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SubjectRe: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call
On Thu, Sep 24, 2020 at 08:04:07PM +0200, Borislav Petkov wrote:
> On Tue, Sep 15, 2020 at 02:28:39PM +0300, Jarkko Sakkinen wrote:
> > diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> > new file mode 100644
> > index 000000000000..adbd59d41517
> > --- /dev/null
> > +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
>
> Why not simply
>
> arch/x86/entry/vdso/sgx.S
>
> ?

I renamed it as vsgx.S (for the sake of convention).

> > @@ -0,0 +1,157 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/export.h>
> > +#include <asm/errno.h>
> > +#include <asm/enclu.h>
> > +
> > +#include "extable.h"
> > +
> > +/* Offset of 'struct sgx_enclave_run' relative to %rbp. */
> > +#define SGX_ENCLAVE_RUN_PTR 2*8
> > +
> > +/* Offsets into 'struct sgx_enclave_run'. */
> > +#define SGX_ENCLAVE_RUN_TSC 0*8
> > +#define SGX_ENCLAVE_RUN_FLAGS 1*8
> > +#define SGX_ENCLAVE_RUN_EXIT_REASON 1*8 + 4
> > +#define SGX_ENCLAVE_RUN_USER_HANDLER 2*8
> > +/* #define SGX_ENCLAVE_RUN_USER_DATA 3*8 */
>
> What's with that guy? Left here as documentation showing what's at 3*8
> offset?

Yes.

> > +#define SGX_ENCLAVE_RUN_EXCEPTION 4*8
> > +
> > +#define SGX_SYNCHRONOUS_EXIT 0
> > +#define SGX_EXCEPTION_EXIT 1
>
> Those are in ...uapi/asm/sgx.h too. Unify?

I have not authored this patch but what I would propose is to use just
raw value in the place of these constants. It is practially just a
boolean value.

I can also add sgx_vdso.h to uapi directory. I just don't see the point.

Holding on for feedback with this one.

> > +
> > +/* Offsets into sgx_enter_enclave.exception. */
> > +#define SGX_EX_LEAF 0*8
> > +#define SGX_EX_TRAPNR 0*8+4
> > +#define SGX_EX_ERROR_CODE 0*8+6
> > +#define SGX_EX_ADDRESS 1*8
> > +
> > +.code64
> > +.section .text, "ax"
> > +
> > +SYM_FUNC_START(__vdso_sgx_enter_enclave)
> > + /* Prolog */
> > + .cfi_startproc
>
> Are the CFI annotations for userspace?

Yes, for stack unwinding. However, I would leave them out. Holding on
for further feedback with this tho.

> > + push %rbp
> > + .cfi_adjust_cfa_offset 8
> > + .cfi_rel_offset %rbp, 0
> > + mov %rsp, %rbp
> > + .cfi_def_cfa_register %rbp
> > + push %rbx
> > + .cfi_rel_offset %rbx, -8
> > +
> > + mov %ecx, %eax
> > +.Lenter_enclave:
> > + /* EENTER <= leaf <= ERESUME */
> > + cmp $EENTER, %eax
> > + jb .Linvalid_input
> > + cmp $ERESUME, %eax
> > + ja .Linvalid_input
> > +
> > + mov SGX_ENCLAVE_RUN_PTR(%rbp), %rcx
> > +
> > + /* No flags are currently defined/supported. */
> > + cmpl $0, SGX_ENCLAVE_RUN_FLAGS(%rcx)
> > + jne .Linvalid_input
> > +
> > + /* Load TCS and AEP */
>
> TSC
>
> I can see why you would write "TCS" though - there's a thread control
> structure thing too in that patch.

Renamed.

