct optee_domain *ctx,
+ struct cpu_user_regs *regs,
+ struct optee_std_call *call,
+ struct shm_rpc *shm_rpc,
+ uint64_t cookie)
+{
+ register_t r1, r2;
+
+ /* In case if guest will forget to update it with meaningful value */
+ shm_rpc->xen_arg->ret = TEEC_ERROR_GENERIC;
+ shm_rpc->xen_arg->cmd = OPTEE_RPC_CMD_SHM_FREE;
+ shm_rpc->xen_arg->num_params = 1;
+ shm_rpc->xen_arg->params[0].attr = OPTEE_MSG_ATTR_TYPE_VALUE_INPUT;
+ shm_rpc->xen_arg->params[0].u.value.a = call->rpc_buffer_type;
+ shm_rpc->xen_arg->params[0].u.value.b = cookie;
+
+ if ( access_guest_memory_by_ipa(current->domain,
+ gfn_to_gaddr(shm_rpc->gfn),
+ shm_rpc->xen_arg,
+ OPTEE_MSG_GET_ARG_SIZE(1),
+ true) )
+ {
+ /*
+ * Well, this is quite bad. We have error in error path.
+ * This can happen only if guest behaves badly, so all
+ * we can do is to return error to OP-TEE and leave
+ * guest's memory leaked.
Could you expand a bit more what you mean by "guest's memory leaked"?
There will be memory leak somewhere in the guest. Yes, looks
like it is misleading...
What I mean, is that OP-TEE requests guest to allocate some
memory. Guest does not know, when OP-TEE finishes using this memory, so
guest will free the memory only by OP-TEE's request. We can't emulate
this request in current circumstances, so guest will keep part of own
memory reserved for OP-TEE infinitely.
What the state of the page from Xen PoV?
From Xen point of view all will be perfectly fine.
I.e. is there any reference
taken by the OP-TEE mediator? Will the page be freed once the guest is
destroyed?...
As I said, it has nothing to do with the page as Xen it sees. Mediator
will call put_page() prior to entering this function. So, no Xen
resources are used.
It makes sense, Thank you for the explanation. Please update the comment
accordingly.
+ */
+ shm_rpc->xen_arg->ret = TEEC_ERROR_GENERIC;
+ shm_rpc->xen_arg->num_params = 0;
+
+ return false;
+ }
+
+ uint64_to_regpair(&r1, &r2, shm_rpc->cookie);
+
+ call->state = OPTEEM_CALL_XEN_RPC;
+ call->rpc_op = OPTEE_SMC_RPC_FUNC_CMD;
+ call->rpc_params[0] = r1;
+ call->rpc_params[1] = r2;
+ call->optee_thread_id = get_user_reg(regs, 3);
+
+ set_user_reg(regs, 0, OPTEE_SMC_RETURN_RPC_CMD);
+ set_user_reg(regs, 1, r1);
+ set_user_reg(regs, 2, r2);
+
+ return true;
+}
+
+/* Handles return from Xen-issued RPC */
+static void handle_xen_rpc_return(struct optee_domain *ctx,
+ struct cpu_user_regs *regs,
+ struct optee_std_call *call,
+ struct shm_rpc *shm_rpc)
+{
+ call->state = OPTEEM_CALL_NORMAL;
+
+ /*
+ * Right now we have only one reason to be there - we asked guest
+ * to free shared buffer and it did it. Now we can tell OP-TEE that
+ * buffer allocation failed.
+ */
Should we add an ASSERT to ensure the command is the one we expect?
It is strange, that it is missing, actually. Looks like I forgot to add
it. But, looking at xen-error-handling, maybe BOG_ON() would be better?
The documentation in xen-error-handling needs some update. IIRC George
had a patch for updating the documentation on the mailing list.
BUG_ON() (and BUG()) should only be used if this is an error the
hypervisor can't recover. I am actually slowly go through the tree and
removing those who are in the guest path as some could be triggered on
new revision of the architecture :(.
In this case, this is in guest path and an error case. If something has
been missed and the guest may trigger the BUG_ON(). While this is a DOS,
this is still not desirable.
So there are three solutions:
1) Crash the guest
2) Add an ASSERT()
3) Print a warning
This is an error path so 2) might be less desirable if we don't do full
coverage of the code in debug mode.
Cheers,
--
Julien Grall
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