> > + mov SGX_ENCLAVE_RUN_TSC(%rcx), %rbx
> > + lea .Lasync_exit_pointer(%rip), %rcx
> > +
> > + /* Single ENCLU serving as both EENTER and AEP (ERESUME) */
> > +.Lasync_exit_pointer:
> > +.Lenclu_eenter_eresume:
> > + enclu
> > +
> > + /* EEXIT jumps here unless the enclave is doing something fancy. */
> > + mov SGX_ENCLAVE_RUN_PTR(%rbp), %rbx
> > +
> > + /* Set exit_reason. */
> > + movl $SGX_SYNCHRONOUS_EXIT, SGX_ENCLAVE_RUN_EXIT_REASON(%rbx)
> > +
> > + /* Invoke userspace's exit handler if one was provided. */
> > +.Lhandle_exit:
> > + cmpq $0, SGX_ENCLAVE_RUN_USER_HANDLER(%rbx)
> > + jne .Linvoke_userspace_handler
> > +
> > + /* Success, in the sense that ENCLU was attempted. */
> > + xor %eax, %eax
> > +
> > +.Lout:
> > + pop %rbx
> > + leave
> > + .cfi_def_cfa %rsp, 8
> > + ret
> > +
> > + /* The out-of-line code runs with the pre-leave stack frame. */
> > + .cfi_def_cfa %rbp, 16
> > +
> > +.Linvalid_input:
> > + mov $(-EINVAL), %eax
> > + jmp .Lout
> > +
> > +.Lhandle_exception:
> > + mov SGX_ENCLAVE_RUN_PTR(%rbp), %rbx
> > +
> > + /* Set the exit_reason and exception info. */
> > + movl $SGX_EXCEPTION_EXIT, SGX_ENCLAVE_RUN_EXIT_REASON(%rbx)
> > +
> > + mov %eax, (SGX_ENCLAVE_RUN_EXCEPTION + SGX_EX_LEAF)(%rbx)
> > + mov %di, (SGX_ENCLAVE_RUN_EXCEPTION + SGX_EX_TRAPNR)(%rbx)
> > + mov %si, (SGX_ENCLAVE_RUN_EXCEPTION + SGX_EX_ERROR_CODE)(%rbx)
> > + mov %rdx, (SGX_ENCLAVE_RUN_EXCEPTION + SGX_EX_ADDRESS)(%rbx)
> > + jmp .Lhandle_exit
> > +
> > +.Linvoke_userspace_handler:
> > + /* Pass the untrusted RSP (at exit) to the callback via %rcx. */
> > + mov %rsp, %rcx
> > +
> > + /* Save @e, %rbx is about to be clobbered. */
>
> I've been wondering what that "@e" thing is and I believe I've found it at the
> end: "... @e, the optional sgx_enclave_exception struct"
>
> Yes?
>
> Please rewrite what that "e" is here - I think you wanna say "struct
> sgx_enclave_run.exception" instead to clarify.

I agree. Open coded them as "struct sgx_enclave_exception".

>
> ...
>
> > diff --git a/arch/x86/include/asm/enclu.h b/arch/x86/include/asm/enclu.h
> > new file mode 100644
> > index 000000000000..06157b3e9ede
> > --- /dev/null
> > +++ b/arch/x86/include/asm/enclu.h
> > @@ -0,0 +1,8 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_X86_ENCLU_H
> > +#define _ASM_X86_ENCLU_H
> > +
> > +#define EENTER 0x02
> > +#define ERESUME 0x03
> > +
> > +#endif /* _ASM_X86_ENCLU_H */
> > diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> > index d0916fb9629e..1564d7f88597 100644
> > --- a/arch/x86/include/uapi/asm/sgx.h
> > +++ b/arch/x86/include/uapi/asm/sgx.h
>
> Are all those defines being added to the uapi header, also actually
> needed by userspace?

I'd remove the two constants, as it is clearly just a boolean value.

> > @@ -72,4 +72,132 @@ struct sgx_enclave_provision {
> > __u64 attribute_fd;
> > };
> >
> > +#define SGX_SYNCHRONOUS_EXIT 0
> > +#define SGX_EXCEPTION_EXIT 1
> > +
> > +struct sgx_enclave_run;
> > +
> > +/**
> > + * typedef sgx_enclave_exit_handler_t - Exit handler function accepted by
> > + * __vdso_sgx_enter_enclave()
> > + *
> > + * @rdi: RDI at the time of enclave exit
> > + * @rsi: RSI at the time of enclave exit
> > + * @rdx: RDX at the time of enclave exit
> > + * @ursp: RSP at the time of enclave exit (untrusted stack)
> > + * @r8: R8 at the time of enclave exit
> > + * @r9: R9 at the time of enclave exit
>
> I'm sure you can avoid that "at the time of enclave exit" repetition.

I edited this as

/**
* typedef sgx_enclave_exit_handler_t - Exit handler function accepted by
* __vdso_sgx_enter_enclave()
* @rdi: RDI snapshot
* @rsi: RSI snapshot
* @rdx: RDX snapshot
* @rsp: RSP snapshot (untrusted stack)
* @r8: R8 snapshot
* @r9: R9 snapshot
* @run: Pointer to the caller provided struct sgx_enclave_run
*
* Return:
* 0 or negative to exit vDSO
* positive to re-enter enclave (must be EENTER or ERESUME leaf)
*/
typedef int (*sgx_enclave_exit_handler_t)(long rdi, long rsi, long rdx,
long rsp, long r8, long r9,
struct sgx_enclave_run *run);


> > +/**
> > + * typedef vdso_sgx_enter_enclave_t - Prototype for __vdso_sgx_enter_enclave(),
> > + * a vDSO function to enter an SGX enclave.
> > + *
> > + * @rdi: Pass-through value for RDI
> > + * @rsi: Pass-through value for RSI
> > + * @rdx: Pass-through value for RDX
> > + * @leaf: ENCLU leaf, must be EENTER or ERESUME
> > + * @r8: Pass-through value for R8
> > + * @r9: Pass-through value for R9
> > + * @r: struct sgx_enclave_run, must be non-NULL
> > + *
> > + * NOTE: __vdso_sgx_enter_enclave() does not ensure full compliance with the
> > + * x86-64 ABI, e.g. doesn't handle XSAVE state. Except for non-volatile
> > + * general purpose registers, EFLAGS.DF, and RSP alignment, preserving/setting
> > + * state in accordance with the x86-64 ABI is the responsibility of the enclave
> > + * and its runtime, i.e. __vdso_sgx_enter_enclave() cannot be called from C
> > + * code without careful consideration by both the enclave and its runtime.
> > + *
> > + * All general purpose registers except RAX, RBX and RCX are passed as-is to
> > + * the enclave. RAX, RBX and RCX are consumed by EENTER and ERESUME and are
> > + * loaded with @leaf, asynchronous exit pointer, and @tcs respectively.
>
> That tcs is probably struct sgx_enclave_run.tcs?

Yes.

> > + * RBP and the stack are used to anchor __vdso_sgx_enter_enclave() to the
> > + * pre-enclave state, e.g. to retrieve @e and @handler after an enclave exit.
>
> Expand @e and do you mean "@user_handler" with "@handler"?

I replaced this and similar fields as @run.user_handler and so forth.

>
> > + * All other registers are available for use by the enclave and its runtime,
> > + * e.g. an enclave can push additional data onto the stack (and modify RSP) to
> > + * pass information to the optional exit handler (see below).
> > + *
> > + * Most exceptions reported on ENCLU, including those that occur within the
> > + * enclave, are fixed up and reported synchronously instead of being delivered
> > + * via a standard signal. Debug Exceptions (#DB) and Breakpoints (#BP) are
> > + * never fixed up and are always delivered via standard signals. On synchrously
> > + * reported exceptions, -EFAULT is returned and details about the exception are
> > + * recorded in @e, the optional sgx_enclave_exception struct.
> > + *
> > + * If an exit handler is provided, the handler will be invoked on synchronous
> > + * exits from the enclave and for all synchronously reported exceptions. In
> > + * latter case, @e is filled prior to invoking the handler.
> > + *
> > + * The exit handler's return value is interpreted as follows:
> > + * >0: continue, restart __vdso_sgx_enter_enclave() with @ret as @leaf
> > + * 0: success, return @ret to the caller
> > + * <0: error, return @ret to the caller
> > + *
> > + * The exit handler may transfer control, e.g. via longjmp() or C++ exception,
> > + * without returning to __vdso_sgx_enter_enclave().
> > + *
> > + * Return:
> > + * 0 on success (ENCLU reached),
> > + * -EINVAL if ENCLU leaf is not allowed,
> > + * -errno for all other negative values returned by the userspace exit handler
> > + */
> > +typedef int (*vdso_sgx_enter_enclave_t)(unsigned long rdi, unsigned long rsi,
> > + unsigned long rdx, unsigned int leaf,
> > + unsigned long r8, unsigned long r9,
> > + struct sgx_enclave_run *r);
> > +
> > #endif /* _UAPI_ASM_X86_SGX_H */
> > --
> > 2.25.1
> >
>
> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

Also, I renamed 'r' as 'run' in some places.

End result:

https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-sgx.git/tree/arch/x86/include/uapi/asm/sgx.h

I'm wondering this sentence:

"The calling convention is custom and does not follow System V x86-64 ABI."

AFAIK, now the vDSO is fully C-callable but waiting for feedback before
removing it.

/Jarkko

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